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Experimental Studies of Molybdenum Physiology and Toxicology

MoS2 nanoparticles induce behavioral alteration and oxidative stress mediated cellular toxicity in the social insect oecophylla smaragdina (asian weaver ant)

The study evaluates molybdenum disulfide (MoS2) nanoparticles (NPs) induced oxidative stress during cellular toxicity in an invertebrate in vivo system, the weaver ant. The lethal concentration was checked and LC50 was obtained as 50 microg/mL. Feeding assay and the photoluminescence activity confirmed the ingestion of MoS2 NPs by the organism. Behavioral assays showed altered grooming behavior in the MoS2 NP fed ants. A drastic decrease in the hemocyte count in the MoS2 NP fed ants revealed the anti-proliferative role of MoS2. This was further confirmed by 5-bromo-2'-deoxyuridine (BrdU) labeling assay. MoS2 NPs induced apoptotic activity was also observed in the hemocytes by acridine orange/ethidium bromide (AO/EB) staining. The level of oxidative stress during cellular toxicity was observed. An increased reactive oxygen species (ROS) level was observed in the MoS2 NP fed ants when compared to the control group. The increased activity of superoxide dismutase (SOD) and the lipid peroxidation (LPO) product were observed. While, the activities of catalase (CAT) and glutathione-s-transferase (GST) and the glutathione content (GSH) were decreased by MoS2 NPs. The transcript levels of SODs, CAT and GST were up regulated in the treated group. Our results suggest that MoS2 NPs induced oxidative stress mediates the cellular toxicity in the foragers of the weaver ant.

S. Cc, A. Anusri, C. Levna, A. Pm, and D. Lekha, Mos2 nanoparticles induce behavioral alteration and oxidative stress mediated cellular toxicity in the social insect oecophylla smaragdina (asian weaver ant), J Hazard Mater, 2020, 385, 121624.

Alterations of mitochondrial antioxidant indexes and apoptosis in duck livers caused by Molybdenum or/and cadmium

Cadmium (Cd) and high Molybdenum (Mo) can lead to adverse reactions on animals, but the co-induced toxicity of Mo and Cd to liver in ducks was not well understood. To investigate the co-induced toxic effects of Mo combined with Cd on mitochondrial oxidative stress and apoptosis in duck livers. 240 healthy 11-day-old ducks were randomly divided into 6 groups (control, LMo group, HMo group, Cd group, LMoCd group and HMoCd group). After being treated for 30, 60, 90 and 120 days, liver mitochondrial antioxidant indexes, ceruloplasmin (CP), metallothionein (MT), Bak-1 and Caspase-3 genes mRNA expression levels, and ultrastructural changes were evaluated. The results showed that total antioxidative capacity (T-AOC), catalase (CAT), superoxide dismutase (SOD) and xanthine oxidase (XOD) activities in experimental groups were decreased, whereas malondialdehyde (MDA) content and nitric oxide synthase (NOS) activity were increased compared with control group, and these changes of co-treated groups were more obvious in the later period of the experiment. The mRNA expression levels of CP, Bak-1 and Caspase-3 were up-regulated in experimental groups compared with control group and showed significant difference between co-treated groups and single treated groups. The mRNA expression level of MT in Cd group was higher than that in co-treated groups. Additionally, ultrastructural changes showed karyopyknosis, mitochondrial swelling, vacuolation and disruption of mitochondrial cristae in co-treated groups. Taken together, it was suggested that dietary Mo and Cd might lead to mitochondrial oxidative stress and apoptosis in duck livers, and it showed a possible synergistic relationship between the two elements.

Dai, X., Xing, C., Cao, H., Luo, J., Wang, T., Liu, P., Guo, X., Hu, G., and Zhang, C.,Alterations of mitochondrial antioxidant indexes and apoptosis in duck livers caused by Molybdenum or/and cadmium, Chemosphere, 2017, 193, 574-580.


