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Toxicological information

Acute Toxicity: inhalation

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Administrative data

acute toxicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2001-02-21 to 2001-03-16
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Guideline study reliable without restrictions.

Data source

Reference Type:
study report
Report date:

Materials and methods

Test guideline
according to guideline
OECD Guideline 403 (Acute Inhalation Toxicity)
GLP compliance:
yes (incl. QA statement)
signed 2001-04-03
Test type:
standard acute method
Limit test:

Test material

Constituent 1
Reference substance name:
molybdenum dioxide
molybdenum dioxide
Constituent 2
Chemical structure
Reference substance name:
Molybdenum dioxide
EC Number:
EC Name:
Molybdenum dioxide
Cas Number:
Molecular formula:
Constituent 3
Reference substance name:
Cas Number:
Details on test material:
- Name of test material (as cited in study report): Molybdenum dioxide
- Chemical name: Molybdenum dioxide (MoO2)
- Physical state: Brown/violet powder
- Analytical purity: > 98 % (Content molybdenum > 74.0 %)
- Expiration date of the batch: 2010-11-30
- Storage condition of test material: In the dark/dry at ambient room temperature (approx. 20 °C) and in the original container
No further information on the test material was stated.

Test animals

Details on test animals or test system and environmental conditions:
- Age at study initiation: approx. 8 and 9 weeks old
- Weight at study initiation: Test group: Average males was 292 g and the average females was 217 g; Control group: Average males was 288 g and the average females was 220 g
- Housing: The animals were housed by sex, in groups of 5. The holding cages were made of stainless steel sheet and wire mesh and were suspended on a movable rack. Plastic trays, lined with absorbent paper, were placed below each cage to collect animal excreta. The paper was changed daily. Clean cages were introduced at approximately 2-weekly intervals. The animals remained in a holding room except for the 4-hour exposure and an overnight post exposure period when the animals in the test group were kept in a ventilated cabinet to allow dispersal of any residual test substance.
- Diet (ad libitum): While in their cages all animals had free access to an excess amount of food, SDS rat and mouse diet (RM1 (E) SQC expanded pellet)
- Water (ad libitum): tap water supplied by Anglian Water
- Acclimation period: 9 days before the day of exposure

- Temperature: 20 °C - 20.5 °C
- Relative humidity : 36% - 65 %
- Air changes: At least 15 air changes per hour
- Photoperiod (hrs dark / hrs light): 12/12
No further information on the test animals was stated.

Administration / exposure

Route of administration:
inhalation: dust
Type of inhalation exposure:
nose only
clean air
Details on inhalation exposure:
- Exposure apparatus: The snout-only exposure chambers (ADG Developments LTD., Hitchin, Hertfordshire, England) used for the exposure were cyclindrical form (30 cm diameter, 45 cm height) and made of aluminium alloy. The internal surfaces of the chamber have a conformal chemically resistant coating.
- Exposure chamber volume: The chambers have an enclosed volume of approximatley 30 litres.
- Method of holding animals in test chamber: The animals were held for exposure in moulded polycarbonate restraining tubes, which were attached at evenly spaced ports in the cylindrical section of the chamber, and were designed to allow only the snout to project into the chamber. Each animal was restrained in a forward position by an adjustable foamed plastic stopper, which also provided a seal for the tube. EAch animal was placed into a separate restraining tube and the tubes were then attached to the exposure chamber.
The conditioned test atmosphere entered through a port at the top centre of the chamber and passed out through a port at the base section below the level of the animals.
The exposure system was positioned inside a large cabinet equipped with an extract fan exhausting to atmosphere through an absolute filter.

- Source and rate of air: A supply of clean, dry air was connected to the generator and the supply prssure was adjusted to give a flow rate of 10 litres/minute, measured at the generator outlet. An in-line flow meter was used to monitor the generator air supply throughout the exposure. The exhaust airflow was calibrated at the point of attachment to the exposure chamber and was adjusted to produce a slightly negative pressure.

- Method of conditioning air: The test atmosphere was passed through a vertical glass elutriator to reduce, by sedimentation, the amount of non-respirable particulate in the test atmosphere.

