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EC number: 200-001-8
CAS number: 50-00-0
There is evidence that formaldehyde induces toxic effects only at the site of contact after oral, dermal or inhalation exposure. Toxicity is not evident at remote sites such that general signs of toxicity occur only secondary to these local lesions. In spite of some recent studies describing effects after inhalation of formaldehyde far of the portal of entry, this assessment is still upheld after comparing these studies with key guideline studies of high validity.In chronic drinking water studies in rats local effects in the forestomach and stomach were induced, the NOAEC is 0.020-0.026% formaldehyde in drinking water. The NOAEL for systemic effects is 82 mg/kg bw/day in males and 109 mg/kg bw/day in females.Studies on repeated dermal dose toxicity in compliance to current Guidelines are not available.Local effects in the upper respiratory tract were induced after repeated inhalation exposure in experimental animals. The most sensitive site in rodents and monkeys following inhalation exposure is the respiratory epithelium in the anterior part of the nasal cavity. At higher exposure levels also the olfactory epithelium, larynx or trachea were affected. Rats are more sensitive than mice or hamsters. The LOAEC is 2 ppm in rats, 3 ppm in monkeys and 6 ppm in mice. The overall NOAEC for local effects in experimental animals is 1 ppm (1.2 mg/m³). The NOAEC for systemic effects not occurring at the site of first contact in long-term inhalation studies in rats and mice is 15 ppm.
In a reliable GLP-conform study similar to OECD TG 453, a combined chronic toxicity/carcinogenicity study, 70 male and 70 female Wistar rats per dose (subgroups of 10 rats/sex/dose killed 12 or 18 months after start of exposure) received via the drinking water 0, 1.2, 15, 82 mg/kg bw/day (males) or 0, 1.8, 21, 109 mg/kg bw/day (females) for 105 weeks (concentration: 0, 20, 260, or 1900 mg/L or 0, 0.002, 0.026, 0.19%).
At the high dose body weight gain was decreased in males & females and food and water consumption decreased. Other parameters (except pathology) were not altered.Pathological alterations in the kidney like the renal papillary necrosis detected in high dose males and females is discussed as an effect of the reduced water intake and is indirectly treatment related. Treatment related lesions were detected in the forestomach (focal papillary epithelial hyperplasia, ulceration and hyperkeratosis) and the glandular stomach (chronic atrophic gastritis, ulceration and hyperplasia) of males and females in the high dose group. No gastric tumours were induced. This study did not provide any evidence of carcinogenicity in rats after oral administration of formaldehyde.
Oral exposure via the drinking water induced local effects in the stomach of rats at a concentration of 0.19% corresponding to 82 mg/kg bw/d in males and 109 mg/kg bw/d in females; the NOAEC is 0.026% corresponding to 15 mg/kg bw/d in males and 21 mg/kg bw/d in females.
In a reliable study similar to OECD TG 452, groups of 20 male and 20 female Wistar rats were given formaldehyde solution in their drinking water at concentrations of 0, 0.02, 0.1 and 0.5 % (0, 10, 50, 300 mg/kg bw/day) for 24 months.
In the high dose group, poor general state, reduction of body weight gain and both food and water consumption, increased mortality (ca. 50 % after 12 months, no rat alive after 24 months), and changes in various clinical parameters were recorded. Histopathology revealed local effects in the stomach/forestomach of the high dose group. Lesions of the forestomach in males and females were squamous and and basal cell hyperplasia, hyperkeratosis, erosions/ulcers, and submucosal cell infiltration. Glandular hyperplasia, erosions/ulcers, and submucosal cell infiltration in the glandular stomach were also found at the high dose level. A high incidence of renal papillary necrosis was observed in male and female animals (about 50 % versus 0 - 10 % in the other groups). This finding is ascribed to the dehydration caused by the considerable decrease of liquid consumption. No other treatment related lesions were reported. However, local effects were also detected at the mid dose level. Administration of 0.10 % resulted in forestomach hyperkeratosis in animals after 18 (1/6 males) and 24 months (1/8 females). According to the authors, the NOAEL was 10 mg/kg bw/day (NOAEC 0.02 %).
Local effects in the forestomach were detected in rats after chronic oral exposure to ≥ 0.10 % in the drinking water (50 mg/kg bw/day); the NOAEC was 0.02 % in the drinking water (10 mg/kg bw/day).
Incidence of lesions in the forestomach and glandular stomach of rats after 4 week treatment via the drinking water
Type of lesion
Dose in mg/kg bw/day
Number of rats examined
Focal hyperkeratosis of forestomachvery slightslightmoderate
Dilated fundic gland
Submucosal mononuclear-cell infiltrate
Focal gastritisvery slightslightmoderate
Focal papillomatous hyperplasia
In a reliable study comparable to OECD TG 407 groups of ten male and ten fenale 5-week-old rats received formaldehyde in the drinking-water over period of 4 weeks at dose levels of 5, 25 and 125 mg/kg body weight/day. A group of 20 males and 20 females served as controls and received unsupplemented drinking-water ad libitum. An additional group of ten males
and ten females was given unsupplemented drinking-water in an amount equal to the amount of liquid consumed by the group given the top.
