Table 2.

Risk of Parkinson disease in relation to use of well water, U.S. Medicare 2009

	All Cases and Controls 89,705a cases/21,526,967b controls	All with Claims Datab 89,705a cases/117,971b controls
Adjustment in addition to age, sex and race/ethnicityd	OR (95% CI)d,e	OR (95% CI)d,e
None	0.87 (0.85–0.89)	0.86 (0.83–0.89)
Geographic region (state)	0.89 (0.87–0.92)	0.88 (0.85–0.91)
Agricultural pesticides (county), kg/acre	0.87 (0.85–0.89)	0.86 (0.83–0.89)
Number of physician visitsf	0.88 (0.85–0.90)	0.86 (0.83–0.89)
Number of diagnosesg	N/A	0.90 (0.87–0.94)
Smokingh	N/A	0.86 (0.83–0.89)
Number of diagnosesg and smokingh	N/A	0.91 (0.88–0.94)
Number of diagnoses,g smoking,h ETS,i UV-related cancer,j agricultural pesticides and state	N/A	0.91 (0.88–0.95)

Abbreviations: OR, odds ratio; CI, confidence interval; International Classification of Diseases Ninth Revision Clinical Modification, ICD-9; kg, kilogram; UV, ultraviolet light; ETS, environmental tobacco smoke.

^aExcludes 85 (0.09%) of cases without data on well water use.

^bExcludes 22,375 (0.10%) of controls without data on well water use (including 124 [0.10%] of controls with claims data), and 58 (0.0002%) additional controls with unknown sex. Sex was known for all subjects with claims data.

^cAll cases and a random sample of 0.5% of controls, who represent all cases and controls for whom complete Medicare claims data for 2004–2009, and therefore data on number of diagnoses and smoking, were available.

^dAdjusted for age (as 2 linear splines with a knot at age 85), sex and race/ethnicity (in 7 categories); and, if noted, additional covariates with all continuous variables modeled linearly.

^eProbability of well water use is a continuous variable ranging from 0 to 1 (0% to 100%), such that a one unit change, and hence the OR, compares beneficiaries with 100% probability of well water use vs. beneficiaries with 0% probability of well water use, i.e. similar to a dichotomous exposure variable. Use of well water means the probability of well water from an individual dug or drilled well, based on residential zip code in the PD diagnosis/control reference year.

^fNumber of physician visits (outpatient institutional visits and visits to physician offices combined) in 2008 (prior to PD diagnosis/control reference).

^gTotal number of unique ICD-9 diagnosis codes up to PD diagnosis/control reference date.

^hProbability of ever/never tobacco smoking, determined based on tobacco-specific ICD-9 diagnosis codes and a probabilistic model based on sex, race/ethnicity, birth cohort and >600 other diagnosis and procedure codes.

ⁱPercent of county residents who smoke tobacco daily.

^jAny UV-related cancer[59] according to ICD-9 diagnosis code, as a proxy for lifetime outdoor physical activity.