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Toxicological information

Carcinogenicity

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Description of key information

In rats, the target tissue for carcinogenesis is the nasal cavity where olfactory epithelial neuroblastoma (a rare malignant tumor) and respiratory epithelial adenoma developed following high long-term inhalation exposure to naphthalene, whereas lung tumors (alveolar/bronchiolar adenomas but one carcinoma) are produced in mice. Both species suffer from early acute and severe chronic inflammation of the mucous membranes in the respiratory tract. The toxicological relevance to humans of the findings in rodents has been under controversial discussion in the scientific world.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Link to relevant study records
Reference
Endpoint:
carcinogenicity: oral
Type of information:
experimental study
Adequacy of study:
supporting study
Study period:
1955
Reliability:
3 (not reliable)
Rationale for reliability incl. deficiencies:
other: Early study not following guidelines or satisfying GLP criteria. Not documented in detail. Low number of animals.
Qualifier:
no guideline followed
Principles of method if other than guideline:
Rats were exposed to naphthalene by the oral route 6 days a week during a period of 700 days. Daily uptake between 10 and 20 mg per animal.
GLP compliance:
no
Species:
rat
Strain:
other: BD I and BD III
Sex:
not specified
Details on test animals and environmental conditions:
TEST ANIMALS
- Source: own laboratory
- Age at study initiation: 100 days
Route of administration:
oral: feed
Vehicle:
other: oil (not specified in detail)
Details on exposure:
DIET PREPARATION
- Rate of preparation of diet (frequency): daily, 6 days a week
- Mixing appropriate amounts with (Type of food): mixed food enriched with protein, vitamin, fat and salt
Analytical verification of doses or concentrations:
no
Duration of treatment / exposure:
700 days
Frequency of treatment:
6 times a week for a period of 700 days
Remarks:
Doses / Concentrations:
10-20 mg per animal daily, total dose 10g per animal
Basis:
nominal in diet
No. of animals per sex per dose:
28
Control animals:
no
Clinical signs:
no effects observed
Mortality:
no mortality observed
Body weight and weight changes:
not examined
Food consumption and compound intake (if feeding study):
not examined
Food efficiency:
not examined
Water consumption and compound intake (if drinking water study):
not examined
Ophthalmological findings:
not examined
Haematological findings:
not examined
Clinical biochemistry findings:
not examined
Urinalysis findings:
not examined
Behaviour (functional findings):
not examined
Organ weight findings including organ / body weight ratios:
no effects observed
Gross pathological findings:
no effects observed
Histopathological findings: non-neoplastic:
no effects observed
Histopathological findings: neoplastic:
no effects observed
Dose descriptor:
NOAEL
Remarks on result:
not determinable
Remarks:
no NOAEL identified
Conclusions:
Considering the small number of animals used, no firm conclusions as to the carcinogenic potential of naphthalene can be drawn from this early study.
Executive summary:

In this study groups of 28 BDI and BDIII rats (sex not specified) were treated with 0 or 10-20 mg/day naphthalene in the diet, 6 days/week for 100 weeks. Animals were kept under observation until they died. Survival was unaffected by treatment and there were no signs of toxicity throughout the study. It was also reported that there was no damage to the eyes. Autopsy was reported to be thorough and any organs showing abnormalities were examined histologically. There was no increase in the incidence of any tumours in treated animals. Overall, considering the small number of animals used, no firm conclusions as to the carcinogenic potential of naphthalene can be drawn from this early study and it is classified as supporting study only.

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
NOAEL
Study duration:
chronic
Species:
rat
Quality of whole database:
low

Carcinogenicity: via inhalation route

Link to relevant study records
Reference
Endpoint:
carcinogenicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Study period:
105 weeks
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: GLP conform study.
Qualifier:
no guideline available
Principles of method if other than guideline:
Groups of 49 male and 49 female rats were exposed to naphthalene by inhalation at concentrations of 0, 10, 30, or 60 ppm for 6 hours per day, 5 days per week for 105 weeks. Survival, body weights, pathology findings and gross observations are reported.
GLP compliance:
yes
Species:
rat
Strain:
Fischer 344
Sex:
male/female
Details on test animals and environmental conditions:
TEST ANIMALS
- Source: Taconic Laboratory Animals and Services (Germantown, NY)
- Age at study initiation: 6 weeks
- Weight at study initiation:
- Fasting period before study:
- Housing: Stainless steel chambers (Harford System, Division of Lab Products, Inc., Aberdeen, MD), changed weekly, 1 animal per chamber
- Diet (e.g. ad libitum): NTP-2000 irradiated pelleted diet (Zeigler Brothers, Inc., Gardners, PA), available ad libitum except during exposure periods, changed weekly
- Water (e.g. ad libitum): Softened tap water (Richland municipal supply) via automatic watering system (Edstrom Industries, Waterford WI), available ad libitum
- Acclimation period: 14 days

ENVIRONMENTAL CONDITIONS
- Temperature: 75° ± 3°F
- Relative humidity: 55% ± 15%
- Room fluorescent light: 12 hours/day
- Chamber air changes: 15 ± 2/hour

- Date of First Exposure: 28 March 1996
- Date of Last Exposure: 27 March 1998
- Necropsy Dates: 30 March-2 April 1998
- Average Age at Necropsy: 110-111 (males) or 111 (females) weeks
Route of administration:
inhalation: vapour
Type of inhalation exposure (if applicable):
whole body
Vehicle:
clean air
Details on exposure:
GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
- Exposure apparatus: The generator consisted of a 2-L glass reaction flask surrounded by a heated mantle. Heated nitrogen metered into the flask carried the vaporized naphthalene out of the generator. The mantle and nitrogen temperatures were adjusted to maintain the temperature of the vapour above the bulk naphthalene between 66°C and 71° C while the bulk chemical was monitored to ensure that its temperature was maintained below the melting point.
- Method of conditioning air: heated, HEPA- and charcoal-filtered air
- Temperature: 75° ± 3°F
- Relative humidity: 55% ± 15%
- Chamber air changes: 15 ± 2/hour

TEST ATMOSPHERE
- Brief description of analytical method used: The naphthalene concentrations in the exposure chambers were monitored by an online gas chromatograph; the average chamber concentrations were maintained within 1% of the target concentrations. Samples were drawn from each exposure chamber approximately every 24 minutes using a 12-port stream select valve. The online gas chromatograph was checked throughout the day for instrument drift against an online standard of naphthalene. The online gas chromatograph was calibrated monthly by a comparison of chamber concentration
data to data from grab samples, which were collected with charcoal sampling tubes and analyzed by an offline gas chromatograph. The offline gas chromatograph was calibrated with gravimetrically prepared standards of naphthalene containing 1-phenylhexane as an internal standard in toluene.
- Samples taken from breathing zone: yes
Analytical verification of doses or concentrations:
yes
Details on analytical verification of doses or concentrations:
On-line gas chromatograph (model 5890, manufactured by Hewlett-Packard (Palo Alto, CA), Flame ionization detector, column: DB-5, 30 m × 0.53 mm, 1.5 µm film (J&W Scientific), carrier gas: nitrogen at approximately 25 mL/minute, oven temperature program: isothermally at 175° C.
Off-line gas chromatograph (model 5890, manufactured by Hewlett-Packard (Palo Alto, CA), Flame ionization detector, column: DB-5, 30 m × 0.53 mm, 1.5 µm film (J&W Scientific), carryer gas: helium at 6 psi head pressure, oven temperature program: 60° C for 1 minute, then 16° C/minute to 200°C
Duration of treatment / exposure:
6 hours plus T90 (12 minutes) per day
Frequency of treatment:
5 days per week, for 105 weeks
Remarks:
Doses / Concentrations:
0, 10, 30, or 60 ppm (0, 52, 157, and 314 mg/m3)
Basis:
analytical conc.
No. of animals per sex per dose:
49 per sex per concentration
Control animals:
yes, concurrent vehicle
Details on study design:
- Dose selection rationale:
The exposure concentrations for the naphthalene study were selected based on the results of a 2-year study in mice in which animals were exposed to 0, 10, or 30 ppm. Additionally, the highest exposure concentration (60 ppm) was selected to allow for variations in the maximum achievable concentration without aerosolization, determined by the study laboratory to be approximately 80 ppm, due to changes in temperature or operating conditions within the exposure system. The lowest concentration of 10 ppm is the threshold limit value for naphthalene (ACGIH, 1999).
Observations and examinations performed and frequency:
CAGE SIDE OBSERVATIONS: Yes
- Time schedule: Observed twice daily

DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule: Clinical findings were recorded every 4 weeks beginning at week 4 and every 2 weeks beginning at week 92.