Food and liquid intake were decreased in the groups on the top dose. Effects observed only in the top-dose group, were yellow discoloration of the fur, decreased protein and albumin levels in the blood plasma, thickening of the limiting ridge and hyperkeratosis in the forestomach, and focal gastritis in the glandular stomach.
It was concluded that in this study the no-observed-adverse-effect levels of formaldehyde were 25 mg/kg body weight/day.
In this reliable review the authors discuss the scientific literature available for cell replication, histopathological alterations and tumour response (polypoid adenomas).
In 2012 the Committee for Risk Assessment (RAC) of the European Chemicals Agency concluded that 2ppm formaldehyde represent a Lowest Observed Adverse Effect Concentration (LOAEC) for polypoid adenomas, histopathological lesions and cell proliferation. An analysis of all data shows that a LOAEC of 2ppm it is not justified for cell proliferation and polypoid adenomas. Higher values are also supported by a new statistical analysis. For histopathological lesions a NOAEC of 1ppm may be defined but the lesions at 2ppm cannot be regarded as pre-stages for tumour development. One major uncertainty exists: the description of polypoid adenomas and the lesions at 2ppm often is insufficient and diagnostic uncertainties can only be resolved by a re-evaluation according to modern histomorphological standards. Although the discrepancy between our assessment and that of RAC may seem rather small we feel the LOAECs proposed by RAC must be challenged taking into consideration the broad data base for formaldehyde and the potential impact of any published RAC opinion on the present discussions about appropriate occupational and indoor exposure limits.
Repeated oral exposure
In a study comparable to OECD Guideline 453 (Til et al., 1989; combined chronic toxicity/ carcinogenicity study) 70 male and 70 female Wistar rats per dose (subgroups of 10 rats/sex/dose killed 12 or 18 months after start of exposure) received formaldehyde via the drinking water at dose levels of 0, 1.2, 15, 82 mg/kg bw/day (males) or 0, 1.8, 21, 109 mg/kg bw/day (females) for 105 weeks (concentration: 0, 20, 260, or 1900 mg/L or 0, 0.002, 0.026, 0.19%). At the high dose body weight gain was decreased in males & females as well as food and water consumption. Other parameters (except pathology) were not altered. Pathological alterations in the kidney, such as the renal papillary necrosis detected in high dose males and females, are attributed to a secondary effect arising from significantly reduced (~40%) water intake. Treatment related lesions were detected only in the forestomach (focal papillary epithelial hyperplasia, ulceration and hyperkeratosis) and the glandular stomach (chronic atrophic gastritis, ulceration and hyperplasia) of males and females in the high dose group. No gastric tumors were induced (see Section Carcinogenicity). In summary, oral exposure via the drinking water induced local effects in the stomach of rats at a concentration of 0.19% corresponding to 82 mg/kg bw/d in males and 109 mg/kg bw/d in females; the NOAEC is 0.026% corresponding to 15 mg/kg bw/d in males and 21 mg/kg bw/d in females.
Very similar results were presented by Tobe et al. (1989; comparable to OECD Guideline 452 with acceptable restrictions). In this drinking water study 20 Wistar rats per dose per sex were exposed to 0, 0.020, 0.10, 0.50% formaldehyde in the drinking water (0, 10, 50, 300 mg/kg bw/day) for up to 24 months (no post exposure observation period; 6 rats per sex per dose sacrificed at each interim sacrifice after 12 and 18 month). In the high dose group, poor general state, reduction of body weight gain and both food and water consumption, increased mortality (ca. 50% after 12 months, no rat alive after 24 months), and changes in various clinical parameters were recorded. Histopathology revealed local effects in the stomach/forestomach of the high dose group. Lesions of the forestomach in males and females were squamous and basal cell hyperplasia, hyperkeratosis, erosions/ulcers, and submucosal cell infiltration. Glandular hyperplasia, erosions/ulcers, and submucosal cell infiltration in the glandular stomach were also found at the high dose level. A high incidence of renal papillary necrosis was also observed in male and female animals (about 50% versus 0-10% in the other groups). This finding is ascribed by the authors to the dehydration caused by the considerable decrease of liquid consumption (significant decrease in the high dose group, data presented in a graph revealed a range of 10-20 mL/rat/day versus 25-35 mL/rat/day in the control). No other treatment related lesions were reported. However, local effects were also detected at the mid dose level: administration of 0.10% resulted in forestomach hyperkeratosis in animals after 18 (1/6 males) and 24 months (1/8 females). According to the authors, the NOAEL was 10 mg/kg/d (NOAEC 0.02%).
In a subacute study (Til et al., 1988) oral exposure of rats via the drinking water for 28 days induced local effects in the stomach at 125 mg/kg bw/day; the NOAEL was 25 mg/kg bw/day, however, no data are given on the formaldehyde concentration in the drinking water.
