Determination of the potential for nickel exposure depends to a large degree on the reliability of analytical data from environmental samples and biological specimens. This is particularly true when trying to differentiate between anthropogenic and natural contributions of nickel to environmental samples. Concentrations of nickel in unpolluted atmospheres and in pristine surface waters are often so low as to be near the limits of current analytical methods. Attention must also be paid to the fact that the amount of nickel identified through analytical techniques is not necessarily equivalent to the amount that is bioavailable (i.e., available for absorption into the body).
Emissions to the atmosphere from the industrial production and use of nickel are approximately 14.5 x 106 kg/year. At the same time, natural emissions from volcanism, dust storms, fires, etc. contribute approximately 8.5 x 106 kg/year. However, natural and industrial emissions combined are substantially less than the emissions from fuel combustion which total approximately 28.6 x 106 kg/year. Eisler (1998) quotes a figure of 16% of the atmospheric nickel burden due to natural sources, and 84% due to anthropogenic sources, which agrees with these figures.
The figure given for emissions of nickel to the atmosphere due to intentional production and use of nickel is approximately 13 x 106 kg Ni/y. There are larger differences in the estimates for the contribution from other anthropogenic sources. These range from 28.6 x 106 kg Ni/y (Bennett, 1984) to a total of 43.4 . 106 kg/year (Niagu, 1989). This difference is however very small compared to the range of estimates for emissions from natural sources which range from 8.5 x 106 kg/year (Bennett, 1984) to 1800 x 106 kg/year (Richardson et al., 2001). The uncertainties in the estimates of nickel emissions from processes not related to intentional nickel production suggest that the relative contribution of nickel emissions associated with intentional nickel production and use may have been overestimated in earlier reviews.
Chemical and physical degradation of rocks and soils, atmospheric deposition of nickel-containing particulates, and discharges of industrial and municipal waste release nickel into ambient waters (US EPA, 1986). The main anthropogenic sources of nickel in water are primary nickel production, metallurgical processes, combustion and incineration of fossil fuels, and chemical and catalyst production (US EPA, 1986). These are the same sources that contribute to emissions to the atmosphere.
The primary anthropogenic source of nickel to soils is disposal of sewage sludge or application of sludge as a fertilizer. Secondary sources include industrial nickel production and use, and emissions from electric power utilities and automobiles. Weathering and erosion of geological materials also release nickel into soils (Eisler, 1998).