BODY WEIGHT: Yes
- Time schedule for examinations: Body weights were recorded on study day 1, every 4 weeks beginning at week 4, and every 2 weeks beginning at week 92.

NECROPSY
Necropsy was performed on all core study animals.

HISTOPATHOLOGY
Complete histopathology was performed on all core study animals. In addition to gross lesions and tissue masses, the following tissues were
examined: adrenal gland, bone with marrow, brain, clitoral gland, esophagus, eyes, heart, large intestine (cecum, colon, rectum), small intestine
(duodenum, jejunum, ileum), kidney, larynx, liver, lung with mainstem bronchi, lymph nodes (mandibular, mesenteric, bronchial, mediastinal),
mammary gland (females), nose, ovary, pancreas, parathyroid gland, pituitary gland, preputial gland, prostate gland, salivary gland, skin, spleen,
stomach (forestomach and glandular), testis (with epididymis and seminal vesicle) thymus, thyroid gland, trachea, urinary bladder, and uterus.
Sacrifice and pathology:
HISTOPATHOLOGY: Yes
Statistics:
Survival Analyses: The probability of survival was estimated by the product-limit procedure of Kaplan and Meier (1958) and is presented in the form of graphs. A missexed animal was censored from the survival analyses; animals dying from natural causes were not censored. Statistical analyses for possible dose-related effects on survival used Cox (1972) method for testing two groups for equality and Tarone (1975) life table test to identify dose-related trends. All reported P values for the survival analyses are two sided.

The Poly-k test (Bailer and Portier, 1988; Portier and Bailer, 1989; Piegorsch and Bailer, 1997) was used to assess neoplasm and nonneoplastic lesion prevalence.

Body weight data, which historically have approximately normal distributions, were analyzed with the parametric multiple comparison procedures of Dunnett (1955) and Williams (1971, 1972). Jonckheeres test (Jonckheere, 1954) was used to assess the significance of the dose-related trends and to determine whether a trend-sensitive test (Williams test) was more appropriate for pairwise comparisons than a test that does not assume a monotonic dose-related trend (Dunnetts test).
Clinical signs:
no effects observed
Description (incidence and severity):
There were no clinical findings related to naphthalene exposure.
Mortality:
no mortality observed
Description (incidence):
There were no clinical findings related to naphthalene exposure.
Body weight and weight changes:
effects observed, treatment-related
Description (incidence and severity):
Mean body weights of all exposed groups of male rats were less than those of the chamber control group throughout most of the study. Mean body weights of exposed groups of females were generally similar to those of the chamber controls.
Food consumption and compound intake (if feeding study):
not examined
Food efficiency:
not examined
Water consumption and compound intake (if drinking water study):
not examined
Ophthalmological findings:
not examined
Haematological findings:
not examined
Clinical biochemistry findings:
not examined
Urinalysis findings:
not examined
Behaviour (functional findings):
not examined
Organ weight findings including organ / body weight ratios:
not examined
Gross pathological findings:
effects observed, treatment-related
Description (incidence and severity):
Malignant nasal neoplasms were observed in several male and female rats. These masses frequently partially occluded the nasal passages or obliterated the normal architecture of the nasal turbinates and, in some affected animals, invaded the brain.
Histopathological findings: non-neoplastic:
effects observed, treatment-related
Description (incidence and severity):
Increased incidences of a variety of non-neoplastic lesions occurred in the nose of exposed male and female rats.
Histopathological findings: neoplastic:
effects observed, treatment-related
Description (incidence and severity):
Increased incidences of a variety of neoplasms occurred in the nose of exposed male and female rats.
Details on results:
Nose: Increased incidences of a variety of neoplasms and nonneoplastic lesions occurred in the nose of exposed male and female rats. These lesions were observed in all three levels of the nasal cavity (level I, excised immediately posterior to the upper incisor teeth; level II, excised through the level of the incisive papilla anterior to the first palatial ridge; and level III, excised through the middle of the second molar teeth). Neuroblastomas of the olfactory epithelium occurred in males exposed to 30 or 60 ppm and in all exposed groups of females. The incidences of neuroblastoma occurred with positive trends in males and females, and the incidence in females exposed to 60 ppm was significantly greater than that in the chamber controls. Neuroblastomas have not been observed in male or female chamber control rats. Neuroblastomas were variably sized, unilateral or bilateral invasive masses that arose in Level III of the nasal cavity and extended into Levels II and I. Larger masses occluded the nasal passages and often obliterated the nasal architecture invading nerves, nasal bones, and the cribriform plate. Other masses extended along the mucosa and replaced the epithelium of the turbinates and nasal septum. The morphology of the neuroblastomas varied. Component neoplastic cells were round, polygonal, or spindleshaped and arranged in variably sized, irregular islands, cords, and rosettes separated by fibrovascular stroma. In other masses, component cells were arranged in a glandular pattern. Some cells had scant eosinophilic to amphophilic cytoplasm with pale oval to polygonal vesicular nuclei and prominent central nucleoli; others had abundant cytoplasm and elongate, intensely basophilic nuclei. Small nests of neoplastic cells were present in the lamina propria of the turbinates and nasal septum, and in olfactory nerve bundles. A few neoplasms had focal irregular areas of squamous metaplasia, sometimes extensive with formation of keratin pearls. Variably sized focal areas of coagulative necrosis were also observed in most neuroblastomas. Mitotic figures were abundant. Neoplasms that invaded the cribriform plate extended into the olfactory lobes of the brain. One male each in the 30 and 60 ppm groups had metastases in the lungs. The incidences of adenoma of the respiratory epithelium occurred with a positive trend in male rats and were significantly increased in all exposed groups; the incidences in female rats exposed to 30 or 60 ppm were also increased, but not significantly. Nasal adenomas have not been observed in male or female chamber control rats. Adenomas arose from the respiratory and transitional epithelia of Levels I and II of the nasal cavity along the medial or lateral aspects or tips of the nasoturbinates or the lateral wall. They were irregular exophytic, polypoid, pedunculated or broad-based sessile masses that varied in size and sometimes partially occluded the nasal passages. Component neoplastic cells were well-differentiated, simple to cuboidal to columnar and arranged primarily as variably sized glands surrounded by scant fibrovascular stroma with few focal solid areas of cells. In some masses, the epithelium appeared to be pseudostratified. The glands were often variably distended by luminal accumulations of proteinaceous secretory material and cellular debris. A few adenomas were composed of less well differentiated cells that were squamoid in morphology; these cell were large, round to polygonal, with scant to moderate amounts of eosinophilic cytoplasm and large round to oval nuclei that contained one or two prominent nucleoli. In addition to the nasal neoplasms, the incidences of a variety of nonneoplastic lesions in exposed males and females were significantly greater than those in the chamber controls. These lesions included atypical (basal cell) hyperplasia, atrophy, chronic inflammation, and hyaline degeneration of the olfactory epithelium; hyperplasia, squamous metaplasia, hyaline degeneration, and goblet cell hyperplasia of the respiratory epithelium; and glandular hyperplasia and squamous metaplasia. In general, the severities of olfactory epithelial and glandular lesions increased with increasing exposure concentration. Atypical hyperplasia of the olfactory epithelium occurred primarily along the nasal septum of the ethmoid region. Atypical hyperplasia consisted of disorganization of olfactory epithelium with proliferation of nests of sensory cells within or beneath the epithelium and multifocal nodular proliferations of basal cells, which extended into the submucosa. Atrophy of olfactory epithelium was characterized by a decrease in the height of the epithelium lining the dorsal meatuses of Level II and the ethmoid turbinates of Level III due to variable loss of epithelial cells. Mild atrophy consisted of only loss of sustentacular cells. Moderate atrophy consisted of loss of mostly sustentacular cells; however, there was also loss of olfactory neurons. In the most severe cases, there was complete loss of sustentacular cells and neurons, leaving only basal epithelial cells. Frequently, ciliated columnar cells replaced normal olfactory epithelium. Although included in the spectrum of changes diagnosed as olfactory epithelial atrophy, the latter alteration is often classified as respiratory epithelial metaplasia. Chronic inflammation of the olfactory region consisted of infiltrates of primarily mononuclear inflammatory cells within the lamina propria invariably accompanied by fibrosis (Plate 10). In affected sites, there was often synechia between adjacent turbinates. Respiratory epithelial hyperplasia involved the lateral wall and medial surface of the naso- and maxilloturbinates, and was mostly focal to segmental but sometimes involved most of the turbinate extending onto the lateral wall in Levels I and II of the nasal cavity. The affected epithelia appeared thickened by increased numbers of disorganized, often pseudostratified, epithelial cells (Plate 11); component epithelial cells were non-ciliated flattened, or ciliated cuboidal to columnar ciliated. Frequently, the hyperplastic ciliated epithelium was folded in rugose fashion sometimes extending into the submucosa forming pseudoglands, or was continuous with the epithelium of submucosal glands. Respiratory epithelial squamous metaplasia involved the lateral surfaces of the nasoturbinates and the lateral wall in Level I of the nasal cavity. Metaplasia consisted of replacement of the normally ciliated respiratory epithelium by one to six layers of polygonal cells with flattening of the more superficial cells. Keratinization was seldom noted. Glandular hyperplasia primarily affected the Bowman.s glands of the nasal septum, in the dorsal meatus, and ethmoid turbinates in Level III of the nasal cavity. Hyperplasia consisted of proliferation of glands that were frequently enlarged or distended with cell debris and proteinaceous material. Frequently, affected glands were lined by hyperplastic ciliated epithelium that was continuous with that of the mucosa. The hyperplastic cells were often distended by intracytoplasmic protein or protein globules. Squamous metaplasia of glands often accompanied hyperplasia. It was characterized by replacement of the normal epithelial lining by several layers of nonkeratinized squamous cells that often obliterated the glandular lumen. Goblet cell hyperplasia was generally of minimal severity and primarily involved the respiratory epithelium of the nasal septum in Level I of the nasal cavity. Goblet cells were increased in number, were swollen with mucus, and often formed in small gland-like clusters within the mucosal epithelium. Hyaline degeneration was a focal or multifocal, minimal to mild change that affected both the respiratory and olfactory epithelia. Affected epithelial cells were swollen by intracytoplasmic homogenous, brightly eosinophilic globules. These globules are commonly observed in aging animals, and the severity may increase with age. In chronic inhalation studies, the incidence and severity of this change are often exacerbated in an exposure-dependent manner. Goblet cell hyperplasia and hyaline degeneration are considered non-specific protective or adaptive responses to chronic inhalation of irritants.