There are several relatively new oral gavage studies the interpretation of which is hampered by several important drawbacks: detailed information is missing on the dosing solution, histopathology of the stomach, the primary portal of entry, was not carried out, and only one dose level of FA was used such that a dose response relationship cannot be assessed. Furthermore, the relatively steep rise in blood levels obtained by gavage leads to some uncertainty in risk assessment for humans exposed via food or liquid intake. Finally, the authors generally failed to discuss their observations in relation to other prominent studies in rodents, like e.g. the chronic study of Til et al. (1989). Therefore, the findings of these studies will only be summarized without going into details. These studies concentrated on effects by combined exposure to FA and other substances and here we will only report on the parts dealing solely with FA.
Soni et al. (2013) described necrosis of the liver after daily exposure of male mice to 25 mg/kg bw/d over 8 weeks. The strain of mice and the source of FA are not given nor the FA concentrations or the dosing volume.
Abd-Elhakim et al. (2016) observed in male Swiss mice alterations in the hematogram, leukogram and in immunological parameters at 25 mg/kg bw/d over 60 consecutive days. A 40% solution of FA (possibly formalin) was diluted in water to “working stock solutions” without giving further details. Taking into account the dosing volume of 0.1 ml/mouse (20-25 g initial weight) a FA concentration in the range of 6000 mg/l can be estimated being by a factor of ~3 higher than the highest drinking water concentration used by Til et al. (1989). Under consideration of the bolus application severe effects on the stomach are to be expected with unknown systemic sequelae and missing data in this respect is a severe deficiency.
Mohamed et al. (2016) observed in Dawley Albino rats (3 months of age) adverse effects on biochemical liver and kidney parameters and histopathological alterations in these organs. The animals received 100 mg formalin/kg bw/d (concentration of FA not given) over 30 consecutive days. For a 40% formalin solution the dose would be 40 mg FA/kg bw/d. As the dosing volume was not specified the concentration of FA cannot be calculated.
Conclusion on repeated oral exposure:
The main effects in chronic drinking water studies with rats are local lesions in the forestomach and the glandular stomach at a concentration of 0.10% in the drinking water corresponding to 50 mg/kg bw/day (Tobe et al., 1989) or 0.19% corresponding to 82 mg/kg bw/day in males and 109 mg/kg bw/day in females (Til et al., 1989). In both studies the NOAEC for local effects is very similar: 0.026% formaldehyde in the drinking water corresponding to 15 mg/kg bw/day in males and 21 mg/kg bw/day in females (Til et al. 1989) or in the study of Tobe and coworkers (1989) 0.02% corresponding to 10 mg/kg bw/day. Til et al. (1989) as well as Tobe et al.(1989) reported an increased incidence in papillary necrosis of the kidney, but this effect was due to the reduced water intake and is only indirectly treatment related. The decrease in body weight is related to the reduced water & food consumption and/or is secondary to lesions of the stomach. The NOAEL for systemic effects is 82 mg/kg bw/day in males and 109 mg/kg bw/day in females (Til et al. 1989). The study of Tobe et al. (1989) is less suitable for final evaluation of systemic effects.
Repeated dermal exposure
Studies on repeated dermal dose toxicity in compliance to current Guidelines are not available. According to column 2 of REACH Annex VIII Section 8.6.1, the study does not need to be conducted as exposure to formaldehyde via inhalation is considered to be the main route of exposure.
The available data on this endpoint are of limited validity due to restricted documentation and methodological shortcomings. In the study of Iversen (1986) the NOAEC for skin effects in mice was 200 μL of 1% formaldehyde applied twice weekly for 60 weeks; at a concentration of 10% formaldehyde hyperplasia of the epidermis was found and a few mice had small ulcers and scratches (no further data given on origin, whether self-inflicted or caused directly by formaldehyde) of the skin; no treatment related tumors at remote tissues were detected (limited documentation; histopathology of the lung, nasal mucosa, brain and all tumors only in the high dose group).
In a subacute pilot study for an initiation/promotion study (Krivanek et al. 1983) daily dermal application of 100 μl 0.5% formaldehyde in acetone/water (50:50) solution resulted in reversible irritation of the skin in mice, no effects were detected at a concentration of 0.1%.
Repeated inhalation exposure
Monticello et al. (1991) presented an inhalation study in male F344 rats which is restricted to histopathology and autoradiography of the respiratory tract. Six rats per group were exposed 6 h/day for 1, 4, 9 days, or 6 weeks to 0, 0.7, 2, 6, 10, 15 ppm formaldehyde (whole body exposure was used in all studies described in detail in this section). Histopathology of the respiratory tract was performed and additionally, autoradiography of the nasal cavity (3H-thymidine injected 2 h prior to sacrifice for DNA labelling) for measurement of cell proliferation. No effects were detected at 0.7 and 2 ppm. At >= 6 ppm lesions of the respiratory epithelium in the nasal cavity were observed; no effects were noted in any other part of the respiratory tract. Effects in the nasal cavity were dose dependent and increased with exposure time. The lesions in nasal cavity exhibited an anterior-posterior severity gradient, more severe lesions were observed in the anterior part. In histoautoradiography significant effects were reported at a dose level of >= 6 ppm and exposure period >= 1 day. In conclusion, repeated inhalation exposure resulted in lesions of the respiratory epithelium in the nasal cavity of rats, the NOAEC was 2 ppm, an anterior-posterior severity gradient was shown and the lesions were associated with epithelial cell proliferation (Monticello et al.,1991).