Induction of fetal abnormalities and genotoxicity by molybdenum nanoparticles in pregnant female mice and fetuses

Increasing the uses of molybdenum (Mo) nanoparticles in a wide range of applications including food, industry, and medicine, resulted in increased human exposure and necessitated the study of their toxic effects. However, almost no studies are available on their genotoxic effects, especially on pregnant females and their fetuses. Therefore, this study was undertaken to estimate the possible induction of genotoxicity and fetal abnormalities, especially fetal malformations and skeletal abnormalities by Mo nanoparticle administration in mice. Oral administration of Mo nanoparticles resulted in significant decreases in the maternal body weight, the number and length of fetuses as well as skeletal abnormalities mainly less ossification and less chondrification. Administration of Mo nanoparticles also caused DNA damage induction which elevated the expression levels of p53, the vital gene in maintaining the genomic stability and cell differentiation in both maternal and fetus tissues. Similarly, the expression levels of E-Cad and N-Cad genes that control skeleton development have also been increased in the tissues of female mice administered Mo nanoparticles and their fetuses. Thus, we concluded that oral administration of Mo nanoparticles induced genotoxic effects and fetal abnormalities that necessitated further studies on the possible toxic effects of Mo nanoparticles.

H. R. H. Mohamed, R. H. El-Atawy, A. M. Ghoneim, and A. A. El-Ghor,Induction of fetal abnormalities and genotoxicity by molybdenum nanoparticles in pregnant female mice and fetuses, Environmental science and pollution research international, 2020. Environmental Science and Pollution Research https://doi.org/10.1007/s11356-020-08137-0.



Sulfite-induced protein radical formation in LPS aerosol-challenged mice: Implications for sulfite sensitivity in human lung disease

Exposure to bisulfite HSO3-and sulfite SO32- has been shown to induce a wide range of adverse reactions in sensitive individuals. Studies have shown that peroxidase-catalyzed oxidation of bisulfite leads to formation of several reactive free radicals, such as sulfur trioxide anion SO3-, peroxymonosulfate -O3SOO., and especially the sulfate SO4- anion radicals. One such peroxidase in neutrophils is myeloperoxidase (MPO), which has been shown to form protein radicals. Although formation of bisulfite-derived protein radicals is documented in isolated neutrophils, its involvement and role in in vivo inflammatory processes, has not been demonstrated. Therefore, we aimed to investigate bisulfite-derived protein radical formation and its mechanism in LPS aerosol-challenged mice, a model of non-atopic asthma. Using immuno-spin trapping to detect protein radical formation, we show that, in the presence of bisulfite, neutrophils present in bronchoalveolar lavage and in the lung parenchyma exhibit, MPO-catalyzed oxidation of MPO to a protein radical. The absence of radical formation in LPS-challenged MPO- or NADPH oxidase-knockout mice indicates that sulfite-derived radical formation is dependent on both MPO and NADPH oxidase activity. In addition to its oxidation by the MPO-catalyzed pathway, bisulfite is efficiently detoxified to sulfate by the sulfite oxidase (SOX) pathway, which forms sulfate in a two-electron oxidation reaction. Since SOX activity in rodents is much higher than in humans, to better model sulfite toxicity in humans, we induced SOX deficiency in mice by feeding them a low molybdenum diet with tungstate. We found that mice treated with the SOX deficiency diet prior to exposure to bisulfite had much higher protein radical formation than mice with normal SOX activity. Altogether, these results demonstrate the role of MPO and NADPH oxidase in bisulfite-derived protein radical formation and show the involvement of protein radicals in a mouse model of human lung disease.

A. Kumar, M. Triquigneaux, J. Madenspacher, K. Ranguelova, J. J. Bang, M. B. Fessler, and R. P. Mason,Sulfite-induced protein radical formation in LPS aerosol-challenged mice: Implications for sulfite sensitivity in human lung disease, Redox biology, 2018, 15, 327-334.

[LPS Aerosol is an industrial chemical, It is a specialized soft-film spray coating designed to prevent rust and corrosion on steel, aluminum and other metals. It contains mineral spirits and mineral oil which can be irritating to skin.

Neutrophils. A type of immune cell that is one of the first cell types to travel to the site of an infection. Neutrophils help fight infection by ingesting microorganisms and releasing enzymes that kill the microorganisms. A neutrophil is a type of white blood cell, a type of granulocyte, and a type of phagocyte. https://www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0022058/]





Reproductive toxicity in male mice after exposure to high molybdenum and low copper concentrations

To evaluate the effects of dietary high molybdenum (HMo) and low copper (LCu) concentrations on reproductive toxicity of male mice, 80 mice were divided into 4 groups of 20. These groups were fed with the following: (1) normal control (NC) diet (NC group); (2) NC and HMo diets (HMo group); (3) LCu diet (LCu group); and (4) HMo and LCu diets (HMoLCu group).