Lung: The incidences of alveolar epithelial hyperplasia in all exposed groups of female rats were greater than that in the chamber controls (chamber control, 4/49; 10 ppm, 11/49; 30 ppm, 11/49; 60 ppm, 9/49); the increased incidences in the 10 and 30 ppm groups were significant. However, in male rats, the incidences of hyperplasia were significantly decreased in the 10 and 30 ppm groups (23/49, 12/49, 9/48, 16/49). The incidences of minimal chronic inflammation of the lungs were significantly increased in male rats exposed to 10 or 60 ppm (2/49, 13/49, 6/48, 15/49). The incidences of lung neoplasms were not affected in exposed males (2/49, 3/49, 1/48, 0/49) or females (1/49, 0/49, 0/49, 0/49). Chronic inflammation consisted of small focal interstitial and intra-alveolar collections of varying numbers of macrophages, neutrophils, and lymphocytes along with minimal interstitial fibrosis. Mixed with the inflammatory cells were multinucleated giant cells, cell debris, and cholesterol clefts. This change occurred subpleurally and/or at the tips of lung lobes. Such minimal inflammatory foci are often found in chamber control rats, as they were in this study. Although the incidences of chronic inflammation were increased in groups exposed to naphthalene, it was not clear whether this change was exposure related.
Dose descriptor:
NOAEC
Remarks on result:
not determinable
Remarks:
no NOAEC identified
Dose descriptor:
NOAEL
Remarks on result:
not determinable
Remarks:
no NOAEL identified
Dose descriptor:
LOAEC
Effect level:
ca. 50 mg/m³ air (nominal)
Based on:
test mat.
Sex:
male/female
Basis for effect level:
other: histopathology of the respiratory tract: lfactory epithelium, neuroblastoma
Remarks on result:
other: Effect type: carcinogenicity (migrated information)

Table 1: Results of the NTP Carcinogenicity Inhalation Study on Rats: Survey on Nasal
               Tumours and Non-neoplastic Effects [NTP, Dec. 2000]

Exposure Concentration

0 ml/m3

10 ml/m3

30 ml/m3

60 ml/m3

Neoplastic changes

Male animals

Alveolar/brochiolar carcinoma

0

3 (6 %)

0

0

Respiratory epithelium, neuroblastoma (metastatic)

0

0

1 (2 %)

1 (2 %)

Olfactory epithelium, neuroblastoma

0

0

4 (8 %)

3 (6 %)

Respiratory epithelium, adenoma

0

6 (12 %)

8 (17 %)

15 (31 %)

Female animals

Olfactory epithelium, neuroblastoma

0

2 (4 %)

 3 (6 %)

11 (22 %)

Olfactory epithelium, neuroblastoma (metastatic, nose)

0

0

0

1 (2 %)

Respiratory epithelium, adenoma

0

0

4 (8 %)

2 (4 %)

Non-neoplastic changes

Male animals

Olfactory epithelium: atrophy

3 (6 %)

49 (100 %)

48 (100 %)

47 (98 %)

Olfactory epithelium: hyaline degeneration

3 (6 %)

45 (92 %)

40 (83 %)

38 (79 %)

Olfactory epithelium: atypical hyperplasia

0

48 (98 %)

45 (94 %)

46 (96 %)

Olfactory epithelium: chronic inflammation

0

49 (100 %)

48 (100 %)

48 (100 %)

Respiratory epithelium: hyaline degeneration

0

20 (41 %)

19 (40 %)

19 (40 %)

Respiratory epithelium: hyperplasia

3 (6 %)

21 (43 %)

29 (60 %)

29 (60 %)

Respiratory epithelium: squamous metaplasia

0

15 (31 %)

23 (48 %)

18 (38 %)

Glands (nose): hyperplasia

1 (2 %)

49 (100 %)

48 (100 %)

48 (100 %)

Glands (nose): squamous metaplasia

0

3 (6 %)

14 (29 %)

26 (54 %)

Goblet cells (resp. epithelium): hyperplasia

0

25 (51 %)

29 (60 %)

26 54 %)

Female animals

Olfactory epithelium: atrophy

0

49 (100 %)

49 (100 %)

47 (96 %)

Olfactory epithelium: hyaline degeneration

13 (27 %)

46 (94 %)

49 (100 %)

45 (92 %)

Olfactory epithelium: atypical hyperplasia

0

48 (98 %)

48 (98 %)

43 (88 %)

Olfactory epithelium: chronic inflammation

0

47 (96 %)

47 (96 %)

45 (92 %)

Respiratory epithelium: hyaline degeneration

8 (16 %)

33 (67 %)

34 (69 %)

28 (57 %)

Respiratory epithelium: hyperplasia

0

18 (37 %)

22 (45 %)

23 (47 %)

Respiratory epithelium: squamous metaplasia

0

21 (43 %)

17 (35 %)

15 (31 %)

Glands (nose): hyperplasia

0

48 (100 %)

48 (100 %)

42 (86 %)

Glands (nose): squamous metaplasia

0

16 (33 %)

29 (59 %)

20 (41 %)

Goblet cells (resp. epithelium): hyperplasia

0

16 (33 %)

29 (59 %)

20 (41 %)

TABLE 2: Summary of the Incidence of Neoplasms in Male Rats in the 2-Year Inhalation Study of Naphthalene a)