Swenberg et al. (1983a) exposed rats to 15 ppm for 1-9 days (6h/d). After 1 day of exposure, acute degeneration of the respiratory epithelium with edema and congestion was evident. This was followed by ulceration, necrosis, and an influx of inflammatory exudates at days 3-9. Early squamous metaplasia was detected after as little as 5 days of exposure. Five days of exposure followed by 2 days without exposure led to considerable regeneration. These initial lesions were most severe on the tip of the maxilloturbinatesand nasoturbinates. By comparing these acute toxicity data with changes observed after prolonged exposure the authors conclude that adaptive changes occur.
In another short-term exposure study male rats were exposed for 1-4 days (6h/d) to 0.5, 2, or 6 ppm.At 0.5 or 2 ppm ultrastructural changes occurred at the cilia of the respiratory epithelial cells (blebbing of the membranes in some cilia) which are not considered to be an epithelial injury. At 6 ppm hypertrophy of the goblet and ciliated cells was noted after an exposure for 1, 2, or 4 days. Non-keratinysed squamous cells were seen as early 4 days after exposure to 6 ppm and at 1 and 2 days after exposure 15 ppm. Neutrophil infiltration was observed after an exposure to 6 ppm (Monteiro-Riviere & Popp, 1986).
Time dependency of early histopathological lesions can be seen from a study primarily designed to investigate the time and concentration dependency of formaldehyde-induced effects on mucociliary function in rats (Morgan et al., 1986). A single 6 h exposure to 15 ppm produced only minimal histopathological lesions in the anterior nasal passages. After 2 days of exposure, epithelial damage and inflammation were more severe with a more posterior extension and cellular proliferation with early squamous metaplasia after 4 days. After 9 days of exposure (6 h/d; 5 d/w) epithelial degeneration was severe with ulceration in some parts of the nasoturbinate. Similar but less severe changes were found at 6 ppm and no epithelial lesions in rats exposed to 0.5 or 2 ppm.
Cell proliferation in the middle portion of respiratory mucosa after a few days of exposure was investigated by Swenberg et al. (1986). A transient increase in cell proliferation was seen in rats exposed to 0.5 or 2 ppm for just one day. This increase disappeared after 3 and 9 days of exposure. At 6 ppm rats had a massive increase in labelling index after a single 6-h exposure, a lesser response after 3 exposures that returned to nearly the control rate after 9 exposures.
A sub-chronic inhalation study in Wistar rats was performed according to current guideline (OECD 413) with acceptable restrictions (haematology without blood clotting parameter & limited clinical chemistry; no ophthalmological examination). Ten male (m) and 10 female (f) rats were exposed for 13 weeks to 0, 1, 10, or 20 ppm formaldehyde (6 h/day for 5 days per week). At the high dose uncoordinated locomotion and excitation (m&f), growth retardation (m&f), decreased total protein and increased serum enzyme activity (ASAT, ALAT, and ALP in m; considered not to be ahepatotoxic effect because there were no effects on liver weight and no histopathological effects in the liver), squamous metaplasia of thelarynx (m) and olfactory epithelium (m&f), focal epithelial thinning (m&f) and keratinisation (m) of olfactory epithelium, and diffuse squamous metaplasia of nasal respiratory epithelium (m&f) were observed. At 10 ppm (LOAEC) focal squamous metaplasia, hyperplasia, and keratinization as well as epithelial disarrangement were seen in the nasal respiratory epithelium of males and females. No effects were detected in other organs. In summary, local effects in the respiratory epithelium of the nasal cavity were induced at 10 ppm (LOAEC) and at the higher dose of 20 ppm also in the olfactory epithelium and larynx. The NOAEC in this study is 1 ppm (NCF 1984).
In the study of Wilmer et al. (1989) it was shown that exposure concentration is more important than exposure duration. The study is limited to toxic effects in the nasal cavity of male Wistar rats after inhalation exposure. Rats were exposed a) continuously: 8 h/day, 5 days per week or b) intermittently: daily 8 x 30 min exposure separated by 30 min non-exposure periods, 5 days per week for 13 weeks in a) and b). Effects were studied by histopathology and histoautoradiography after thymidine-labelling. Intermittent exposure to a concentration of 4 ppm (product of concentration [4 ppm] x time [4 h/day] is 16 ppm x h/day) but not 2 ppm resulted in treatment related effects in the respiratory epithelium confined to the nasal cavity of rats which included: increased degree and incidence of epithelial disarrangement, squamous metaplasia with and without keratinisation, and basal cell hyperplasia. No effects were seen with continuous exposure to 1 or 2 ppm although the product of concentration (2 ppm) x time (8 h/day) is the same than with interrupted exposure to 4 ppm (16 ppm x h/day) suggesting that the concentration rather than the 'dose' is responsible for the effects. No significant, treatment related effects were found on cell proliferation. The authors suggested a NOAEC of 2 ppm for effects on the nasal epithelium.
