Once absorbed into the blood, nickel is predominantly extracted by the kidneys and excreted in urine. Urinary excretion of nickel is thought to follow a first-order kinetic reaction (Christensen and Lagesson, 1981).
Urinary half-times in workers exposed to nickel via inhalation have been reported to vary from 17 to 39 hours in nickel platers who were largely exposed to soluble nickel (Tossavainen et al., 1980).
Relatively short urinary half-times of 30 to 53 hours have also been reported in glass workers and welders exposed to relatively insoluble nickel (Raithel et al., 1982; cited in IARC, 1990; Zober et al., 1984). It should be noted, however, that in these cases the insoluble nickel that workers were exposed to – probably NiO or complex oxides – was likely in the form of welding fumes or fine particles (Zober et al., 1984; Raithel et al., 1981). Such particles may be absorbed more readily than large particles. Difference in particle size may account for why other researchers have estimated much longer biological half-times of months to years for exposures to presumably relatively insoluble nickel compounds of larger particle size (Torjussen and Andersen, 1979: Boysen et al., 1984; Åkesson and Skerfving, 1985). The precise role that particle size or dose may play in the absorption and excretion of insoluble nickel compounds in humans is still uncertain (Sunderman et al., 1986). Reported urinary excretion half-times following oral exposures are similar to those reported for inhalation (Christensen and Lagesson, 1981; Sunderman et al., 1989). Christensen and Lagesson (1981) reported that maximal excretion of nickel in urine occurred within the first 8 hours of ingesting soluble nickel compounds. The highest daily maximum renal excretion reported by the authors was 0.5 mg Ni/day.
Excretion via other routes is somewhat dependent on the form of the nickel compound absorbed and the route of exposure. Unabsorbed dietary nickel is lost in feces. Insoluble particles cleared from the lung via mucociliary action and deposited in the gastrointestinal tract are also excreted in the feces.
Sweat constitutes another elimination route of nickel from the body; nickel concentrations in sweat have been reported to be 10 to 20 times higher than concentrations in urine (Cohn and Emmett, 1978; Christensen et al., 1979). Sunderman et al. (1986) state that profuse sweating may account for the elimination of a significant amount of nickel.
Bile has been shown to be an elimination route in laboratory animals, but its importance as an excretory route in humans is unknown.
Hair is also an excretory tissue of nickel. However, use of hair as an internal exposure index has not gained wide acceptance due to problems associated with external surface contamination and non-standardized cleaning methods (IPCS, 1991).
Nickel may also be excreted in human breast milk leading to dietary exposure of breast-fed infants. On a body weight basis, such exposures are believed to be similar to average adult dietary nickel intake (Grandjean, 1984).