Table 1.

Summary of epidemiological evidence ^a on selected potential airborne environmental “risk factors” for PD in relation to olfaction

	Study design	Study setting and participants	Exposure assessment	Outcome assessment	Primary findings	Comments
Pesticides	Cross-sectional, case-control, & longitudinal studies	Mostly occupational setting (e.g., farmers or farm workers vs. others); a few population-based studies; mostly working age population or older; primarily men	Self-reported pesticide exposures; farmer as a surrogate for pesticide exposure; biospecimen concentration (e.g., urinary sample)	Sniffin-Stick odor identification and threshold tests; self-reported olfactory impairment	Mostly positive associations (OR: 1.02–1.49), but data are not entirely consistent, especially on which olfaction domain was impaired.	Exposure assessment errors; self-reported olfaction not accurate; no data on whether the impairment was transitory, progressive or permanent (i.e., age related decline)
Air pollutants	Cross-sectional, & case-control studies	Population-based studies; wide age range (i.e., young, middle, & old age); both men and women	Ecological (e.g., residents of Mexico city vs. another city); criteria air pollutants (e.g., NO2 or PM2.5)	Sniffin-Stick odor identification and threshold tests; clinical anosmia based on ICD codes	Mostly positive associations (OR: 1.20–1.73), but some null association findings	No longitudinal studies; unsure if the impairment was transitory or age-related; possible effect modification by age
Cigarette smoking	Cross-sectional, & longitudinal studies	Population-based studies; PD patients vs. others; wide age range, including young, middle, and old age, both men and women	Self-reported smoking history (e.g., never, past, current), pack-years, years of smoking, smoking cessation	Sniffin-Stick odor threshold and odor identification; UPSIT; MODSIT; SDOIT; self-reported OI; B-SIT; SOIT; Connecticut Chemosensory Clinical Research Center test; other non-commercial tests (e.g., identification of series dilution of common odors)	Mostly positive associations (OR: 1.36–2.43) especially for current smoking, but not entirely consistent. In PD patients, smoking was not positively associated with poor olfaction	Unsure if the impairment was transitory or age-related; analyses limited to PD may lead to a biased association of smoking with poor olfaction
Heavy metals	Cross-sectional studies	Predominantly occupational settings (e.g., welders vs. others); a few population-based studies; mostly working age population or older, occasionally children	Surrogate (e.g., welder); biospecimen concentrations (blood, urine, hair, fingernail, bone); most studied metals include manganese, cadmium, chromium and lead	Olfactory acuity by olfactometer; Sniffin-Stick odor threshold and odor identification; UPSIT; self-reported olfaction other non-commercial tests (e.g., identification of series dilution of common odors)	Manganese: inconsistent - positive, negative or null associations; cadmium, & lead: inconsistent - positive or null associations; Chromium: positive associations, but data are limited	Small sample sizes, cross-sectional, and occupational setting; unsure if the impairment was transitory (due to occupational hazards) or age-related.
Organic solvents	Cross-sectional studies	Occupational settings (e.g., workers in paint manufacturing facilities, printing workers, or medical laboratory employee vs others); working age; men and women	Surrogate (e.g., job-title based exposure assessment)	Self-reported olfaction; Sniffin-Stick odor threshold and odor identification; UPSIT	Mostly positive associations, but some show null associations. There is a potential effect modification by smoking	Small sample sizes, cross-sectional, and occupational setting; unsure if the impairment was transitory (due to occupational hazards) or age-related.

Abbreviations: OR: odds ratio; UPSIT: University of Pennsylvania Smell Identification Test; MODSIT: Modular Smell Identification Test; SDOIT: The San Diego Odor Identification Test; B-SIT: Brief Smell Identification Test; SOIT: Scandinavian Odor Identification Test

^aSee references and citations in the text due to space limit.