The major routes of nickel intake are dietary ingestion and inhalation. In most individuals, even some who are occupationally exposed, diet constitutes the main source of nickel intake. The average daily dietary nickel intake for U.S. diets is 69-162 µg Ni/day (NAS 2002; O’Rourke et al., 1999; Pennington and Jones 1987; Thomas et al., 1999). These values agree with those from European studies. However, consumption of foodstuffs naturally high in nickel, such as oatmeal, cocoa, chocolate, nuts, and soy products, may result in higher nickel intake (Nielsen and Flyvolm, 1984; Grandjean et al., 1989).
Nickel in potable water also is generally quite low, averaging from <0.001 to <0.010 mg Ni/L (Grandjean et al., 1989). Assuming an intake of 2 L/day, either as drinking water or water used in beverages, nickel in water may add 0.002 to 0.02 mg Ni to total daily intake.
For individuals who are not occupationally exposed to nickel, nickel intake via inhalation is considerably less than dietary intake. The Ni concentration of particulate matter in the atmosphere of the United States ranges from 0.01 to 60, 0.6 to 78, and 1 to 328 ng/m3 in remote, rural, and urban areas, respectively (Schroeder et al., 1987). Average ambient air Ni concentrations in U.S. and Canadian cities range from 5 to 50 ng/m3 and 1 to 20 ng/m3, respectively. Nickel concentrations in indoor air are typically <10 ng/m3 (Graney et al., 2004; Kinney et al., 2002; Koutrakis et al., 1992; Van Winkle and Scheff 2001).
Higher nickel air values have been recorded in heavily industrialized areas and larger cities (IPCS, 1991). An average urban dweller (70 kg man breathing 20 m3 of 20 ng Ni/m3/day) would inhale around 0.4 µg Ni/day (Bennett, 1984). For rural dwellers, daily intake of airborne nickel would be even lower.
Ultimately, the general population absorbs the greatest amount of nickel through food. Typical daily intakes of nickel from drinking water and inhalation of air are approximately 20 µg and 0.4 µg, respectively.
For occupationally exposed individuals, total nickel intake is likely to be higher than it is for the general populace. Whether diet or workplace exposures constitute the main source of nickel intake in workers depends upon a number of factors. These factors include the aerodynamic size of the particle and whether it is inhalable, the concentration of the nickel that is inhaled, the minute ventilation rate of a worker, whether breathing is nasal or oronasal, the use of respiratory protection equipment, personal hygiene practices, and general work patterns.
Based upon the exposure estimates presented in Section 3 and assuming that a total of 12 m3 of air is inhaled in an eight-hour work day (the as- sumption being that industrial workers have a higher inhalation rate than the average citizen), the approximate amount of nickel likely to be inhaled in nickel-producing industries would range from 0.036 to 0.72 mg Ni/day. The average amount of nickel likely to be inhaled in most nickel-using industries would range from ~0 to 1.1 mg Ni/day depending upon the industry. Battery production with metallic nickel and metallic nickel powder metallurgy operations are an exception, with average airborne nickel concentrations (based on reports that have been made occasionally) ranging from 0.3 to 0.5 mg Ni/m3, respectively.
Other sources of exposure include contact with nickel-containing items (e.g., jewelry), medical applications (e.g., prostheses), and tobacco smoke. Dermal exposure to nickel-containing articles constitutes one of the most important routes of exposure for the public with respect to allergic contact dermatitis. Likewise, tobacco smoking may also be a source of nickel exposure. Some researchers have suggested that smoking a pack of 20 cigarettes a day may contribute up to 0.004 mg Ni/day (Grandjean, 1984). While this would contribute little to total nickel intake, smoking cigarettes with nickel-contaminated hands can significantly increase the potential for oral nickel exposures.