revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 389
5. POTENTIAL FOR HUMAN EXPOSURE
rings, as well as in soil and sediment cores, suggest that a four- to five-fold increase in mercury levels in
air has occurred since the beginning of the industrial revolution.
A degree of uncertainty exists with respect to estimates of the relative contributions of natural and
anthropogenic sources of mercury emissions to the environment reported in the scientific literature.
Nriagu and Pacnya (1988) estimated anthropogenic emissions to be more than half of the total global
emissions of 6,000 tons/year. Nriagu (1989) estimated mercury emissions from natural sources to be 2,500
tons/year. In contrast, WHO (1990, 1991) reported that the major source of atmospheric mercury is global
degassing of mineral mercury from the lithosphere and hydrosphere at an estimated rate of 2,700–6,000
metric tons/year, which is approximately 1.3 to 3 times the rate of release from anthropogenic sources.
Lindqvist (1991b) estimated world anthropogenic emissions at 4,500 tons with an additional 3,000 tons
attributed to natural sources. Most recently, Pirrone et al. (1996) estimated world emissions of mercury at
2,200 metric tons/year and concluded that natural sources, industrial sources, and the recycling of
anthropogenic mercury each contribute about one-third of the current mercury burden in the global
atmosphere. A major source of the uncertainty is that emissions from terrestrial and marine systems
include a “recycled” anthropogenic source component (WHO 1990).
Recent estimates of anthropogenic releases of mercury to the atmosphere range from 2,000–4,500 metric
tons/year, mostly from the mining and smelting of mercury and other metal sulfide ores. An estimated
10,000 metric tons of mercury are mined each year, although there is considerable year-to-year variation
(WHO 1990). Other anthropogenic sources include: industrial processes involving the use of mercury,
including chloralkali manufacturing facilities; combustion of fossil fuels, primarily coal; production of
cement; and medical and municipal waste incineration and commercial/ industrial boilers (Bache et al.
1991; EPA 1987f, 1996b; Lindberg 1984; Lindqvist 1991b; Nriagu and Pacyna 1988; WHO 1990, 1991).
Stein et al. (1996) estimated that approximately 80% of the anthropogenic sources of mercury are
emissions of elemental mercury to the air, primarily from fossil fuel combustion, mining, smelting, and
from solid waste incineration. Another 15% of the anthropogenic emissions occur via direct application of
fertilizers and fungicides and municipal solid waste (e.g., batteries and thermometers) to the land.
Recently, Carpi et al. (1998) studied the contamination of sludge-amended soil with inorganic and methylmercury
and the subsequent emission of this mercury contamination into the atmosphere. These authors
reported the routine application of municipal sewage sludge to crop land significantly increased the
concentration of both total mercury and methylmercury in surface soil from 80 to 6,1000 µ/kg (ppb) and
0.3 to 8.3 µ/kg (ppb), respectively. Both inorganic and methylmercury were transported from the