association with cutting, cooling, or lubricating oils achieved statistical significance (Table 3). Analyses stratified by smoking status showed that ORs were generally larger among non- smokers than smokers, especially for cutting, cooling, or lubricating oils; antifreeze and cool- ants; and mineral or white spirits (Table 3). In an additional model including all three expo- sures among the nonsmokers, the ORs were 3.9 for cutting, cooling, or lubricating oils; 1.9 for antifreeze and coolants; and 2.9 for mineral or white spirits; none was statistically significant (data not shown). Some interaction with smok- ing was suggested for antifreeze and coolants (p-value for interaction = 0.06) as well as cut- ting, cooling, or lubricating oils (p-value for interaction = 0.10); none of the other workplace exposures had a significant interaction with smoking (all p-values for interaction > 0.05). No exposure except lead (Kamel et al. 2002) showed a clear-cut dose–response with lifetime days of exposure (Table 4). The trend for cutting, cooling, or lubricating oils was also significant (p = 0.04), but the dose–response curve was not monotonic. Other exposures had a similar nonmonotonic trend, with elevated ORs at medium but not high levels of expo- sure duration. We found substantial although imprecise elevations in the risk of ALS for 400- to 1,999-day exposure to cutting, cooling, or lubricating oils and for ≥ 2,000-day exposure to dyes or printing inks (Table 4). Table 5 shows the association between ALS and chemicals inferred from the work- place exposures. Aliphatic chlorinated hydrocarbons, ethylene/propylene glycols, glycol ethers, heptane, and hexane were each associated with a 1.5- to 1.7-fold risk of ALS. Analyses stratified by smoking status showed generally larger ORs among nonsmokers than smokers (Table 5). The interaction of smok- ing with exposure was statistically significant for glycol ethers (p = 0.02) and xylene (p = 0.03) and was borderline significant for ali- phatic chlorinated hydrocarbons (p = 0.06) and ethylene/propylene glycols (p = 0.06). Because workers may have been exposed to several chemicals simultaneously, we exam- ined associations of ALS with each chemi- cal among the nonsmokers while adjusting for the five chemicals associated with ALS in the entire population (aliphatic chlorinated hydrocarbons, ethylene/propylene glycols, glycol ethers, heptane, and hexane; one per each model). Associations between ALS and most chemicals diminished in these analyses except for aliphatic chlorinated hydrocarbons, ethylene/propylene glycols, glycol ethers, and hexane (data not shown). Exposure to formaldehyde was not associ- ated with the risk of ALS overall (OR = 0.8; 95% CI, 0.5–1.5), and the ORs did not vary greatly by exposure probability (0–1, 1, and 2) or weighted exposure duration (≤ 10,000, 10,001–40,000, and > 40,000 hr) (Table 6). An additional analysis was conducted compar- ing individuals who had a weighted exposure to formaldehyde > 60,000 hr (four cases and four controls) to unexposed individuals, giving an OR of 3.0 (95% CI, 0.7–12.9). Workplace exposures and ALS In the interview, the subjects were asked two additional questions: “Did you usually clean your hands with solvents or thinners on any job?” and “Did you ever feel sick or high from an exposure at work?” In total, 78 sub- jects (26 cases and 52 controls) responded positively to the first question, giving an OR of 1.2 (95% CI, 0.7–2.2), and 57 subjects (16 cases and 41 controls) responded posi- tively to the second question, giving an OR of 0.7 (95% CI, 0.3–1.3). Because