The predominance of exposure concentration over cumulative dose can also be derived from a comparison of effects in rats observed after 6 months with different exposure schedules. Striking differences in the pathological response to 15 ppm formaldehyde (6h/d, 5d/week corresponding to 450 ppm x h/week) in comparison to 3 ppm (22h/d, 7d/week corresponding to 462 ppm x h/week). The first exposure scenario corresponded to that of Kerns et al. (1983) and the latter to that of Rusch et al. (1983). Although the exposure-time product was comparable, much less toxicity was observed by exposure to 3 ppm indicating that the concentration is more important than the total dose for the observed nasal injury.
Similar findings were also noted after short-term exposure when investigating the influence on cell proliferation by exposing rats to either 3 ppm x 12 h, 6 ppm x 6 h, or 12 ppm x 3 h (each concentration x time product = 36 ppm x h) over 3 or 10 days. In the most anterior part of the nose where mucociliary clearance is minimal, the total dose was more important than the concentration concerning cell proliferation. In contrast, at level II, where mucociliary clearance is prominent and where most of the tumors originate, cell proliferation was concentration dependent. Cell proliferation rates after 3 days of exposure (suggested combination of hyperplasia due to thickening of the epithelium and compensatory proliferation for cell death)were clearly higher than after 10 days of exposure (only compensatory proliferation; Swenberg et al., 1983b). These latter findings indicate to an adaptation as initial cell proliferation rates diminish with prolongation of exposure.
Mice are less sensitive than rats. Early experiments demonstrated that the enhanced sensitivity of rats compared to mice for tumor induction is also reflected by increased cell proliferation as measured by pulse labelling with [3H]-thymidine. A single 6-h exposure to 15 ppm caused a 13-fold increase in cell proliferation in rats and only an 8-fold increase in mice. The labelling index was further increased to 23-fold when rats were exposed to 15 ppm for 5 days, but only a small additional increase was noted for mice (Chang et al., 1983).
In a sub-chronic inhalation study 10 male and 10 female B6C3F1 mice per group were exposed 6 h/day, 5 days per week for 13 weeks to 0, 2, 4, 10, 20, or 40 ppm formaldehyde. Lesions of the anterior part of the nasal cavity (squamous metaplasia [m&f] and rhinitis [m]) were reported at >= 10 ppm. Severity and site of lesions (also more posterior parts of the nasal cavity) increased with rising concentrations. Higher dose levels of >= 20 ppm in males & females also affected the larynx and trachea; lesions of the bronchus occurred at 40 ppm. No effects of toxicological relevance were detected at 4 ppm (distant site tissues not examined). Comparison with rat studies (see NCF, 1984 or Wilmer et al., 1989; or direct comparison by CIIT 1981) revealed that mice are less sensitive than rats. Conclusion: Inhalation exposure for 13 weeks induced local effects in the anterior nasal cavity of mice at 10 ppm, higher dose levels induced also effects in more proximal parts of the upper respiratory tract; the NOAEC was 4 ppm (Maronpot et al., 1986).
Rusch et al.(1983) compared toxic effects after inhalation exposure in 3 different species: rat, hamster, and monkey. The study was restricted to effects on the respiratory tract. F344 rats (20 m and 20 f per dose); Syrian golden hamster (10 m and 10 f per dose), and monkeys (Cynomolgus; 6 males per dose) were exposed to 0, 0.2, 1, or 3 ppm, 22 h per day, 7 days per week for 26 weeks. Rats revealed no treatment related effects at 0.2 and 1 ppm. At 3 ppm clinical symptoms were not reported but body weight was decreased in males. This level induced also effects in the epithelium of nasoturbinates in males and females including increased incidence in squamous metaplasia and hyperplasia at the middle section level of the nasoturbinates. The LOAEC was 3 ppm and the NOAEC was 1 ppm. Very similar results were found in monkeys. This species revealed no treatment related effects at 0.2 and 1 ppm. At 3 ppm clinical symptoms were reported. This level induced also effects in the nasoturbinates: squamous metaplasia and hyperplasia. No effects were detected in other tissues of the respiratory tract. In contrast, Syrian golden hamsters revealed no treatment related effects in any studied respiratory tract tissue (lung, trachea, nasal cavity) even at a level of 3 ppm. Hamsters seem to be a less sensitive animal model for formaldehyde exposure compared to rats.
Supporting evidence for a NOAEC of 1 ppm in rats came also from a study using transmission electron microscopy (SOCMA, 1980) after exposure for 26 weeks to 0 or 1 ppm (5 rats per sex, 22 h/day, 5 days/week). Rats exposed to 1 ppm did not show any ultrastructural alterations of the nasal epithelium, tracheal epithelium or epithelium of terminal bronchioles (SOCMA 1980).
