Human exposure to borates may occur through ingestion of food and water or insecticides used to control cockroaches, inhalation of boron-containing powders or dusts, or absorption of boron from cosmetics or medical preparations through mucous membranes or damaged skin. The most appreciable boron exposure to the general population is likely to be ingestion of food and to a lesser extent in water. Estimates of average daily boron ingestion by humans range from 10 to 25 mg (Beyer et al. 1983; Waggott 1969).
Occupational exposure to boron compounds may be higher. Workers in industries producing or using boron or boron compounds may be exposed by inhalation to boron-containing dusts or gaseous boron compounds due to process upsets or faulty equipment. Dermal absorption of boron may also occur if damaged skin is in contact with these materials, but this is considered a minor pathway (Stokinger 1981).
Borate dusts have been monitored in workplace air. Reported concentrations of borax dust in different areas of a large borax mining and refining plant ranged from 1.1 to 14.6 mg/m (Garabrant et al. 1985) and the mean boric acid/boron oxide dust concentration in a boric acid manufacturing plant was 4.1 mg/m (Garabrant et al. 1984). These values indicate that permissible exposure limits (PELs) set by OSHA, or threshold limit values (TLVs) recommended by the ACGIH, for boron-containing dusts in workplace air (Table 7-1) may, at times, be exceeded. Other industries include manufacture of fiberglass and other glass products, cleaning and laundry products, fertilizers, pesticides, and cosmetics (U.S. Borax and Chemical Corporation 1991; Stokinger 1981). Median normal values of boron in human blood (9.76 mg/100 g) and urine samples from these workers (720 mg boron/L) were reported (Stokinger 1981). Boron was not detected in a national survey of human adipose tissue (Stanley 1986). The National Institute for Occupational Safety and Health (NIOSH) estimated that the number of workers potentially exposed to boron increased from 6,500 in the early 1970s (NOHS 1989) to 35,600 in the early 1980s (NOES) 1989). Neither the NOHS nor the NOES databases contain information on the frequency, concentration, or duration of exposures of workers to any of the chemicals listed therein. These surveys provide only estimates of the number of workers potentially exposed to chemicals in the workplace. Sittig (1985) reports that NIOSH estimated the number of workers potentially exposed to borax at 2,490,000, to boron oxide at 21,000, and to boron trifluoride at 50,000.
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