 Chamber Control   10 ppm   30 ppm   60 ppm 
Disposition Summary 
 Animals initially in study   49   49   49   49 
 Early deaths         
 Moribund   21   22   19   25 
 Natural deaths   4   5   6   3 
 Survivors         
 Terminal sacrifice   24   22   23   21 
 Missexed       1   
 Animals examined microscopically   49   49   48   49 
Alimentary System 
 Intestine large, colon   (48)   (49)   (48)   (48) 
 Polyp adenomatous       1 (2%)   
 Intestine large, cecum   (46)   (49)   (47)   (48) 
 Intestine small, jejunum   (45)   (47)   (43)   (47) 
 Carcinoma         1 (2%) 
 Leiomyosarcoma         1 (2%) 
 Intestine small, ileum   (45)   (47)   (45)   (47) 
 Liver   (49)   (49)   (48)   (49) 
 Hepatocellular carcinoma   1 (2%)       1 (2%) 
 Hepatocellular adenoma   1 (2%)       3 (6%) 
 Mesentery   (13)   (6)   (9)   (8) 
 Hemangiosarcoma         1 (13%) 
 Sarcoma   1 (8%)       
 Pancreas   (49)   (49)   (48)   (49) 
 Adenoma   1 (2%)       2 (4%) 
 Carcinoma         1 (2%) 
 Mixed tumor benign       1 (2%)   
 Salivary glands   (49)   (49)   (47)   (49) 
 Stomach, forestomach   (49)   (49)   (48)   (49) 
 Stomach, glandular   (49)   (49)   (48)   (49) 
 Tongue     (1)   (1)   
 Squamous cell carcinoma     1 (100%)     
Cardiovascular System 
 Heart   (49)   (49)   (48)  (49)
 Schwannoma benign     1 (2%)   2 (4%)   
 Schwannoma malignant, metastatic, skin       1 (2%)   
Endocrine System 
 Adrenal cortex   (49)   (49)   (48)   (49) 
 Adenoma   2 (4%)     1 (2%)   2 (4%) 
 Adrenal medulla   (49)   (49)   (47)   (49) 
 Pheochromocytoma malignant   1 (2%)   3 (6%)   1 (2%)   1 (2%) 
 Pheochromocytoma benign   4 (8%)   6 (12%)   6 (13%)   8 (16%) 
 Bilateral, pheochromocytoma benign     1 (2%)     
 Islets, pancreatic   (49)   (49)   (48)   (49) 
 Adenoma   2 (4%)   5 (10%)   3 (6%)   2 (4%) 
 Carcinoma   4(8%)   4(8%)   4(8%)   4(8%) 
 Pituitary gland   (49)   (49)   (47)   (49) 
 Pars distalis, adenoma   31 (63%)   31 (63%)   35 (74%)   29 (59%) 
 Thyroid gland   (46)   (47)   (45)   (47) 
 Bilateral, C-cell, adenoma     1 (2%)     
 C-cell, adenoma   9 (20%)   5 (11%)   4 (9%)   4 (9%) 
 C-cell, carcinoma   1 (2%)   2 (4%)   1 (2%)   1 (2%) 
 Follicular cell, adenoma       1 (2%)   
 Follicular cell, carcinoma     1 (2%)     
Genital System 
 Epididymis   (49)   (49)   (48)   (49) 
 Preputial gland   (48)   (49)   (47)   (49) 
 Adenoma   3 (6%)     1 (2%)   1 (2%) 
 Carcinoma   3(6%)   1(2%)   1(2%)   1(2%) 
 Prostate   (49)   (49)   (48)   (49) 
 Seminal vesicle   (47)   (49)   (47)   (47) 
 Carcinoma       1 (2%)   
 Testes   (49)   (49)   (48)   (49) 
 Bilateral, interstitial cell, adenoma   24 (49%)   22 (45%)   19 (40%)   20 (41%) 
 Interstitial cell, adenoma   14 (29%)   10 (20%)   17 (35%)   11 (22%) 
Hematopoietic System 
 Bone marrow   (49)   (49)   (48)   (49) 
 Lymph node   (3)   (3)   (8)   (4) 
 Lymph node, bronchial   (29)   (36)   (38)   (35) 
 Lymph node, mandibular   (40)   (45)   (46)   (44) 
 Lymph node, mesenteric   (47)   (49)   (48)   (49) 
 Lymph node, mediastinal   (24)   (28)   (44)   (41) 
 Spleen   (49)   (49)   (48)   (49) 
 Hemangiosarcoma     1 (2%)     1 (2%) 
 Thymus   (47)   (46)   (43)   (46) 
 Schwannoma malignant, metastatic, skin       1 (2%)   
Musculoskeletal System 
 Bone   (49)   (49)   (48)  -49
 Osteosarcoma   1 (2%)   1 (2%)     
Nervous System 
 Brain   (49)   (49)   (48)   (49) 
 Neuroblastoma, metastatic, nose         2 (4%) 
 Spinal cord     (1)     
Special Senses System 
 Eye   (48)   (48)   (48)   (48) 
 Zymbal's gland       (1)   (1) 
 Carcinoma       1 (100%)   
 Bilateral, carcinoma         1 (100%) 
Urinary System 
 Kidney   (49)   (49)   (48)   (49) 
 Schwannoma malignant, metastatic, skin       1 (2%)   
 Renal tubule, carcinoma         1 (2%) 
 Transitional epithelium, carcinoma   1 (2%)       
 Urinary bladder   (48)   (49)   (48)   (49) 
 Transitional epithelium, papilloma   1 (2%)       2 (4%) 
Systemic Lesions
Multiple organs b)   (49)   (49)   (48)   (49) 
 Leukemia mononuclear   26 (53%)   21 (43%)   24 (50%)   17 (35%) 
 Mesothelioma benign   2 (4%)     1 (2%)   1 (2%) 
Neoplasm Summary
Total animals with primary neoplasmsc)   48   49   48   49 
 Total primary neoplasms   149   139   152   148 
 Total animals with benign neoplasms   46   47   47   47 
 Total benign neoplasms   107   96   110   108 
 Total animals with malignant neoplasms   34   32   34   32 
 Total malignant neoplasms   42   43   42   39 
 Total animals with metastatic neoplasms   4   3   4   2 
 Total metastatic neoplasms   4   3   8   2 
 Total animals with uncertain neoplasms 
 benign or malignant 
       1 
 Total uncertain neoplasms         1 

a) Number of animals examined microscopically at the site and the number of animals with neoplasm

b) Number of animals with any tissue examined microscopically

c) Primary neoplasms: all neoplasms except metastatic neoplasms

TABLE 3 Summary of the Incidence of Nonneoplastic Lesions in Male Rats in the 2-Year Inhalation Study of Naphthalene a)