In a long-term inhalation study restricted to the investigation of effects in the nasal cavity 90-147 male F344 rats per dose were exposed to 0, 0.7, 2, 6, 10, 15 ppm formaldehyde gas 6 h/day, 5 days per week for 24 months. Rats exposed to 0.7 or 2 ppm did not show any histopathological changes in the nasal cavity and no effects on cell proliferation index. Nonneoplastic effects were detected at a dose level of 6 ppm (mixed cell infiltrate, squamous metaplasia, hyperplasia in the anterior part of the nasal cavity) as well as a minimal carcinogenic response (incidence 1/90 versus 0/90 in control). A sharp increase in tumor incidences in the nasal cavity (mainly squamous cell carcinoma) was reported at 10 and 15 ppm. Dose dependent increases on cell proliferation were detected also only at >=10 ppm. In conclusion, inhalation exposure for 2 years induced local effects in the nasal cavity of male F344 rats at a dose level of 6 ppm and clearly carcinogenic effects at 10 ppm; the NOAEC was 2 ppm (Monticello et al., 1996).
In the inhalation study of Kamata et al. (1997; reliable with acceptable restrictions concerning local effects) 32 male F344 rats per dose were exposed to 0, 0.3, 2.2, or 15 ppm formaldehyde 6 h/day, 5 days per week for 28 months; 5 rats per dose were sacrificed after 12, 18, or 24 months of exposure (no post exposure observation period). Rats exposed to 0.3 ppm for 28 months did not show any histopathological changes in the nasal cavity. At 2.2 ppm a significant increase was noted in the incidence of squamous cell metaplasia with and without hyperplasia but no carcinogenic effects. Clear carcinogenic effects were noted at a level of 15 ppm but only in the nasal cavity; no tumors were found in non-site-of-contact tissues. There is some evidence that this study is less suitable for deriving a LOAEL (more variable exposure concentrations than in other studies). In summary, inhalation exposure for 28 months induced rats non-neoplastic effects in the respiratory epithelium of the nasal cavity at ≥ 2.2 ppm and neoplastic effects at 15 ppm.
In a long-term study comparable to OECD guideline 453 with acceptable restrictions (no histopathology of sternum, larynx and pharynx) the inhalation exposure of male and female F344 rats for 2 years at dose levels of 0, 2, 6, 15 ppm (120 rats per sex per dose; 24 months exposure, 6 h per day, 5 days per week; post exposure period 6 months; interim sacrifices) resulted exclusively in local effects of the nasal cavity and of the proximal trachea in males and females. Only non-neoplastic effects were reported in the high dose group in the proximal trachea but a high incidence in squamous cell carcinomas of the nasal cavity was observed. Carcinogenic effects in the mid dose group (squamous cell carcinomas in nasal cavity) are not of statistical significance but possibly of toxicological relevance. Non-neoplastic effects in the mid dose group like dysplasia and squamous cell metaplasia in the nasal cavity are considered to be treatment related since these effects occurred in more posterior parts or the nasal cavity. At 2 ppm purulent rhinitis, epithelial dysplasia, and squamous metaplasia were present in the anterior part of the nasal cavity. Dysplasia occurred earlier than metaplasia. Generally the location of the histopathological lesions moved from the anterior parts of the nose to more posterior sites with increasing exposure duration and concentration. There is some evidence that effects at the low dose level are less suitable for derivation of a LOAEC due to more variable exposure concentrations than in other studies particularly at the 2 ppm exposure level. In conclusion, clearly carcinogenic effects in the nasal cavity of rats were reported after inhalation exposure to 15 ppm formaldehyde for up to 2 years; local effects in the nasal cavity were found even at a level of 2 ppm. The exposure period of 24 months was followed by 3 months without exposure. At month 27 there was a significant decrease in the incidence of squamous metaplasia at all exposure concentrations regressing predominantly from the more posterior parts of the nose to the anterior sections (CIIT, 1981; Kerns et al. 1983).
In this study also B6C3F1 mice were exposed using the same experimental design. Formaldehyde exposure resulted only in local effects in the nasal cavity (males and females), predominantly in the respiratory epithelium; no tumors were seen in other organs. Rhinitis, dysplasia, squamous metaplasia were induced, but also atrophy of olfactory epithelium with increasing exposure periods. Except for reduced body weight no further effects were detected. The NOAEC was 2 ppm and the LOAEC 6 ppm. In contrast to rats only a few tumors were detected in the nasal cavity at 15 ppm. Generally the lesions in mice were less severe than in rats.At 27 months (after a 3 months observation period without exposure) dysplastic epithelial lesions were only present in the 15 ppm group. Squamous metaplasia was not present at this time interval and the low- and intermediate-exposure groups were free of treatment-related effects (CIIT, 1981; Kerns et al. 1983).The NOAEC for systemic effects in these long-term inhalation studies in rats and mice is 15 ppm.