On the 50th and 100th day, superoxide dismutase (SOD), malondialdehyde (MDA), glutathione peroxidase (GSH-Px), and total antioxidant capacity (T-AOC) were analyzed to determine oxidative stress states.

Morphological changes in testicular tissue were evaluated with hematoxylin and eosin staining and ultrastructural changes were monitored by transmission electron microscopy.

The results showed that administration of HMo, LCu, and HMoLCu not only decreased sperm density and motility but also increased the rate of teratosperm occurrence.

A significant increase in MDA content and a decrease in SOD, GSH-Px, and T-AOC contents were observed in LCu, HMo, and HMoLCu groups.

Testicular tissues and cells of mice were damaged by HMo and the damages were more serious in the case of Cu deficiency.

Exposure to HMo adversely affected the reproductive system of male mice, and dietary LCu plays key roles in HMo-induced reproductive toxicity.

Wang, H. W., Zhou, B. H., Zhang, S., Guo, H. W., Zhang, J. L., Zhao, J., and Tian, E. J.,Reproductive toxicity in male mice after exposure to high molybdenum and low copper concentrations, Toxicology and industrial health, 2016, 32, 1598-606.


Effect of Molybdenum Nanoparticles on Blood Cells, Liver Enzymes, and Sexual Hormones in Male Rats

Despite an increasing surge in application of nanoparticles in industries, there is a serious lack of information concerning their impact on human health and the environment. The present study investigated effects of molybdenum nanoparticles (Mo NPs) injected intraperitoneally into Sprague-Dawley rats at different doses of Mo NPs (5, 10, and 15 mg/kg BW per day) during a period of 28 days. Hematological and biochemical parameters as well as sexual hormones and histopathological examinations of the liver and testis were assessed and compared with control group.

The results showed that the serum levels of testosterone decreased significantly in both groups of 10 and 15 mg (Mo NPs)/kg BW in comparison with the control group (p < 0.05). However, there were insignificant differences observed in luteinizing hormone (LH) levels and hematological parameters when compared with the control group (p > 0.05). The results of liver enzymes showed that serum levels of aspartate aminotransferase (AST) decreased significantly in both dosage groups of 5 and 10 mg/kg BW (Mo NPs) when compared with the control group (p < 0.05), and significant decrease obtained in lactate dehydrogenase (LDH) levels at dose of 5 mg/kg BW in comparison with the control group (p < 0.05). The histopathological examination of testis showed a decrease in number of Leydig cells. Also, the number of chronic inflammatory cells increased in portal triad and parenchyma in liver tissue of rats exposed to Mo NPs.

Asadi, F., Mohseni, M., Dadashi Noshahr, K., Soleymani, F. H., Jalilvand, A., and Heidari, A.,Effect of Molybdenum Nanoparticles on Blood Cells, Liver Enzymes, and Sexual Hormones in Male Rats, Biological trace element research, 2016.. doi:10.1007/s12011-016-0765-5

Acute Toxicities of Molybdenum Compounds towards Animals

Acute Toxicities of Molybdenum Compounds towards Animalsa,b
LD50/mg/l (mg/kg)
MoO3c guinea pig LD75 400   [1,2]
  rat LD50190   [1,2]
MoO3(pure) rat LD50 273   [3]
    LD50 296-352   [4]
MoO3 (technical) rat LD50 666   [5]
Ammonium molybdated guinea pig LD 2200 LD100 800 [1,2]
  rat   MLD 203 [1,2]
  rabbit LD 1870   [1,2]
  cat LD1600-3200   [1,2]
Sodium molybdate rat MLD 290   [1,2]
    LD 671    
    LD50 503    
  dog   LD 3200  
  cat   LD1600-3200  
      LD50 344 [6]
Molybdate aq solutione chicken   LD100517+/-83 [7]
  dog   LD1001372+/-441  
  pigeon   LD100 607+/-119  
  rat   LD100 413+/-106  
Mo(VI) rat   MLD 223 [8]
Mo(V) + glucose rat   MLD 132 [8]
Mo(V ) + ascorbic acid rat   MLD 1999 [8]
MoS2 rat   LD50 > 15 000 [9]

Notes and references

aThe definition of toxicity is according to the U.S. Federal Hazardous Substances Labelling Act. The toxicity of the molybdenum compounds listed as toxic is "slight" (e.g., compare sodium arsenate, MLD10 for the rat by intraperitoneal injections). Abbreviations: LD, lethal dose for one animal; LDn, dose killing n% of a group of test animals; MLD, minimum lethal dose, i.e., the smallest of a number of doses which killed one or a group of test animals, all in mg/kg body weight.