  Chamber Control  10 ppm 30 ppm 60 ppm
 Disposition Summary 
 Animals initially in study   49   49   49   49 
 Early deaths                                                  
 Moribund   21   22   19   25 
 Natural deaths   4   5   6   3 
 Survivors                                                        
 Terminal sacrifice   24   22   23   21 
 Missexed       1   
 Animals examined microscopically   49   49   48   49 
 Alimentary System 
 Liver   (49)   (49)   (48)   (49) 
 Angiectasis    1 (2%)    1 (2%)    1 (2%)   
 Basophilic focus    34 (69%)    31 (63%)    28 (58%)    32 (65%) 
 Clear cell focus    14 (29%)    14 (29%)    14 (29%)    11 (22%) 
 Degeneration, cystic    3 (6%)    3 (6%)    2 (4%)    2 (4%) 
 Eosinophilic focus    3 (6%)    2 (4%)    1 (2%)    2 (4%) 
 Fatty change    2 (4%)    2 (4%)    4 (8%)    5 (10%) 
 Hepatodiaphragmatic nodule    1 (2%)    3 (6%)    2 (4%)    2 (4%) 
 Inflammation, granulomatous      1 (2%)     
 Mixed cell focus    3 (6%)    2 (4%)    2 (4%)    2 (4%) 
 Necrosis        1 (2%)   
 Regeneration    1 (2%)    2 (4%)    1 (2%)    1 (2%) 
 Syncytial alteration          1 (2%) 
 Tension lipidosis        1 (2%)   
 Artery, inflammation    1 (2%)       
 Bile duct, hyperplasia    35 (71%)    32 (65%)    28 (58%)    21 (43%) 
 Centrilobular, necrosis    13 (27%)    11 (22%)    7 (15%)    4 (8%) 
 Mesentery   (13)    (6)    (9)    (8) 
 Artery, inflammation, chronic active    3 (23%)        1 (13%) 
 Artery, mineralization        1 (11%)   
 Fat, haemorrhage      1 (17%)     
 Fat, inflammation      1 (17%)     
 Fat, necrosis    10 (77%)    4 (67%)    8 (89%)    6 (75%) 
 Pancreas   (49)   (49)   (48)   (49) 
 Atrophy    19 (39%)    17 (35%)    17 (35%)    14 (29%) 
 Basophilic focus      1 (2%)    1 (2%)    1 (2%) 
 Hyperplasia      3 (6%)    2 (4%)    1 (2%) 
 Artery, inflammation    1 (2%)      1 (2%)   
 Duct, cyst          1 (2%) 
 Salivary glands   (49)   (49)   (47)   (49) 
 Atrophy      1 (2%)     
 Metaplasia, squamous    1 (2%)       
 Necrosis    1 (2%)       
 Stomach, forestomach   (49)   (49)   (48)   (49) 
 Diverticulum    1 (2%)       
 Hyperplasia, squamous    2 (4%)    2 (4%)    2 (4%)    1 (2%) 
 Inflammation, acute        1 (2%)   
 Necrosis          1 (2%) 
 Ulcer    1 (2%)    6 (12%)    3 (6%)    2 (4%) 
 Stomach, glandular   (49)   (49)   (48)   (49) 
 Inflammation, acute        1 (2%)   
 Mineralization      2 (4%)    1 (2%)   
 Necrosis    7 (14%)    2 (4%)    2 (4%)    3 (6%) 
 Ulcer      1 (2%)      1 (2%) 
 Artery, inflammation    1 (2%)       
 Tongue      (1)    (1)   
 Epithelium, hyperplasia        1 (100%)   
 Tooth    (1)    (2)    (4)    (2) 
 Inflammation, chronic active    1 (100%)    1 (50%)    4 (100%)   
 Malformation      1 (50%)      2 (100% 
 Cardiovascular System 
 Heart   (49)   (49)   (48)   (49) 
 Cardiomyopathy    42 (86%)    44 (90%)    37 (77%)    42 (86%) 
 Necrosis    1 (2%)       
 Atrium, thrombosis    5 (10%)    2 (4%)    3 (6%)    2 (4%) 
 Valve, thrombosis, chronic      1 (2%)     
 Endocrine System 
 Adrenal cortex   (49)   (49)   (48)   (49) 
 Angiectasis    1 (2%)    1 (2%)     
 Degeneration, cystic      2 (4%)    1 (2%)    1 (2%) 
 Hyperplasia    30 (61%)    28 (57%)    23 (48%)    36 (73%) 
 Hypertrophy    7 (14%)    6 (12%)    9 (19%)    4 (8%) 
 Necrosis      1 (2%)    2 (4%)    1 (2%) 
 Vacuolization cytoplasmic    1 (2%)    1 (2%)    3 (6%)   
 Adrenal medulla   (49)   (49)   (47)   (49) 
 Hyperplasia    26 (53%)    13 (27%)    23 (49%)    12 (24%) 
 Necrosis      1 (2%)     
 Islets, pancreatic   (49)   (49)   (48)   (49) 
 Hyperplasia      1 (2%)    2 (4%)    1 (2%) 
 Pituitary gland   (49)   (49)   (47)   (49) 
 Angiectasis    1 (2%)        1 (2%) 
 Cyst      1 (2%)      1 (2%) 
 Pars distalis, hyperplasia    11 (22%)    12 (24%)    10 (21%)    15 (31%) 
 Thyroid gland   (46)   (47)   (45)   (47) 
 C-cell, hyperplasia    32 (70%)    36 (77%)    31 (69%)    33 (70%) 
 Follicular cell, hyperplasia    2 (4%)      3 (7%)   
 General Body System 
 Peritoneum  (1)  
 Inflammation, suppurative      1 (100%)  
 Genital System 
 Epididymis   (49)   (49)   (48)   (49) 
 Angiectasis      1 (2%)     
 Granuloma sperm      1 (2%)     
 Preputial gland   (48)   (49)   (47)   (49) 
 Cyst        1 (2%)    1 (2%) 
 Hyperplasia, squamous    1 (2%)       
 Inflammation, chronic active    2 (4%)      2 (4%)    2 (4%) 
 Prostate   (49)   (49)   (48)   (49) 
 Hyperplasia    11 (22%)    8 (16%)    16 (33%)    8 (16%) 
 Inflammation, chronic active    3 (6%)    2 (4%)    3 (6%)    2 (4%) 
 Epithelium, hyperplasia    1 (2%)       
 Seminal vesicle   (47)   (49)   (47)   (47) 
 Inflammation, chronic active          1 (2%) 
 Testes   (49)   (49)   (48)   (49) 
 Atrophy    2 (4%)    4 (8%)    2 (4%)    4 (8%) 
 Artery, inflammation, chronic active      2 (4%)      2 (4%) 
 Interstitial cell, hyperplasia    5 (10%)    9 (18%)    2 (4%)    11 (22%) 
 Hematopoietic System 
 Lymph node    (3)    (3)    (8)    (4) 
 Iliac, hemorrhage    1 (33%)       
 Lymph node, mandibular   (40)   (45)   (46)   (44) 
 Infiltration cellular, plasma cell    1 (3%)    1 (2%)    1 (2%)    1 (2%) 
 Infiltration cellular, polymorphonuclear    1 (3%)       
 Spleen   (49)   (49)   (48)   (49) 
 Fibrosis    7 (14%)    12 (24%)    6 (13%)    6 (12%) 
 Hematopoietic cell proliferation    4 (8%)    3 (6%)    1 (2%)    4 (8%) 
 Hemorrhage    3 (6%)    1 (2%)    2 (4%)    2 (4%) 
 Necrosis    3 (6%)    2 (4%)    1 (2%)    2 (4%) 
 Thrombosis    1 (2%)        1 (2%) 
 Musculoskeletal System 
 Bone   (49)   (49)   (48)  (49)
 Osteopetrosis        1 (2%)   
 Nervous System 
 Brain   (49)   (49)   (48)   (49) 
 Degeneration    1 (2%)      1 (2%)   
 Necrosis        1 (2%)   
 Artery, inflammation    1 (2%)       
 Special Senses System 
 Eye   (48)   (48)   (48)   (48) 
 Cataract    4 (8%)    2 (4%)    1 (2%)    3 (6%) 
 Hemorrhage          1 (2%) 
 Inflammation, suppurative    1 (2%)       
 Retina, atrophy    2 (4%)    1 (2%)    1 (2%)    2 (4%) 
 Urinary System 
 Kidney   (49)   (49)   (48)   (49) 
 Infarct    3 (6%)      1 (2%)    3 (6%) 
 Inflammation, suppurative          1 (2%) 
 Metaplasia, osseous        1 (2%)   
 Nephropathy    43 (88%)    44 (90%)    45 (94%)    43 (88%) 
 Renal tubule, hyperplasia        1 (2%)    1 (2%) 
 Urinary bladder   (48)   (49)   (48)   (49) 
 Hemorrhage    1 (2%)       
 Transitional epithelium, hyperplasia          1 (2%) 

a) Number of animals examined microscopically at the site and the number of animals with lesion

TABLE 3: Summary of the Incidence of Neoplasms in Female Rats in the 2-Year Inhalation Study of Naphthalene a)