- System of generating particulates/aerosols: A Wright Dust Feed mechanism (WDF) was used to produce the test atmosphere containing a particulate aerosol generated from Molybdenum dioxide. The WDF was designed to produce and maintain atmosphere containing a particulate aersol by suspending material scraped from the surface of a compressed powder in a stream of dry air. The concentration of particulate aerosol in the air is determined by the rate at which the scraper blade is advanced into the compressed powder.
Two models of a 'belt-driven' WDF are used for acute inhalation exposures, each with a different gearing mechanism and designated as 'Slow' or 'Fast'. The 'Fast WDF' was used for generation of the test atmosphere. A jet was fitted to the WDF to break-up aggregates emitted from the aerosol generator.
A sample of the test substance was packed into the container of the WDF mechanism using a hydraulic bench press to assist packing. An even density of the test substance was achieved by packing the container in stages and applying a force of 3.0 ton to compress the powder. The applied force was sufficient to prevent disintegration of the packed powder during the generation procedure. The packed container was weighed.
The WDF was positioned vertically on a stand at the side of th exposure chamber and the output was connected to the top inlet port of the chamber via the elutriation column. An initial speed controller setting of '58% of the maximum speed' of the WDF was selected, as a result of preliminary generation trials, to generate a concentration of total particulate at the target concentration of 5 mg/l. The WDF speed setting was adjusted as necessary during the exposure in order to ensure generation of the test aerosol close to target.
The powder container of the WDF was advanced manually until a trace of suspended dust was seen to emerge from the WDF outlet. The gearing on the generator was then engaged and the generator motor switched on to start the exposure.
After an equilibration period of 7 minutes, the exposure was timed for 4 hours. The WDF canister was replaced with further packed canisters as required during the exposure. The generator was then swithced off and the chamber was allowed to clear before the animals were removed for examination.
Following exposure, the animals were returned to the holding cages where food and water were available. The test animals were kept in a ventilated cabinet overnight and then returned to the holding room for the remainder of the observation period.
The control group was treated similarly but received clean air only for 4-hours. The control animals were returned to the holding room at the end of the exposure period.

- Method of particle size determination: Two air samples were taken during the exposure at a sampling rate of 2 litres/minute using a Marple cascade impactor ( Model 298, Graseby Andersen Inc., Atlanta, Georgia) to determine particle size distribution. the samples were taken at 89 and 200 minutes into exposure. The volume of air sampled was measured using a wet-type gas meter (Model DM3B, G.H. Zeal Ltd., London, England).
The amount of material collected on the stages of the sampler was determined gravimetrically. the particle size distribution of the test atmosphere was assessed using linear regression analysis. The probit of the cumulative percentage of the total particles collected, smaller than the cut-point of each stage, was plotted against the logarithm of the cut-point of each stage.

- Temperature, humidity, pressure in air chamber: The air temperature in the exposure chamber was mesaured using an alcohol-in-glass thermometer and the relative humidity was determined using an Analytical Development Comapny Ltd., infra red water vapour analyser. The temperature and relative humidity were determined at the start of exposure and then at 30-minute intervals during the 4-hour exposure. The mean chamber air temperature, the relative humidity and the standard deviation (sd) of the means during the exposure of the groups were: Control group: 19.4 °C +/- 0.22 °C and 29 % +/- 3.7 %; Test group: 19.9 °C +/- 0.17 °C and 35 % +/- 4.4 %. The mean relaive humidity of the test chamber was slightly higher than the control value and was considerd not to have affected the outcome of the study. The mean chamber air temperature was similar for the control and test atmospheres.

- Brief description of analytical method used: Ten samples of air were removed from the test chamber during exposure in order to determine the concentration of the test aerosol. In the first instance, samples were obtained following equilibration and generally at approximately hourly intervals thereafter. Additional samples were obtained as necessary to monitor the chamber concentration following adjustment to the exposure system in order to ensure satisfactory generation. A time-weighted average was calculated from the individual data in order to prevent undue biasing of repeat samples on the overall mean.
Each air sample was withdrawn, at a rate of 2 litres/minute, through a pre-weighed glass fibre filter (Schleicher & Schuell GF/50 filters) mounted in an open face filter holder. The volume of air sampled was measured using a wet-type gas meter (Model DM3B, G.H. Zeal Ltd., London, England). The filters were re-weighed following sampling for gravimetric analysis of the test aerosol.