b In tests of toxicity by skin absorption, skin irritation, and eye irritation (Test carried out for Climax Molybdenum Co. by Scientific Associates and New Drug Institute) the compounds listed were nontoxic except that ammonium and calcium molybdates were slight eye irritants.

c Some reports of high toxicity of MoO3 are based on the results of long term feeding studies wrongly presented as acute single dose studies

(e.g. LD50 125 mg/kg from L. Fairhall, R. Dunn, N. Sharpless, E. Pritchard, The toxicity of molybdenum, US Public Health Bulletin 293, 1945) (G.G. van Riper and J.C. Gilliland , ULLMANN’S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY, 1990, A16, Ch. 12 )

d The description "ammonium molybdate" includes the compounds (NH4)2MoO4, (NH4)2Mo2O7 and (NH4)6Mo7O24.4H2O. There is unlikely to be any difference in the behaviour of the various molybdates.

e After a 60 min intravenous infusion.

[1] Saunders, W. B., Handbook of Toxicology, 1956, London.
[2] Fairhall, L. T., Dunn, R. C., Sharpless, N. E. and E. A. Pritchard, The Toxicity of Molybdenum, U.S. Public Health Service, Public Health Bulletin, 1945, 293
[3] Acute oral LD50 assay in rats FDRL-ID:81-0393 for AMAX Inc. Aug. 1981
[4] Food and Drug Research Laboratories Inc. Acute oral LD50 assay in rats for molybdenum trioxide pure grade, 1981, Waverly New York.
[5] Acute oral LD50 assay in rats FDRL-ID:81-0394 for AMAX Inc. Aug. 1981
[6] Irving Sax, N., Dangerous Properties of Industrial Materials, 1984, 6th Ed. Van Nostrand Reinhold Co. New York, 1953.
[7] Caujolle, F. and Changh, P. H., Agressologie, 1967, 8, 265 (Quoted in Lener, J. and Bibr, B., J. Hygiene, Epidemiology, Microbiology and Immunology, 1984, 28, 405).
[8] Lener, J. and Bibr, B., Proc. Int. Conf.: Heavy Metals in the Environment. Amsterdam 1981, 462. (Quoted in Lener, J. and Bibr, B., J. Hygiene, Epidemiology, Microbiology and Immunology, 1984, 28, 405).
[9] Acute oral LD50 assay in rats of MoS2 FDRL-ID 9589A for AMAX Inc. Nov 29 1987.

Inhalation of dusts of molybdenum trioxide

Inhalation of dusts of molybdenum trioxide (Table below) and the more soluble molybdates produced toxic effects. [Fairhall, 1945] The toxic effect of molybdenum trioxide dust was aggravated by silica [Mogilevskaya, 1961]. Precautions should be taken against the inhalation of dusts of molybdenum trioxide and the more soluble molybdates; maximum permissible concentrations in air recommended by the American Industrial Hygiene Association are 5 mg Mo/m3 (cf. Be, 0.002; V, 0.5; arsine, 0.3) over an eight-hour period. The World Health Organisation recommended in 1969 a "safe concentration zone" of molybdenum as 4-5 mg/m3.

Fairhall, L. T., Dunn, R. C., Sharpless, N. E. and Pritchard, E. A., The Toxicity of Molybdenum, U. S. Public Health Service, Public Health Bulletin, 1945, 293.

Mogilevskaya, O. Ya., Gig. Sanit., 1961, 26, 18.

Effects of MoO3 inhalation on mice and rats compared with controls
ObservableMice, maleMice, femaleRats, maleRats, female
Survival 0 0 0 0
Mean body weights 0 0 0 0
Blood Mo (b) + + + +
Mo toxicity symptoms (c) 0 0 0 0
Bone density or curvature 0 0 0 0
Nasal and larynx lesions + + + +
Chronic inflammation in lungs 0 0 +(d) +(d)
Lung carcinoma + + 0?(e) 0 (e)

(a) Groups of 50 male and 50 femaleF344/N rats and B6C3F1 mice were exposed to MoO3 by inhalation at 0, 10, 30, or 100 mg/m3, 6 h/day, 5 days/week, for 2 years. Controls were not exposed. Plus (+) means a positive effect compared with the controls which increases with the level of exposure. Zero (0) means no effect.
(b) Mo concentrations (mg/g): male mice <0.10 (no MoO3) to 0.77 (highest MoO3), female mice 0.04 to 0.52, male rats 0.22 to 6.0, female rats,0.06 to 2.4
(c) e.g. diarrhoea, alopecia, dermatosis, anaemia.
(d) At highest exposures.
(e) For 1/50 animals.