 Chamber Control 

 10 ppm  30 ppm   60 ppm 
 Disposition Summary 
 Animals initially in study   49   49   49   49 
 Early deaths         
 Moribund   18   22   16   21 
 Natural deaths   3   6   5   4 
 Survivors         
 Terminal sacrifice   28   21   28   24 
 Animals examined microscopically   49   49   49   49 
 Alimentary System 
 Intestine large, colon   (49)   (49)   (49)   (49) 
 Intestine small, jejunum   (49)   (48)   (48)   (46) 
 Intestine small, ileum   (49)   (48)   (47)   (46) 
 Hepatocellular carcinoma, metastatic, liver     1 (2%)     
 Liver   (49)   (49)   (49)   (49) 
 Hepatocellular carcinoma     1 (2%)   1 (2%)   
 Mesentery   (13)   (8)   (7)   (5) 
 Pancreas   (49)   (49)   (49)   (49) 
 Salivary glands   (49)   (49)   (49)   (49) 
 Adenoma         1 (2%) 
 Stomach, forestomach   (49)   (49)   (49)   (49) 
 Stomach, glandular   (49)   (48)   (49)   (49) 
 Hepatocellular carcinoma, metastatic, liver     1 (2%)     
 Tongue   (1)   (1)     (1) 
 Squamous cell papilloma         1 (100%) 
 Epithelium, squamous cell papilloma     1 (100%)     
 Cardiovascular System 
 Heart   (49)   (49)   (49)  -49
 Schwannoma benign       1 (2%)   
 Endocrine System 
 Adrenal cortex   (49)   (49)   (49)   (49) 
 Adenoma   1 (2%)   2 (4%)   1 (2%)   
 Adrenal medulla   (48)   (49)   (49)   (49) 
 Pheochromocytoma benign   2 (4%)     1 (2%)   2 (4%) 
 Bilateral, pheochromocytoma benign   1 (2%)       
 Islets, pancreatic   (49)   (49)   (49)   (49) 
 Adenoma   1(2%)   1(2%)   1(2%)   1(2%) 
 Carcinoma   1 (2%)       
 Parathyroid gland   (42)   (40)   (41)   (48) 
 Pituitary gland   (49)   (49)   (49)   (48) 
 Pars distalis, adenoma   23 (47%)   27 (55%)   24 (49%)   20 (42%) 
 Pars distalis, carcinoma       1 (2%)   
 Thyroid gland   (47)   (46)   (48)   (48) 
 Bilateral, C-cell, adenoma     1 (2%)     
 C-cell, adenoma   4 (9%)   3 (7%)   2 (4%)   1 (2%) 
 C-cell, carcinoma   3 (6%)   2 (4%)   2 (4%)   
 Follicular cell, carcinoma   1 (2%)       
 General Body System 
 None 
 Genital System 
 Clitoral gland   (49)   (47)   (49)   (48) 
 Adenoma   3 (6%)   7 (15%)   4 (8%)   2 (4%) 
 Carcinoma     1 (2%)   1 (2%)   1 (2%) 
 Bilateral, adenoma   1 (2%)       
 Ovary   (49)   (49)   (49)   (49) 
 Granulosa cell tumor malignant   2 (4%)   2 (4%)     
 Granulosa-theca tumor malignant   1 (2%)       
 Hepatocellular carcinoma, metastatic, liver     1 (2%)     
 Uterus   (49)   (49)   (49)   (49) 
 Carcinoma       1 (2%)   
 Polyp stromal   14 (29%)   5 (10%)   8 (16%)   7 (14%) 
 Bilateral, polyp stromal   1 (2%)   2 (4%)   1 (2%)   
 Hematopoietic System 
 Bone marrow   (49)   (49)   (49)   (49) 
 Lymph node   (2)   (3)   (2)   (3) 
 Lymph node, bronchial   (42)   (33)   (34)   (36) 
 Lymph node, mandibular   (47)   (39)   (46)   (47) 
 Lymph node, mesenteric   (49)   (49)   (49)   (49) 
 Lymph node, mediastinal   (40)   (39)   (41)   (31) 
 Spleen   (49)   (49)   (49)   (49) 
 Hemangiosarcoma   1 (2%)       
 Osteosarcoma, metastatic, bone     1 (2%)     
 Thymus   (46)   (45)   (48)   (41) 
 Musculoskeletal System 
 Bone   (49)   (49)   (49)  -49
 Osteosarcoma     1 (2%)     
 Skeletal muscle     (2)     
 Nervous System 
 Brain   (49)   (49)   (49)   (49) 
 Carcinoma, metastatic, pituitary gland       1 (2%)   
 Glioma malignant       1 (2%)   
 Neuroblastoma, metastatic, nose     1 (2%)     4 (8%) 
 Special Senses System 
 Zymbals gland   (2)   (1) 
 Carcinoma   2 (100%)   1 (100%)     
 Urinary System 
 Kidney   (48)   (49)   (49)   (49) 
 Renal tubule, carcinoma   1 (2%)       
 Urinary bladder   (48)   (49)   (49)   (49) 
 Transitional epithelium, papilloma   1 (2%)     1 (2%)   
 Systemic Lesions
 Multiple organsb)   (49)   (49)   (49)   (49) 
 Leukemia mononuclear   16 (33%)   21 (43%)   15 (31%)   15 (31%) 
 Mesothelioma benign       1 (2%)   
 Neoplasm Summary
 Total animals with primary neoplasmsc)   44   48   47   44 
 Total primary neoplasms   104   106   97   89 
 Total animals with benign neoplasms   38   41   43   35 
 Total benign neoplasms   72   69   69   53 
 Total animals with malignant neoplasms   26   34   26   27 
 Total malignant neoplasms   32   37   28   36 
 Total animals with metastatic neoplasms     3   1   
 Total metastatic neoplasms     7   1   

a) Number of animals examined microscopically at the site and the number of animals with neoplasm

b) Number of animals with any tissue examined microscopically

c) Primary neoplasms: all neoplasms except metastatic neoplasms

TABLE 5: Summary of the Incidence of Nonneoplastic Lesions in Female Rats in the 2-Year Inhalation Study of Naphthalene a)

 Chamber Control  10 ppm   30 ppm   60 ppm 
 Disposition Summary 
 Animals initially in study   49   49   49   49 
 Early deaths                                                                
 Moribund   18   22   16   21 
 Natural deaths   3   6   5   4 
 Survivors                                                                                
 Terminal sacrifice   28   21   28   24 
 Animals examined microscopically   49   49   49   49 
 Alimentary System 
 Intestine large, cecum   (49)   (48)   (48)   (48) 
 Inflammation, acute          1 (2%) 
 Liver   (49)   (49)   (49)   (49) 
 Angiectasis    2 (4%)    4(8%)    2(4%)    1(2%) 
 Basophilic focus    46 (94%)    44 (90%)    46 (94%)    44 (90%) 
 Clear cell focus 

  7 (14%) 

