controls relative to cases. Other study limitations are discussed in the section above on insecticides and PD.
Two studies found an association between past exposure to solvents and PD, but both studies were likely to have been subject to recall bias. It should be noted that little attention has been focused on solvent exposure as a risk factor for the occurrence of PD.
The committee concludes, from its assessment of the epidemiologic literature, that there is inadequate/insufficient evidence to determine whether an association exists between exposure to the solvents under review and Parkinson’s disease.
This section addresses the association between solvent exposure and ALS. Four case-control studies (Table 7.10) were evaluated by the committee (Chio et al., 1991; Gunnarsson et al., 1992; McGuire et al., 1997; Strickland et al., 1996). One was a pilot study of only 25 cases and 50 controls (Strickland et al., 1996). Mortality studies using only death certificates (Hawkes et al., 1989; Neilson et al., 1994) and a study of the occupational distribution of ALS cases in Greece (Kalfakis et al., 1991) were excluded because the committee could not ascertain the nature of the exposure in those studies.
Gunnarsson and colleagues (1992) studied cases and controls in a nine-county region of Sweden. Cases, which were recruited from all departments of neurology and internal medicine, had any one of the three diagnoses subsumed under “motor neuron disease.” A self-administered questionnaire was mailed to study subjects to gather information on current and past occupational, physical, and chemical exposure. Exposure of controls occurring within 5 years before the date of completion of the questionnaire was excluded, as were case exposures occurring within 1 year of symptom onset. Additional potential confounders were contact with animals, physical trauma, use of aluminum utensils, and lack of exercise. Exposure to solvents was reported to be rare in women. For men, none of the occupational solvent-exposure categories (including the umbrella category of “any solvent” exposure) yielded positive associations. A strong association was found for the combination of male sex, any occupational exposure to solvents, and heritability (a family history of neurodegenerative disease or thyroid disease) (OR=15.6, 95% CI=2.8–87.0). Because that combination occurred in seven cases and three controls, the result, if valid, is unlikely to be responsible for a large proportion of cases in the population.