Chan, P.C., Herbert, R.A., Roycroft, J.H., Haseman, J.K., Grumbein, S.L., Miller, R.A., Chou, B.J., Lung tumor induction by inhalation exposure to molybdenum trioxide in rats and mice, Toxicological Sciences, 1998, 45, 58-65.

Molybdenum trioxide inhalation studies in male rats

The following is a detailed summary of a recent study by K. Ozaki et al.

Correlations between non-neoplastic chronic pulmonary lesions and adrenal pheochromocytoma in 9 recent NTP, 2 year particulate inhalation studies in male F344 rats have been investigated. The re-evaluation of other lesions revealed significant associations of pheochromocytoma only with the severity of inflammation and fibrosis. The particulates investigated were all metal compounds and included molybdenum trioxide.

Pheochromocytomas are benign or malignant tumours which give a yellow-brown colour when stained with chromates. These tumours arise as a result of the increased production of catecholamines, such as dopamine, epinephrine and norepinephrine (adrenaline and noradrenaline) in the adrenal medulla in response to a stress which can be an environmental insult. (Catecholamines are synthesised in chromaffin cells.)

Generally exogenous agents that induce adrenal medullary neoplasia do not cause DNA damage so a carcinogenic response is due to an indirect mechanism.

Mammalian tissues are primarily aerobic and hence dependent upon a continuous supply of oxygen. Inadequate O2 delivery results in hypoxemia. Reduction in arterial O2 tension leads to the release of dopamine and other catecholamines. Activity of tyrosine hydroxlyase, the rate limiting step in the biosynthesis of catecholamines is enhanced in the carotid body and the adrenal gland during hypoxemia.

The hypothesis is that the area of lung tissue damaged by the inhalation of certain particulates is correlated positively with the degree of hypoxemia which leads to the pathological increase in tyrosine hydroxylase activity and the production of catecholamines.

The results of the studies relating to MoO3 are given in the following Table.

Results from the re-evaluation of 9 inhalation studies of the particulate compound MoO3 as reported by NTP.

MoO3 inhalation study
0 mg m-3
10 mg m-3
30 mg m-3
100 mg m-3
Proteinosis 0/50 0/50 0/50 0/50
Fibrosis, interstitium 5/50 2.0 4/50 1.0 7/50 1.0 45/50 1.8
Inflammation, chronic active 10/50 1.8 7/50 1.6 37/50 1.2 48/50 2.1
Alveolar epithelium hyperplasia 12/50 1.8 14/50 1.6 16/50 1.2 7/50 1.3
Metaplasia, squamous epithelium 2/50 1.0 0/50 2/50 1.0 27/50 1.1
HistiocytosisIncrease in cells associatedwith inflammation 11/50 1.9 13/50 1.3 35/50 1.3 46/50 1.7
Alveolar/bronchial adenoma 0/50 0/50 0/50 3/50
Alveolar/bronchial carcinoma 0/50 1/50 1/50 1/50
Adrenal Medulla        
PheochromocytomaBenign + malignant 15/50 13/50 18/50 18/50
Hyperplasia 32/50 2.1 27/50 2.1 28/50 2.1 29/50 2.5

Second number in each cell indicate the mean grade of severity

0 no involvement
1 min. up to 5 small lesions 10% of area
2 mild 10 - 25 % of area
3 moderate 26 - 50% of area
4 marked 51 - 75% of area
5 severe > 75% of area.

Terms used in the Table:

Proteinosis: Aggregates of homogeneous to granular material which stains with eosin within alveolar lumina.
Fibrosis: Dense fibrous tissue - thickening and scaring of connective tissue. This is considered secondary to inflammation.
Hyperplasia: Enlargement of an organ or tissue arising from the increased production of cells. In the case of the adrenal medulla in this study this is related to the increase in the production of catecholamines.
Metaplasia: Abnormal change in tissue.
Histiocytosis: Accumulation of alveolar macrophages with foamy cytoplasm and occasional multinucleated giant cells but few neutrophils. (Macrophages and neutrophils are associated with inflammation).
Adenoma: A gland-like benign tumour.
Carcinoma: A cancer.