  16 (33%)    8 (16%)    6 (12%) 
 Cyst          1 (2%) 
 Eosinophilic focus    1 (2%)      6 (12%)    2 (4%) 
 Fatty change    10 (20%)    3 (6%)    2 (4%)    4 (8%) 
 Hepatodiaphragmatic nodule    4 (8%)    1 (2%)    6 (12%)    5 (10%) 
 Inflammation, chronic    2 (4%)       
 Mixed cell focus    6 (12%)    6 (12%)    7 (14%)    6 (12%) 
 Necrosis    1 (2%)    1 (2%)    1 (2%)   
 Regeneration    1 (2%)    2 (4%)    2 (4%)    2 (4%) 
 Vacuolization cytoplasmic, focal    1 (2%)    1 (2%)     
 Bile duct, hyperplasia    5 (10%)    5 (10%)    5 (10%)    6 (12%) 
 Centrilobular, necrosis    11 (22%)    11 (22%)    7 (14%)    9 (18%) 
 Hepatocyte, atrophy        1 (2%)   
 Mesentery   (13)    (8)    (7)    (5) 
 Fat, hemorrhage    1 (8%)       
 Fat, inflammation      1 (13%)     
 Fat, necrosis    13 (100%)    7 (88%)    6 (86%)    5 (100%) 
 Pancreas   (49)   (49)   (49)   (49) 
 Atrophy    18 (37%)    9 (18%)    11 (22%)    10 (20%) 
 Basophilic focus      1 (2%)    1 (2%)   
 Hyperplasia      1 (2%)     
 Duct, cyst      1 (2%)     
 Salivary glands   (49)   (49)   (49)   (49) 
 Atrophy    1 (2%)        2 (4%) 
 Basophilic focus    1 (2%)       
 Stomach, forestomach   (49)   (49)   (49)   (49) 
 Hyperplasia, squamous          1 (2%) 
 Inflammation, acute    1 (2%)       
 Ulcer    3 (6%)    2 (4%)    2 (4%)   
 Stomach, glandular   (49)   (48)   (49)   (49) 
 Hyperplasia        1 (2%)   
 Mineralization    2 (4%)    2 (4%)      2 (4%) 
 Necrosis    3 (6%)    2 (4%)    1 (2%)   
 Ulcer    1 (2%)      1 (2%)   
 Tongue    (1)    (1)      (1) 
 Epithelium, hyperplasia    1 (100%)       
 Tooth        (1)    (1) 
 Malformation        1 (100%)    1 (100%) 
 Cardiovascular System 
 Heart   (49)   (49)   (49)   (49) 
 Cardiomyopathy    32 (65%)    31 (63%)    31 (63%)    34 (69%) 
 Atrium, thrombosis    2 (4%)    2 (4%)    1 (2%)    1 (2%) 
 Endocrine System 
 Adrenal cortex   (49)   (49)   (49)   (49) 
 Atrophy    2 (4%)       
 Degeneration, cystic    4 (8%)    4 (8%)    3 (6%)    3 (6%) 
 Hyperplasia    23 (47%)    12 (24%)    18 (37%)    24 (49%) 
 Hypertrophy    7 (14%)    4 (8%)    12 (24%)    6 (12%) 
 Necrosis    4 (8%)    2 (4%)      1 (2%) 
 Thrombosis    1 (2%)       
 Vacuolization cytoplasmic      2 (4%)      1 (2%) 
 Adrenal medulla   (48)   (49)   (49)   (49) 
 Hyperplasia    10 (21%)    3 (6%)    9 (18%)    5 (10%) 
 Necrosis    2 (4%)    1 (2%)     
 Thrombosis    1 (2%)       
 Islets, pancreatic   (49)   (49)   (49)   (49) 
 Hyperplasia        1 (2%)   
 Parathyroid gland   (42)   (40)   (41)   (48) 
 Hyperplasia        1 (2%)   
 Pituitary gland   (49)   (49)   (49)   (48) 
 Angiectasis    2(4%)    2(4%)    3(6%)    2(4%) 
 Cyst          1 (2%) 
 Pars distalis, hyperplasia    24 (49%)    13 (27%)    18 (37%)    15 (31%) 
 Thyroid gland   (47)   (46)   (48)   (48) 
 C-cell, hyperplasia    39 (83%)    37 (80%)    37 (77%)    42 (88%) 
 Follicular cell, hyperplasia        1 (2%)   
 General Body System 
None 
 Genital System 
 Clitoral gland   (49)   (47)   (49)   (48) 
 Hyperplasia    1 (2%)    2 (4%)    2 (4%)    3 (6%) 
 Inflammation, chronic active    2 (4%)      1 (2%)    1 (2%) 
 Ovary   (49)   (49)   (49)   (49) 
 Cyst    7 (14%)    9 (18%)    11 (22%)    8 (16%) 
 Inflammation, granulomatous    1 (2%)    1 (2%)      2 (4%) 
 Uterus   (49)   (49)   (49)   (49) 
 Cyst    1 (2%)       
 Vagina        (1)   
 Inflammation, suppurative        1 (100%)   
 Hematopoietic System 
 Bone marrow   (49)   (49)   (49)   (49) 
 Atrophy          1 (2%) 
 Hyperplasia, reticulum cell    1 (2%)      1 (2%)   
 Myelofibrosis    1 (2%)       
 Lymph node, mediastinal   (40)   (39)   (41)   (31) 
 Congestion        1 (2%)   
 Hemorrhage        1 (2%)   
 Spleen   (49)   (49)   (49)   (49) 
 Fibrosis    3 (6%)    3 (6%)    3 (6%)    2 (4%) 
 Hematopoietic cell proliferation    2 (4%)    4 (8%)    1 (2%)    4 (8%) 
 Hemorrhage      2 (4%)      1 (2%) 
 Metaplasia, osseous          1 (2%) 
 Necrosis    2 (4%)    1 (2%)    1 (2%)    1 (2%) 
 Thymus   (46)   (45)   (48)   (41) 
 Cyst        1 (2%)   
 Musculoskeletal System 
 Bone   (49)   (49)   (49)   (49) 
 Osteopetrosis    10 (20%)    4 (8%)    7 (14%)    5 (10%) 
 Nervous System 
 Brain   (49)   (49)   (49)   (49) 
 Angiectasis        1 (2%)   
 Degeneration    1 (2%)       
 Thrombosis    1 (2%)       
 Special Senses System 
 Eye   (48)   (47)   (46)   (48) 
 Cataract    5 (10%)    2 (4%)    6 (13%)    3 (6%) 
 Cornea, infiltration cellular, polymorphonuclear    2 (4%)       
 Retina, atrophy    5 (10%)    2 (4%)    4 (9%)    2 (4%) 
 Harderian gland          (1) 
 Inflammation, chronic          1 (100%) 
 Urinary System 
 Kidney   (48)   (49)   (49)   (49) 
 Cyst      1 (2%)     
 Infarct        1 (2%)    1 (2%) 
 Nephropathy    41 (85%)    38 (78%)    34 (69%)    31 (63%) 
 Renal tubule, necrosis    1 (2%)    1 (2%)     
Conclusions:
Under the conditions of this 2-year inhalation study, there was clear evidence of carcinogenic activity of naphthalene in male and female F344/N rats based on increased incidences of respiratory epithelial adenoma and olfactory epithelial neuroblastoma of the nose. In male and female rats, exposure to naphthalene caused significant increases in the incidences of non-neoplastic lesions of the nose.
Executive summary:

Groups of 49 male and 49 female rats were exposed to naphthalene by inhalation at concentrations of 0, 10, 30, or 60 ppm for 6 hours plus T90 (12 minutes) per day, 5 days per week for 105 weeks. The survival of all exposed groups of male and female rats was similar to that of the chamber controls. Mean body weights of all exposed groups of males were less than those of the chamber control group throughout most of the study. Masses were observed in the nose of male and female rats. These masses frequently partially occluded the nasal passages or obliterated the normal architecture of the nasal turbinates.

The incidences of neuroblastoma of the olfactory epithelium, a rare neoplasm, occurred with positive trends in males and females. Because this neoplasm did not occur in chamber control rats or in male rats exposed to 10 ppm and because this neoplasm has not been seen in the historical chamber control rats in NTP 2-year inhalation studies, the increased incidences of neuroblastoma were considered to be related to naphthalene exposure. In males, the incidences of adenoma of the respiratory epithelium of the nose, another rare neoplasm, occurred with a positive trend and were significantly increased in all exposed groups; none occurred in the chamber controls. In females, these neoplasms occurred in the 30 and 60 ppm group but not in the chamber control or 10 ppm groups. Because these neoplasms did not occur in the chamber controls and have not been observed in the historical chamber control rats in NTP 2-year inhalation studies, the incidences of nasal adenoma were considered to be related to naphthalene exposure.

Increased incidences of nonneoplastic lesions of the nose associated with exposure to naphthalene included atypical hyperplasia, atrophy, chronic inflammation, and hyaline degeneration of the olfactory epithelium; hyperplasia, squamous metaplasia, hyaline degeneration, and goblet cell hyperplasia of the respiratory epithelium; and glandular hyperplasia and squamous metaplasia.

Under the conditions of this 2-year inhalation study, there was clear evidence of carcinogenic activity of naphthalene in male and female F344/N rats based on increased incidences of respiratory epithelial adenoma and olfactory epithelial neuroblastoma of the nose. In male and female rats, exposure to naphthalene caused significant increases in the incidences of nonneoplastic lesions of the nose.

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
LOAEC
50 mg/m³
Study duration:
chronic
Species:
rat
Quality of whole database:
acceptable for assessment, relevance for humans being debated in the scientific world and regulatory bodies
(see also Rhomberg LR et al. 2010: Hypothesis-based weight of evidence: A tool for evaluating and communicating uncertainties and inconsistencies in the large body of evidence in proposing a carcinogenic mode of action—naphthalene as an example. Crit. Rev. Toxicol., 40, 671-696; and
Bailey, L.A et al. 2016: Hypothesis-based weight-of-evidence evaluation and risk assessment for naphthalene carcinogenesis. Crit. Rev. Toxicol., 46, 1-42, and HC-NL/DECOS 2012: Naphthalene - Evaluation of the carcinogenicity and genotoxicity. The Hague: Health Council of the Netherlands, 07 Dec. 2012, publication no. 2012/30) [http://www.gezondheidsraad.nl/en/publications/naphthalene] / CCSG 2012_Naphthalene_mode of action.pdf / ACGIH 2014: TLV list.)

Carcinogenicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

The following discussion is quoted from the "Toxicological Profile for Naphthalene, 1 -Methylnaphthalene and 2 -Methylnaphthalene" published by U.S. Department of Health and Human Services (Public Health Service), Agency for Toxic Substances and Disease Registry (ATSDR) in 2005 [ATSDR 2005]:

Chronic inhalation studies found increased incidences of non-neoplastic and neoplastic lesions in the nose of rats, non-neoplastic lesions in the nose of mice, and neoplastic and non-neoplastic lesions in the lungs of mice (2 year carcinogenicity NTP studies). In mice of both sexes, chronic inhalation of 10 or 30 ppm naphthalene induced inflammation of the nose and lung, metaplasia of the olfactory epithelium, and hyperplasia of the nasal respiratory epithelium. In female mice (but not male mice), exposure to 30 ppm (but not 10 ppm) increased the incidence of benign lung tumors (alveolar/bronchiolar adenomas) compared with controls. One other female mouse exposed to 30 ppm showed a malignant lung tumor (alveolar/bronchiolar carcinoma). In rats of both sexes, inhalation of 10, 30, or 60 ppm naphthalene induced non-neoplastic and neoplastic lesions only in the nasal cavity. Non-neoplastic nasal lesions included (1) hyperplasia, atrophy, chronic inflammation, and hyaline degeneration of the olfactory epithelium and (2) hyperplasia, metaplasia or degeneration of the respiratory epithelium or glands. Neoplastic lesions associated with naphthalene exposure in rats were olfactory epithelial neuroblastoma (a rare malignant tumor) and respiratory epithelial adenoma.

The mechanisms by which naphthalene causes non-neoplastic or neoplastic lesions in the respiratory tract of rodents are not completely understood, but are thought to involve reactive metabolites of naphthalene, including 1,2-naphthalene oxide, 1,2-naphthoquinone, 1,4 -naphthoquinone, and possibly 1,2-dihydroxy-3,4-epoxy-1,2,3,4-tetrahydronaphthalene (see Endpoint Summary 7.1 Toxicokinetics).

Comparison of species susceptibility to naphthalene-induced non-neoplastic lung damage suggests that mice are much more sensitive than rats (e.g., non-neoplastic or neoplastic lung lesions were not found in chronically exposed rats in the NTP study) and that differences in rates and stereoselectivity of naphthalene metabolism to epoxide intermediates may be involved in this species difference. Acute (4-hour) inhalation exposure of mice to naphthalene concentrations as low as 2–10 ppm induced lung injury, whereas rats exposed to naphthalene concentrations as high as 110 ppm showed no signs of lung injury. Some evidence has been reported that rates and stereoselectivity of naphthalene metabolism in primate lung tissue may be more like rats than mice. In in vitro studies with microsomes from lymphoblastoid cells, which expressed recombinant human CYP2F1, metabolism of naphthalene to epoxide intermediates was demonstrated, but the predominant enantiomeric form produced (1S,2R-oxide) was different from the form (1R,2S-oxide) produced by mouse CYP2F2. Although these observations on epoxide formation may suggest that mice may be more sensitive than humans to acute naphthalene lung toxicity from epoxide intermediates, the possible role of other potentially reactive metabolites of naphthalene (e.g., the naphthoquinone metabolites) is unknown with chronic exposure scenarios. To date, mechanistic understanding of species differences in naphthalene bioactivation in the lung is too incomplete to definitively rule out the possible human relevance of naphthalene-induced lung lesions in mice (see Endpoint Summary 7.1 Toxicokinetics).

In contrast, the olfactory epithelium and respiratory epithelium of the nose of rats and mice do not appear to differ in sensitivity to naphthalene non-neoplastic toxicity from chronic inhalation exposure. Non-neoplastic nasal lesions were found in nearly all exposed animals of both species at the lowest exposure level, 10 ppm, in both chronic studies. CYP monooxygenases, which might be involved in naphthalene metabolism and bioactivation, have been demonstrated to exist in nasal respiratory epithelial and olfactory epithelial tissue from rodents and humans. Studies designed to specifically characterize metabolism of naphthalene in nasal tissue, however, have not been conducted, with the exception of a single study, which examined in vitro rates of metabolism of naphthalene to naphthalene oxides in postmitochondrial supernatants from mouse, rat, and hamster olfactory tissue. Metabolic rates (units of nmol/min/mg protein) showed the following order: mouse (87.1) > rat (43.5) > hamster (3.9). This order did not correspond with species differences in sensitivity to single intraperitoneal injections of naphthalene in a companion study. The lowest dose levels producing substantial necrosis and exfoliation in olfactory epithelium were 200 mg/kg in rats and 400 mg/kg in mice and hamsters. To date, mechanistic understanding of species differences in naphthalene bioactivation in the respiratory tissues is too incomplete to definitely rule out the possible human relevance of naphthalene-induced nasal lesions in rodents (non-neoplastic lesions in rats and mice and neoplastic lesions in rats; see Endpoint Summary 7.1 Toxicokinetics).

It is unknown whether the naphthalene-induced neoplastic lesions found in mice (lung adenomas) and rats (nose respiratory epithelial adenomas and olfactory epithelial neuroblastomas) are produced via a genotoxic mode of action or a non-genotoxic mode requiring tissue damage and regenerative responses as precursor events. Results from genotoxicity tests for naphthalene have been predominately (but not completely) negative (see Endpoint Summary 7.6 Genetic Toxicity), and the general sites of neoplastic lesions, the nose in rats and the lungs in mice, show some correspondence (but not complete) with the general sites of non-neoplastic lesions. However, mechanistic understanding of naphthalene’s carcinogenic mode of action is too incomplete to rule out the possibility of a genotoxic mode of action.

References:

[ATSDR 2005] Toxicological Profile for Naphthalene, 1-Methylnaphthalene, and 2-Methylnaphthalene No. 67, Agency for Toxic Substances and Disease Registry (ATSDR), Atlanta, Georgia 30333; U.S. Dep. Health & Human Services [http://www.atsdr.cdc.gov]

Justification for selection of carcinogenicity via oral route endpoint:
historical study, only long-term study by the oral route

Justification for selection of carcinogenicity via inhalation route endpoint:
Key study for risk assessment and stipulation of limit values (see also CCSG: Review on mode of action 2010 as attached document)

Carcinogenicity: via inhalation route (target organ): respiratory: nose

Justification for classification or non-classification

The International Agency for Research on Cancer (IARC 2002) has classified naphthalene as “possibly carcinogenic to humans” (Group 2B) on the basis of “inadequate evidence in humans” and “sufficient evidence in experimental animals” for determination of carcinogenicity.

The European Commission (ECB 2003) has classified naphthalene as Category 3 for carcinogenicity (“causes concerns for humans owing to possible carcinogenic effects”). It was noted by the European Commission that a satisfactory assessment of carcinogenic effects in humans was not possible based on inadequate available information and that evidence for carcinogenicity from appropriate animal studies was insufficient to classify the substance in Category 2 (“regarded as if carcinogenic to humans”) (ECB 2003).

The National Toxicology Program (NTP) has classified naphthalene as being “reasonably anticipated to be a human carcinogen based on sufficient evidence from studies in experimental animals”. The United States Environmental Protection Agency (US EPA 1998) has classified naphthalene as Group C (“possible human carcinogen”) based on inadequate human carcinogenicity data and limited evidence of carcinogenicity after oral and inhalation exposures in experimental animals.

There is no reason for changing (in terrms of tightening) the actual classification of naphthalene regarding carcinogenicity.

[IARC 2002] Working Group on the Evaluation of Carcinogenic Risks to Humans. 2002. Naphthalene. IARC Monogr Eval Carcinog Risks Hum. 82: 367-435.

[ECB 2003] European Union risk assessment report: CAS: 91-20-3: naphthalene. 2003. Luxembourg: Office for Official Publications of the European Communities. Report No.: EUR 20763 EN. Available from: http://ecb.jrc.it/DOCUMENTS/Existing-Chemicals/RISK_ASSESSMENT/REPORT/naphthalenereport020.pdf On the cover, European Commission Joint Research Centre. [European Communities, European Commission, Joint Research Centre, Institute for Health and Consumer Protection, European Chemicals Bureau].

[US EPA] US Environmental Protection Agency. 1998. Toxicological review of naphthalene (CAS No. 91-20-3) in support of summary information on the Integrated Risk Information System (IRIS). Washington (DC): United States Environmental Protection Agency. 116 pp. Available from: http://www.epa.gov/iris/toxreviews/0436-tr.pdf.