Recently a subacute inhalation study in rats at exposure concentrations of 0, 0.7, 2, and 6 ppm up to 3 weeks (6 h/d; 5 d/week) was reported with focus on effects in the nasal cavity including genomic signatures (Andersen et al., 2008). Neither cell proliferation nor histopathological alterations were found in nasal cavity at 0.7 ppm. At 2 ppm a few animals exhibited epithelial hyperplasia that occurred in all animals exposed to 6 ppm. A significantly increased cell proliferation was only found at 6 ppm at the end of the first week (day 5) but not at the end of week 3. This is in line with former studies showing that cell proliferation decreases as exposure duration increases. Microarray analysis was conducted on respiratory epithelium of those nasal sites that receive a high formaldehyde flux corresponding to regions with a high tumor incidence in chronic studies. In microarray studies, no genes were altered at 0.7 ppm. At 2 ppm 15 genes were changed on day 5; 28 genes were changed at 6 ppm on day 5. No genes were changed at 2 ppm at day 15. The majority of changes was observed at 6 ppm. The pattern of gene changes at 2 and 6 ppm, with transient squamous metaplasia at day 5, indicated tissue adaptation and reduced tissue sensitivity by day 15 (compare with Swenberg 1983 a/b & 1986). In an acute part of this study rats were exposed to 15 ppm; in addition a group was included receiving a formaldehyde solution by nasal instillation (40 µLper nostril, 400 mM solution). Both instillation of 400 mM and inhalation of 15 ppm formaldehyde altered many more genes than were affected at 6 ppm. Three times more genes were affected by instillation than by inhalation exposure to 15 ppm. U-shaped dose responses were noted in the acute study for many genes that were also altered at 2 ppm on day 5. On the basis of cellular component gene ontology benchmark dose analysis, the most sensitive changes were for genes associated with extracellular components and plasma membrane. Generally,genomic markers provide, at best, only modestly more sensitive measures of tissue response compared with histology.
A summary of other reviewed data on repeated inhalation toxicity is presented in a supporting data record (BfR, 2006).
A literature search after the last IUCLID update was carried out up to May 2022.
Gelbke et al. (2014, key) assessed the NOAELs/LOAELs for the nose of rats after inhalation for histopathological lesions, cell proliferation, and the formation of polypoid adenomas. A new statistical analysis combined with a detailed review of the studies showed that the LOAEL is >2 ppm for cell proliferation and polypoid adenomas. Polypoid adenomas cannot be considered as pre-stages to the development of squamous cell carcinomas, the predominant tumor type in rats and mice after inhalation of formaldehyde. The NOAEL for histopathological lesions in the upper respiratory tract is 1 ppm but lesions observed at 2 ppm are not pre-stages to tumor development. It is noted that descriptions of histopathological lesions, including polypoid adenomas, in former studies often are insufficient and sometimes do not allow a final decision.
These investigations warrant a separate consideration because of the known skin sensitisation by formaldehyde and its alleged implication in respiratory sensitisation. In addition, Rager et al. (2014, see Section Genetic toxicity in vitro) analysed miRNA profiles and noted transcriptional changes of mRNAs involved in the immune/inflammation system in response to changes of miRNAs.
Sapmaz et al. (2015, supporting) studied the effect of formaldehyde inhalation on humoral immunity in rats (5 and 10 ppm, 8 h/d, 5 d/week over 4 weeks). Exposure significantly increased IgA, IgM, and complement 3 levels in blood and decreased IgG dose dependently starting already at 5 ppm. These findings would need to be supported by an independent experiment.
Conclusions on repeated inhalation exposure toxicity:
Local effects in the upper respiratory tract were induced after repeated inhalation exposure in experimental animals. It has been shown in rats, mice, and monkeys that the respiratory epithelium in the nasal cavity is the most sensitive site. In rats and monkeys squamous metaplasia and hyperplasia were reported, in mice rhinitis, dysplasia and squamous metaplasia.
The LOAEC is 3 ppm in monkeys (NOAEC 1 ppm) and 6 ppm in mice (NOAEC 4 ppm). In rats, however, the LOAEC was considered to be 2 ppm (CIIT, 1981; Kamata et al., 1997), although this dose level was the NOAEC in other studies (Monticello et al., 1991; Wilmer et al., 1989; Monticello et al., 1996). These contradictory results most probably related to differences in the accuracy of constant exposure concentrations because formaldehyde induced lesions are predominantly concentration dependent and thereby fluctuations in concentration may become important, such as observed in the CIIT study.
Using the same experimental design mice are less sensitive than rats (CIIT, 1981); this might be related to the lower RD50 value in mice compared to rats (Section Actute Toxicity: inhalation) leading to a decreased minute volume for mice.
In chronic studies with rats as well as in subacute studies with only a few days duration (measuring cell proliferation; see supporting data record BfR, 2006) or weeks duration (measuring epithelial lesions; see data above) the NOAEC was the same range as that in long term studies. Effects were noted in the respiratory epithelium at a concentration of 2 ppm. At dose levels above the LOAEL the severity of the lesion in respiratory epithelium increased with the concentration and the duration of the exposure period (cf. Monticello et al., 1991).However, studies in rats (using the constant concentration x exposure duration) revealed that the concentration rather than the “total dose” is responsible for the effects (Wilmer et al., 1989). At high dose levels (10-20 ppm) no complete reversibility of lesions in the nasal cavity was reported in rats after 13 weeks of exposure and a post exposure observation period of up to 126 weeks (Feron et al., 1988; see supporting data record BfR, 2006). Not only the anterior part of the nasal cavity but also more proximal parts of the upper respiratory tract were affected with increasing formaldehyde concentrations in rats and mice (e.g. CIIT, 1981; Maronpot et al., 1986): olfactory epithelium, larynx, trachea, and bronchus. Increase in toxicity and severity is not linear but shows a sharp increase at 6-10 ppm. By microarray analysis no significant gene changes were observed at 0.7 ppm and minimal changes at 2 ppm only on day 5 that had returned to control level at day 15.The majority of changes were found at 6 ppm (highest exposure level for repeated exposure). However, authors concluded that genomic markers provide, at best, only modestly more sensitive measures of tissue response compared with histology (Andersen et al., 2008).