General: In the controls inflammation with a smaller contribution from fibrosis was associated significantly (p<0.01) with the occurrence of pheochromocytomas.

None of the other non-neoplastic lung lesions were significantly correlated with the incidence of adrenal pheochromocytomas.

Molybdenum trioxide showed no dose related increase in pheochromocytoma incidence but the correlation between the incidence of pheochromocytoma and the severities of fibrosis and inflammation was highly significant (p < 0.01) and the strongest correlation was with inflammation.

Molybdenum trioxide showed one of the strongest correlations with inflammation of most test compounds. However, it was not understood why the increase in the incidence of pheochromocytomas did not follow the treatment-related increase in the severity of inflammation and fibrosis.

Pheochromocytomas can also be related to aging or in response to a wide variety of xenobiotic agents BUT these lesions are rare in humans and other animal species. They occur more frequently in male than female rats.

Ozaki, K., Haseman, J. K., Hailey, J. R., Maronpot, R. R., and Nyska, A., Association of adrenal pheochromocytoma and lung pathology in inhalation studies with particulate compounds in the male F344 rat - The national toxicology program experience, Toxicologic Pathology, 2002, 30, 263-270.

International Molybdenum Association toxicology testing programme

Toxicology Testing Programme: Rats
SubstanceLD50 or LC50/ mg/l
Acute oralaAcute inhalationbAcute dermalc
LD50 ratsLC50 ratsLD50 rats
Molybdenum trioxide (pure) 3260 > 5.84 > 2000
Molybdenum trioxide (tech) > 5000 > 3.93 > 2000
Ammonium dimolybdate 3883 > 2.08 > 2000
Sodium molybdate 4233 > 1.93 > 2000
Molybdenum disulfide >2000 > 2.82 > 2000
Risk phrase required if:d <2000 < 5 < 2000


a Acute oral is representative of relative toxicity by ingestion. Various doses were administered to rats (10/dose) by infusion into the stomach. Animals were observed for 14 days. For pure molybdenum trioxide there was a statistically significant difference between the oral LD50 in male and female rats: (male, 2689; female, 3830 mg/kg). The value given above is an average and is likely a low estimate.

b Acute inhalation is representative of relative toxicity by inhalation. Dust administered to rats (10) for 4 hours. Animals observed for 14 days.

cAcute dermal is representative of relative toxicity due to skin absorption. Various concentrations adminstered topically to rats (10). Animals observed for 14 days.

Skin irritation: Material applied to rabbit (6) skin for 4 hours. Animals observed for erythema and oedema daily for 4 days.

Eye irritation: Material applied to rabbit (6) eyes (100 mg). Animals observed for 7 days.

Skin sensitization: Guinea pigs exposed to material intradermally and topically twice (induction and challenge).

d All acute oral and dermal results were well above the level that requires a harmful classification and precautionary labelling. In the case of inhalation, in some cases it was not possible to generate a dust cloud of 5 mg/l. However, even at the highest concentrations generated there were no animal deaths and there is no basis to assume that labelling of these materials is indicated.

Toxicological studies on MoS2

Acute inhalation study in rats

The inhalation hazard associated with acute exposure to MoS2is low.
The LC50 (4 hour) of MoS2is > 2.82 mg/l in air.
Labelling of MoS2with the risk phrase R20 Harmful by inhalation is not indicated.

MoS2was administered as a particulate aerosol at a concentration of 2.82 mg/l of air for 4 h. The rats were observed for 14 d post exposure.

There were no deaths. No clinical signs and no changes of bodyweight compared with controls. Food and water consumption was similar to that of controls.

The ratio lung/body weight was similar to the ratio found for controls.
A grey appearance of the lungs was noted for all rats post mortem.

Acute oral toxicity to the rat

The acute lethal oral dose to rats of molybdenum disulphide was demonstrated to be greater than 2000 mg/kg bodyweight.

Molybdenum disulphide will not require labelling with the risk phrase R22, "Harmful if swallowed", in accordance with Commission Directive 93/2 I/EEC.

A group of ten fasted rats (five males and five females) received a single oral gavage dose of MoS2formulated in 1% w/v aqueous methylcellulose and administered at a dose level of 2000 mg/kg bodyweight.

