thors, this observation added support to the hypothesis that exposure to VC outside the industrial setting may be an important factor in the etiology of angiosarcoma of the liver. In addition to the above study, Christine et al. (1974) noted six cases of angiosarcoma of the liver in Connecticut. Of these, one lived within two miles of a plant producing wire coated with polyvinyl chloride while a second lived within 0.5 mile of a plant producing vinyl sheets. None of the individuals was known to have had occupational exposure to VC or arsenic or diagnostic exposure to thorium dioxide. Rosenman et al. (1989) recently reported higher odds ratios for central nervous system birth defects in areas around two VC polymerization facilities in New Jersey.

One review of age-adjusted, sex-, race- and site-specific cancer mortality rates in U.S. counties for three time periods found significantly elevated rates of bladder cancer in males in counties surrounding the Drake Superfund site in Pennsylvania (Budnick et al., 1984). Elevations of birth defects also occurred, but were not statistically significant. This site, in Clinton County, Pennsylvania, included the volatile human carcinogens beta-naphthylamine, benzidine, and benzene dispersed over a 46-acre area. Airborne exposure was suspected to have occurred, given the nature of the chemicals involved. Subsequent investigation determined that males in these counties had between 20 and 30 times the rate of bladder cancer as nonexposed males; their primary employment was in the manufacture of beta-naphthylamine, a potent bladder carcinogen (Marsh et al., 1991). No general evidence of environmental effects was obtained, as Chapter 3 noted.

CHRONIC MORBIDITY STUDIES
LONGITUDINAL ANALYSIS

These studies have been of three types: long-term studies of actively exposed persons, prospective studies of a distinct group, and follow-up studies of exposed children.

Long-term studies of individuals acutely exposed to an initial single dose of a pollutant and then subjected to some chronic exposure have included a follow-up of the population exposed to dioxins at Seveso, Italy (Bertazzi et al., 1989). Follow-up studies of a small group of people acutely exposed to chlorine from an accidental release (Weill et al., 1969) have been published. These studies are required if long-term changes or risks are to be identified. In some instances, full recovery occurs; in others, involving sulfur dioxide, sulfuric acid, oxides of nitrogen, or ammonia, recovery may be very slow and in-



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