Related to the studies described above the overall NOAEC for non-neoplastic tissue changes based on chronic studies with rats, mice, hamsters and monkeys is 1 ppm. corresponding to 1.2 mg/m³.
A detailed review of former studies in rats confirmed that the NOAEL for regenerative cell proliferation is between 2 and 6 ppm and for histopathological lesions 1 ppm. Lesions observed at 2 ppm are not prestages to tumor development. Also the polypoid adenomas observed in some inhalation studies are not prestages to squamous cell carcinomas, the most prominent tumor type observed in the nose.
In the recent literature several non-guideline studies report lesions in the lower respiratory tract, in other organs apart from the respiratory tract, or claim indications for immunological effects. The evidence reported in these studies has to be weighed against findings in former guideline studies up to 2 years of exposure with large number of experimental animals, none of which gave indications for lesions apart from the nose. Therefore, by weighting the validity of the different studies, the conclusion is still upheld that formaldehyde inhalation will only lead to lesions in the upper respiratory tract.
Studies reporting immunological effects may warrant a separate evaluation because of the known skin sensitising properties of formaldehyde and indications that formaldehyde inhalation may lead to transcriptional changes by miRNA involved in the immune system. However, the majority of these non-guideline studies had to be disregarded due to missing dose response relationships, investigation of tissues far of the portal of entry, irrelevant exposure routes, or unclear exposure concentrations.
Effects in humans occupationally exposed via inhalation (supporting data record BfR, 2006): There is some evidence from a number of studies that formaldehyde exposure may also induce squamous cell metaplasia and hyperplasia in the respiratory epithelium of the nasal cavity in long-term exposed humans. However, the data base is not sufficient for conclusions on dose or time response relationships. None of the studies were conducted with sufficient methodological accuracy and no firm conclusion can be drawn (WHO, 2002; BfR, 2006).
Chronic respiratory effects of indoor formaldehyde exposures reported by Krzyzanowski et al. 1990 were amongst other publications reviewed by the German UBA. Recently, German UBA (2016) performed a comprehensive review of epidemiological studies investigating the association between formaldehyde exposure and the induction or exacerbation of asthma in children including several of the studies mentioned above. UBA concluded that there is no clear association between formaldehyde exposure in the indoor environment and asthma in children. It was stated that the above mentioned epidemiological studies (e.g. Krzyzanowski et al., 1990; Annesi-Maesano et al. 2012 etc.) suffer from small sample size, implausible formaldehyde concentrations, and the fact that other substances or factors initiating asthma and asthma-like complaints were not adequately considered. Results derived from controlled human exposure studies as well as animal experiments support their opinion. It should also be noted that the Krzyzanowski et al. (1990) findings are inherently limited by the cross-sectional nature of their study and found that the study design is not sufficiently described in the published report.
In the literature search up to April 20, 2015, following the last IUCLID update an epidemiological study was identified unrelated to potential cancer risks. Pinkerton et al. (2013, supporting) assessed mortality from amyotrophic lateral sclerosis (ALS) in a cohort of formaldehyde exposed garment workers that had already been studies for cancer incidence rates by Pinkerton et al. (2004), and Meyers et al. (2013). In their introduction they referred to two former studies related to ALS. The large American Cancer Society’s Cancer Prevention Study II cohort comprised about 1 million subjects with 22 exposed cases and 1120 unexposed. The rate ratio for self-reported formaldehyde exposure was 2.47 (95% CI 1.58, 3.86). The rate ratios were strongly associated with exposure duration. Reliance on self-reported exposure was a mojor limitation with no information on exposure frequency or intensity (Weisskopf et al., 2009). In a case control study with 109 cases and 253 controls, formaldehyde exposure inferred from occupation was not associated with ALS (Fang F, Quiulan P, Ye W, Bumer MK, Umbach DM,Sandler DP,et al. Work place exposures and the risk of amyotrophic lateral sclerosis. Environ Health Perspect,2009;117:1387-92). The study of Pinkerton et al. (2013) comprised 11098 employees exposed to formaldehyde for at least for 3 months. In the early 1980s the overall geometric mean formaldehyde concentration was 0.15 ppm. In total 8 ALS cases were observed with a SMR of 0.89 (95% CI 0.38, 1.75). No association for ALS with formaldehyde exposure was found when analysed for year of first exposure, duration of exposure, or time since first exposure. The main limitations of this study are the small number of cases and the low exposure levels.
According to EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation (EC) No. 1272/2008, classification and labelling is not needed for repeated dose toxicity. However, under consideration of the RAC (2012) proposal for classification and labelling, formaldehyde is classified as Carc. Cat 1B (H350) according to EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation (EC) No. 1272/2008, Annex VI.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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