There were no deaths. Clinical signs of reaction to treatment in the main study, comprised piloerection, hunched posture and ungroomed appearance observed in all rats. There were no other clinical signs and recovery was complete in all instances by Day 4.

All rats were considered to have achieved satisfactory bodyweight gains throughout the study.
All animals were killed and examined macroscopically on Day 15, the end of the observation period. This examination revealed no abnormalities.

Eye irritation to rabbits

Molybdenum disulfide is not irritating to the eyes.
100 mg of MoS2was instilled into the eyes of three rabbits. The animals were observed for three days. The MoS2caused very slight conjunctaval irritation.

Acute dermal toxicity to the rat

Molybdenum disulfide will not require labelling with the risk phrase R21 (EU Commission Directive 93/21/EEC: ‘Harmful in contact with skin.’

The acute lethal dermal dose to rats of MoS2> 2000 mg/kg bodyweight.
Ten rats received a topical application of MoS2in 1% w/v aqueous methylcelulose, 2000mg/kg bodyweight.
There was no systemic response to MoS2.
Four rats showed some dermal irritation which had disappeared 4 d after exposue was stopped.

Skin sensitisation in the guinea-pig

Molybdenum disulphide did not produce evidence of skin sensitisation (delayed contact hypersensitivity) in any of the ten test animals.

Molybdenum disulphide does not require labelling with the risk phrase R43 "May cause sensitisation by skin contact in accordance with Commission Directive 93/21/EEC.

The following dose levels were selected:
Intradermal injection:
10% w/v in Alembicol D
Topical application:
70% w/v in Alembicol D
Challenge application:
70 and 35% w1/v in Alembicol D
Ten test and five control guinea-pigs were used in this study.

Skin irritation to the rabbit

No dermal reactions were observed following a single, semi-occlusive application of molybdenurn disulphide to intact rabbit skin for four hours.

Molybdenum disulphide will not require labelling with the risk phrase R38 ‘Irritating to skin', in accordance with Commission Directive 93/21 /EEC.

Three rabbits were each administered a single dermal dose of 0.5 g of MoS2and observed for four days.

Huntingdon Life Sciences Ltd Research Laboratory March 1998

Molybdenum metal and metal powder

As with other fine powders inhalation of quantities of molybdenum dust can be dangerous. Safety precautions have been described [Lamprey and Ripley, 1962]. Molybdenum metal as such and in alloys is not toxic and has been incorporated in stainless steels used in bone and joint surgery where resistance to corrosion by body fluids is required [Bechtol et al., 1959]. Molybdenum alloyed with cobalt and chromium has been used for some 40 years for surgical implants, dental prostheses etc. As with the stainless steel, molybdenum confers corrosion resistance.

Lamprey H. and Ripley, R. L., J. of the Electrochemical Soc., 1962, 109, 713.
Bechtol, C. O., Ferguson, B. and Laing, P. G., Metals and Engineering in Bone and Joint Surgery, The Williams and Wilkins Co., Baltimore, 1959.

Workers handling metals in welding and steelmaking are expected to have higher levels of trace metals. Interpretation of levels requires some knowledge of the toxicokinetics of a metal and the preferred medium for analysis for each: serum, whole blood or urine (preferably a 24-hour collection). Trends are often more informative than concentrations at one time. Molybdenum and vanadium are often found to be elevated among workers exposed to metals who show no evidence of clinical illness. The Mo level in the general population not occupationally exposed to Mo (the whole blood Mo reference range) is 5 - 50 nmol/l as used by the Trace Element and Environmental Toxicology Laboratory at the University of Alberta Hospitals. Mo is described as having 'very low toxicity' [Guidotti et al., 1997.]

Guidotti, T.L., Audette, R.J., Martin, C.J., Interpretation of the trace metal analysis profile for patients occupationally exposed to metals, Occupational Medicine-Oxford, 1997, 47 , 497-503.

Molybdenum carbide and silicide

Aqueous suspensions of MoC and MoSi2 administered intraperitoneally to mice in doses of 3000 mg/kg did not cause death [Chem. Abs., 1969]. Intratracheal administration of suspensions of these compounds to rats in doses of 50 mg/animal had pathological effects derived from the fibrogenic nature of the compounds. Maximum permissible concentrations in air are recommended as 6 mg Mo/m3

Chem. Abs., 1969, 71, 68977v.
Brakhnova, I. T. and Samsonov, G. V., Gig. Sanit., 1970, 35, 42.

