CAS No. 7439-98-7
Elemental molybdenum is a silver-white, hard metal widely used to add strength and hardness and retard corrosion in metal alloys. Compounds of molybdenum are also used as corrosion inhibitors, hydrogenation catalysts, lubricants, chemical reagents in hospital laboratories, and in pigments for ceramics, inks, and paints. More recently, semiconductor and battery industries have begun to use molybdenum. Molybdenum occurs in natural waters and may be present in concentrations of several hundred micrograms per liter or higher in ground and surface water near mining operations or ore deposits.
Molybdenum is a nutritionally essential trace element that enters the body primarily from dietary sources. In humans, molybdenum is a cofactor for three enzyme classes—sulfite oxidase, aldehyde dehydrogenase, and xanthine oxidase (Kisker et al., 1997). The recommended dietary allowance for adult men and women is 45 µg/day (IOM, 2001), and the average dietary daily intake of molybdenum is approximately 100 µg/day (IOM, 2001; WHO, 1996). Gastrointestinal absorption of molybdenum averages 88-93% for dietary intakes of 22-1490 µg/day. Excretion occurs predominantly via the kidneys, which exert homeostatic regulation over molybdenum balance. At a daily oral molybdenum dose of 24 µg, urinary excretion over six days was 18% of the ingested dose; at daily oral doses of 95 µg and 428 µg, urinary excretion over six days rose to 50% and 67%, respectively, of the ingested dose (Turnlund et al., 1995). In industry, dust and other fine particles produced during refining or shaping of molybdenum or molybdenum-containing alloys are inhalational pathways of exposure.
Human health effects from molybdenum at low environmental doses or at biomonitored levels from low environmental exposures are unknown. Molybdenum is generally considered to be of low human toxicity, and clinical or epidemiologic evidence of adverse effects is limited. Chronic exposure to very high levels may result in higher serum uric acid levels and a gout-like illness (Koval'skiy et al., 1961; U.S. EPA, 1993). Based on studies finding adverse reproductive effects in rats and mice, the Panel on Micronutrients of the Institute of Medicine identified a no observed adverse effect level (NOAEL) of 0.9 mg/kg/day and established a tolerable upper intake level of 0.03 mg/kg/day in humans (IOM, 2001). A long term inhalation bioassay of molybdenum trioxide in mice yielded "some evidence" of carcinogenicity (NTP, 1997). One case-control study suggested a possible link between occupational exposure to molybdenum and lung cancer (Droste et al., 1999), but available epidemiologic data are scant, and molybdenum has not been systematically evaluated for carcinogenicity by IARC.
Molybdenum is an essential element for health, and urinary levels reflect intake from all sources. Levels of molybdenum in urine for the U.S. population were well characterized in NHANES since 1999-2000 (CDC, 2012); these levels were comparable to those reported for adults in smaller European population surveys (Iversen et al., 1998; Minoia et al., 2002; White and Sabbioni, 1998). Urinary molybdenum concentrations in infants may be slightly lower than those in other age groups (Sievers et al., 2001).
Finding a measurable amount of molybdenum in the urine does not imply that the level of molybdenum causes an adverse health effect. Biomonitoring studies on levels of molybdenum can provide physicians and public health officials with reference values so that they can determine whether people have been exposed to higher levels of molybdenum than are found in the general population. Biomonitoring data can also help scientists plan and conduct research on exposure and health effects.
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