- Nominal concentration: The nominal concentration of the test substance in the exposure chamber was calculated from the total mass of Molybdenum dioxide dispersed by the dust generator and the total volume of air flowing through the exposure system during the period of generation.

TEST ATMOSPHERE (if not tabulated)
- Particle size distribution: Approximately 79 % of the particulate were considered of respirable size (< 7 µm in aerodynamic diameter).
- MMAD / GSD: 3.7 µm (GSD = 2.20)
No further information on the inhalation exposure was stated.
Analytical verification of test atmosphere concentrations:
see "details on inhalation exposure" above and "Concentration" below
Duration of exposure:
4 h
Actual concentration: 5.10 mg/l (time-weighed average chamber concentration)
Nominal concentration: 97.9 mg/l
The mean chamber concentration was 5.2 % of the nominal concentration and reflects losses of the test material due to impaction and deposition within the exposure system. The relatively low efficiency observed in this study is not unusual for the exposure system and test compound employed.
No. of animals per sex per dose:
Control group: 5 males /5 females
Test group: 5 males / 5 females
Control animals:
Details on study design:
- Duration of observation period following administration: 14 days
- Frequency of observations and weighing: Throughout the study, all cages were checked at least twice daily, once in the morning and again towards the end of the normal working day, for dead or moribund animals. Any animals found dead were subjected to a detailed macroscopic examination. The animals were observed intermittently for signs of reaction to the test substance during exposure and at least twice daily throughout the observation period. The clinical signs were recorded at the end of the chamber equilibrium period, at 0.25, 0.5 and 1.0 hours then at hourly intervals during the exposure. Clinical signs were recorded immediately following exposure and then at 1.0 and 2.0 hours post exposure During the observation period, the clinical signs were recorded once in the morning and then as necessary following a later check for survival.
All animals were weighed twice during the week prior to exposure, prior to exposure (Day =), weekly during the observation period and on the day of death. A visual inspection of water bottles was conducted daily.
- Necropsy of survivors performed: Yes; At the end of the 14-day observation period, the animals were killed by intraperitoneal injection of pentobarbitone sodium followed by exsanguination from the brachial blood vessels.
All animals were subjected to a detailed macroscopic examination. The lungs (including the larynx and trachea) were removed, dissected clear of surrounding tissue, weighed and the weights recorded.
Macroscopic abnormalites noted at necropsy were fixed in 10 % neutral buffered fromalin. Lungs were infused with 10% neutral buffered formalin prior to immersion in fixative. No further examination was performed. The tissue were otherwise discarded following necropsy.
No further information on the study design was stated.

Results and discussion

Effect levels
Dose descriptor:
Effect level:
> 5.1 mg/L air (analytical)
Based on:
test mat.
Exp. duration:
4 h
There was one unscheduled death. One animal was found dead 3h 33 min into exposure.
Clinical signs:
other: During exposure: Exaggerated breathing was evident in two male test rats during the equilibration period, and all test rats from 15 minutes into exposure. A black substance on fur was observed for all test rats from 15 minutes into exposure. Soiling of th
Body weight:
There were no treatment-related effects.
Gross pathology:
Macroscopic pathology:
Decedent: A mottled grey/pink discolouration was evident on all the lobes of the lungs of the decedent test rat together with large areas of severe congestion on the lungs. the stomach of the decedent test rat was gas filled.
The external finding of fur soiled with excreta and black substance on the fur (head and body) was noted for the decedent test rat prior to necropsy.
Rats surviving the 14-day observation period:
A grey colouration was evident on the lungs of all test rats. Small dark foci were noted on the lungs of a control and a test rat.
The external finding of grey staining on the tail was noted for all surviving test rats prior to necropsy.
Other findings:
- Water consumption:
There were no treatment-related effects.
A visual appraisal of the water bottles indicated that the amount of water consumed by test animals was similar to that of the control animals.
- Organ weights: There were not treatment-related effects on the lung weight.

Applicant's summary and conclusion

Interpretation of results:
not classified
Migrated information Criteria used for interpretation of results: EU
The LC50 (4-hour) for Molybdenum dioxide is in excess of 5.10 mg/l in air.
According to the criteria specified by Directive 67/548/EEC and subsequent regulations, the test item is not classified as acute toxic by the inhalation route.
According to the EC Regulation No. 1272/2008 and subsequent regulations, the test item is not classified as acute toxic by the inhalation route.