Molybdenum pentachloride

Molybdenum pentachloride has a marked irritant effect on the tissues of rats [Spiridonova and Surorov, 1967]. The toxicity is due to hydrochloric acid produced during hydrolysis of the compound and not to molybdenum. Molybdenum pentachloride is a hazardous chemical and, if proper precautions are not taken, its industrial use involves danger of acute intoxication. There were no health problems in the production of MoCl5 at the former Climax Langeloth refinery and the Ann Arbor Laboratory.

Spiridonova, V. S. and Suvorov, S. V., Gig. Sanit., 1967, 32, 79.

Molybdenum pentachloride should be considered as a non-immunotoxic and a weak, nonspecific contact irritant. Immunotoxicity was studied in sheep by a 14 day subchronic exposure to MoCl5 at 1- 100 ppm in food. Contact hypersensitivity was determined by topical ear exposure to MoCl5 [Abdouh et al., 1995].

Abdouh, M., Krzystyniak, K., Flipo, D., Therien, H.M., Fournier, M., Cytometric Profile Of Molybdenum-Induced Contact Sensitization Versus A Strong Allergen Reaction To Oxazolone In Murine Auricular Lymph-Node (Aln) Test, International Journal Of Immunopharmacology, 1995, 17, 545-554.

Molybdenum hexacarbonyl and organomolybdenum compounds

Molybdenum hexacarbonyl is a solid which may be handled in the laboratory without special precautions. It is less toxic than the more volatile iron and nickel carbonyls. However, little is known about the toxic effects of molybdenum hexacarbonyl and organomolybdenum compounds [Vignoli and Defretin, 1963] and it is wise to assume that they are likely to be more toxic than MoO3 and molybdates and they should be handled in such a way that there is no danger of absorption through the skin or inhalation of their dusts and vapours.

Vignoli, L. and Defretin, J. P., Biologie medicale, 1963, 52, 319.

Volatile Mo(CO)6 and W(CO)6 are present in landfill gas from three different municipal waste deposits in concentrations of about 0.2-0.3 microg of Mo/m3 and 0.005-0.01 microg of W/m3. The chemical reactions which give rise to these carbonyls under landfill site conditions are not known. Because volatile metal carbonyls are toxic, questions about the occupational health of workers on landfills needs to be addressed [Feldmann and Cullen, 1997].

Feldmann, J., Cullen, W.R..,Occurrence of volatile transition metal compounds in landfill gas: Synthesis of molybdenum and Tungsten carbonyls in the environment, Environmental Science & Technology, 1997, 31, 2125-2129.

Comparative effects of molybdenum and other elements

In comparative toxicity studies with rats and mice sodium molybdate (LD50 1.45 m mol/kg body weight) was less toxic than sodium tungstate, sodium chromate and sodium vanadate [Pham Huu Chanh, 1965]. The irritant effect of sodium molybdate on skin, mucous membranes, and eyes was much smaller than that of sodium dichromate (an extreme irritant) [Climax Molybdenum Test].

Pham-Huu-Chanh, Arch. Intern. Pharmacodyn., 1965, 154, 243.

The ecotoxicological effects expressed as effect on respiratory rate and chlorophyll a content of the freshwater, algaScenedesmus quadricauda and the bioaccumulation of six ions (Cu2+, Cu+, MoO42-, Mn2+, VO43-, Ni2+) and their associations were determined in comparison with a control [Fargasova, 1998]. The ion concentrations were equal to their EC50 concentrations: for Mo, ammonium heptamolybdate at 2.5 mg l-1. The pH’s of the solutions were not reported. Molybdate had no effect on the respiratory rate; the other ions caused the rate to increase. Combination of moybdate with nickel and copper decreased the rate (but not with manganese). Molybdate and the other ions caused the content of chlorophyll a to decrease. Bioaccumulation of molybdate was negligible MoO42- (< 0.2 %). Molybdate greatly decreased the uptake of Mn, Ni and Cu although these ions had no effect on the uptake of Mo.

Fargasova, A, Accumulation and toxic effects of Cu2+, Cu+, MoO42-, Mn2+, VO43-, Ni2+ and their associations: influence on respiratory rate and chlorophyll (a) content of the green alga Scenedesmus quadricauda, Journal Of Trace And Microprobe Techniques, 1998, 16, 481-490.

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