tetrachloroethylene), on mixtures of specific organic solvents, or on mixtures of unspecified solvents. In many studies, exposure to solvents was not assessed specifically; rather, surrogates of exposure were used, such as job title, industry type, or occupation.

As is discussed in Chapter 2, a study’s ability to determine exposure accurately and specifically is critical in evaluating its overall quality. For the purposes of this report, the committee used studies that assessed exposure to specific organic solvents or to solvent mixtures as the primary evidence for its conclusions. The committee also included surrogates of exposure in drawing its conclusions, but it viewed those studies as supportive or supplemental evidence. Those studies included exposure of painters, printers, aircraft maintenance and manufacturing workers, service-station attendants, and shoe and boot manufacturers. All those studies are included in the data-analysis tables that accompany discussions of each cancer outcome.

The committee found most of the cancer literature focused on the following compounds: benzene, trichloroethylene, tetrachloroethylene, methylene chloride, and mixtures of unspecified organic solvents. Therefore, most of the committee’s conclusions on cancer outcomes are focused on exposure to those compounds. A smaller number of studies analyzed associations between cancer outcomes and toluene, xylene, isopropyl alcohol, methyl ethyl ketone, phenol, and other individual solvents, but for most agents, there was insufficient evidence for the committee to draw conclusions.

For exposure to tetrachloroethylene, the committee included studies of dry-cleaning and laundry workers as part of the primary evidence in drawing conclusions of associations. As a result, the conclusions related to exposure to tetrachloroethylene are also related to exposure to “dry-cleaning solvents.” The committee acknowledges that dry cleaners and launderers are likely exposed to other organic solvents and chemicals, including naphtha, Stoddard solvent, carbon tetrachloride, trichloroethylene, and 1,1,1-trichloroethane (IARC, 1995). As a result, the committee decided to consider studies of both tetrachloroethylene and dry-cleaning solvents in forming their conclusions, thereby including the possibility that one of the other solvents used in that industry contributed to the risks observed in some of the studies on dry cleaners and launderers.

The committee based its review of cancer outcomes only on studies of humans that had a comparison or control group (cohort and case-control studies). Case reports, case series, review articles, and meta-analyses related to cancer were excluded from the committee’s review. Although the committee reviewed ecologic, cross-sectional, proportionate mortality ratio (PMR), and mortality odds ratio studies, it did not consider them critical to its decision and excluded them from the discussions. Chapter 2 describes their specific limitations.

Toxicity and Determination of Carcinogenicity

Excess incidence of cancer has been observed in animals exposed to the specific organic solvents reviewed by the committee. Benzene, perhaps the most thoroughly investigated solvent, is a well-established carcinogen and has repeatedly been shown to induce hematopoietic cancers and cancers of the ovaries, mammary glands, pancreas, and liver (ATSDR, 1997a). The International Agency for Research on Cancer (IARC) has determined that benzene is “carcinogenic to humans” as determined in studies of both humans and animals. IARC bases its determination of benzene’s carcinogenicity on evidence from human studies that is considered “sufficient,” whereas the available animal

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