there may still be residual con- founding by age in models where it was included as a categorical matching variable, we ran additional models further adjusted for age as a continuous variable. Estimates of the rela- tive risks of ALS were substantially unchanged in these models (data not shown). Finally, we repeated all analyses presented above in three separate sets of sensitivity analyses after exclud- ing a) five subjects whose interviews were characterized as questionable; b) seven cases who had reported a family history of ALS; or c) 19 cases who had reported a previous trip to islands in the Western Pacific. These analyses did not give substantially changed conclusions (data not shown). Discussion In the present study, we found that construc- tion workers excluding supervisors and pre- cision metalworkers were at a higher risk of ALS. These findings are consistent with earlier findings that heavy labor (Breland and Currier 1967; Chio 2000; Nelson 1995) and metal Table 3. Self-reported workplace exposures and the risk of ALS, overall and stratified by smoking.a Overall Smokers Nonsmokers No. of No. of No. of No. of No. of No. of controls cases controls cases controls cases Exposure (n = 253) (n = 109) OR (95% CI)b (n = 148) (n =77) OR (95% CI)c (n = 105) (n = 32) OR (95% CI)c Lead 55 35 1.9 (1.1–3.4) 33 25 1.9 (0.9–3.8) 22 10 2.8 (1.0–7.8) Mercury 15 7 1.0 (0.4–2.7) 10 5 — 5 2 — Oil-based paints 38 15 0.7 (0.4–1.4) 26 12 0.7 (0.3–1.6) 12 3 0.7 (0.2–3.1) Paint thinners 43 22 1.1 (0.6–2.0) 28 15 0.8 (0.4–1.8) 15 7 2.0 (0.7–5.9) Paint strippers 19 14 1.7 (0.8–3.6) 11 9 1.5 (0.5–4.0) 8 5 2.5 (0.7–9.0) Varnishes 21 9 0.8 (0.3–2.0) 13 6 — 8 3 — Adhesive 36 23 1.5 (0.8–2.7) 22 15 1.2 (0.5–2.6) 14 8 2.4 (0.9–6.9) Dyes or printing inks 37 21 1.4 (0.8–2.7) 22 13 1.1 (0.5–2.4) 15 8 2.5 (0.9–7.2) Cutting, cooling, or lubricating oils 43 32 1.8 (1.0–3.3) 32 22 1.2 (0.6–2.5) 11 10 6.3 (1.9–20.4) Gas, diesel fuel, motor or fuel oil 56 29 1.1 (0.6–1.9) 37 22 1.0 (0.5–1.9) 19 7 1.5 (0.5–4.3) Antifreeze or coolants 23 17 1.6 (0.8–3.3) 17 10 0.9 (0.3–2.4) 6 7 6.3 (1.6–24.1) Degreasers or cleaning agents 59 28 1.0 (0.6–1.7) 40 20 0.8 (0.4–1.5) 19 8 1.5 (0.5–4.2) Mineral spirits or white spirits 20 13 1.7 (0.8–3.7) 13 7 1.0 (0.4–2.8) 7 6 4.1 (1.2–14.4) Solvents (e.g., toluene or xylene) 28 13 1.0 (0.5–2.1) 20 10 0.9 (0.4–2.2) 8 3 1.6 (0.4–7.1) Dry cleaning agents 4 5 1.9 (0.5–7.8) 3 5 — 1 0 — Anesthetic gases 6 1 0.4 (0.0–3.2) 4 1 — 2 0 — Electrical or electronic equipment or machinery 103 53 1.4 (0.9–2.3) 62 37 1.3 (0.7–2.5) 41 16 1.9 (0.8–4.7) Insecticides 29 15 1.1 (0.5–2.2) 13 11 1.3 (0.5–3.2) 16 4 1.0 (0.3–3.3) Herbicides 7 4 1.1 (0.3–4.1) 3 2 — 4 2 — Fungicides 10 1 0.2 (0.0–1.6) 5 1 — 5 0 — Fumigants 10 5 0.9 (0.3–2.8) 7 4 — 3 1 — aIndividuals with missing information on a specific workplace exposure were excluded from the analysis of that exposure; individuals who did not report a specific exposure served as the reference group for that exposure; for exposure with < 10 cases, stratified analysis was not performed. bCalculated from logistic regression models; all models were adjusted for age (< 56, 56–65, and > 65 years), sex, area of residence (Boston metropolitan area, eastern Massachusetts, and rest of New England), smoking, and educational level. cCalculated from logistic regression models; all models were adjusted for age (< 56, 56–65, and > 65 years), sex, area of residence (Boston metropolitan area, eastern Massachusetts, and rest of New England), and educational level. Environmental Health Perspectives • volume 117 | number 9 | September 2009 1389