American Journal of Epidemiology
ªThe Author 2009. Published by the Johns Hopkins Bloomberg School of Public Health.
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Vol. 169, No. 8
Advance Access publication March 6, 2009
Parkinson’s Disease and Residential Exposure to Maneb and Paraquat From
Agricultural Applications in the Central Valley of California
Sadie Costello, Myles Cockburn, Jeff Bronstein, Xinbo Zhang, and Beate Ritz
Initially submitted September 12, 2008; accepted for publication January 6, 2009.
Evidence from animal and cell models suggests that pesticides cause a neurodegenerative process leading to
Parkinson’s disease (PD). Human data are insufﬁcient to support this claim for any speciﬁc pesticide, largely
because of challenges in exposure assessment. The authors developed and validated an exposure assessment
tool based on geographic information systems that integrated information from California Pesticide Use Reports
and land-use maps to estimate historical exposure to agricultural pesticides in the residential environment. In
1998–2007, the authors enrolled 368 incident PD cases and 341 population controls from the Central Valley of
California in a case-control study. They generated estimates for maneb and paraquat exposures incurred between
1974 and 1999. Exposure to both pesticides within 500 m of the home increased PD risk by 75% (95% conﬁdence
interval (CI): 1.13, 2.73). Persons aged 60 years at the time of diagnosis were at much higher risk when exposed
to either maneb or paraquat alone (odds ratio ¼2.27, 95% CI: 0.91, 5.70) or to both pesticides in combination
(odds ratio ¼4.17, 95% CI: 1.15, 15.16) in 1974–1989. This study provides evidence that exposure to a combi-
nation of maneb and paraquat increases PD risk, particularly in younger subjects and/or when exposure occurs at
case-control studies; fungicides, industrial; geographic information systems; herbicides; maneb; paraquat;
Parkinson disease; pesticides
Abbreviations: CI, conﬁdence interval; DDE, dichlorodiphenyldichloroethylene; GIS, geographic information system; MPPþ, toxic
metabolite of 1-methyl-4-phenylpyridinium; OR, odds ratio; PD, Parkinson’s disease; PLSS, Public Land Survey System; PUR,
Pesticide Use Reporting.
Parkinson’s disease (PD) has been reported to occur at
high rates among farmers and in rural populations, contrib-
uting to the hypothesis that agricultural pesticides might be
causal agents (1–4). Animal studies have linked certain pes-
ticides to Parkinsonism and dopaminergic cell death. The
pesticide rotenone can produce the behavioral and neuro-
pathologic features of PD in some rodent models through
chronic systemic inhibition of mitochondrial complex I (5,
6). Exposure to a combination of the fungicide maneb and
the herbicide paraquat in mice leads to increased substantia
nigra neuronal pathology (7), age-dependent motor degen-
eration, progressive reductions in dopamine metabolites and
turnover (8), and reduced tyrosine hydroxylase and dopa-
mine transporter immunoreactivity (9, 10).
Human evidence is insufﬁcient to identify any particular
pesticide compound, including those implicated by animal
studies, as being responsible for causing PD (11). Method-
ological limitations have clouded the interpretation of most
epidemiologic studies exploring pesticide exposures and PD
in humans. Past studies have generally relied on self-reports
and recall of chemical usage, making them vulnerable to
information bias and differential recall bias (12).
Because pesticides applied from the air or ground may
drift from their intended treatment sites, with measurable
concentrations subsequently detected in the air, in plants,
and in animals up to several hundred meters from applica-
tion sites (13–15), accurate methods of estimating environ-
mental exposures in rural communities are sorely needed.
Correspondence to Dr. Sadie Costello, Department of Environmental Health Sciences, School of Public Health, University of California, Berkeley,
50 University Hall, #7360, Berkeley, CA 94720-7360 (e-mail: firstname.lastname@example.org).
919 Am J Epidemiol 2009;169:919–926
Geographic information system (GIS)-based methods of as-
sessing exposure to pesticides have become popular in re-
cent years and may prove an effective solution when
pesticide data exist. We developed and employed a validated
GIS-based exposure assessment tool to estimate pesticide
exposure from applications to agricultural crops, relying
on California Pesticide Use Reporting (PUR) data, land-
use maps, and geocoded residential historical locations
(16). We investigated whether exposure to the pesticides
maneb and paraquat, alone and in combination, increased
the risk of incident PD among residents of the Central Valley
of California, an area well-known for its intensive agricul-
ture and potential for pesticide exposure.
MATERIALS AND METHODS
All procedures described have been approved by the
University of California, Los Angeles, institutional review
board for human subjects, and informed consent was ob-
tained from all participants.
We used a population-based approach for recruiting cases
and controls from a largely agricultural population in Cali-
fornia. Details are provided elsewhere (17). Brieﬂy, persons
with PD newly diagnosed between January 1998 and January
2007 who resided in 1 of 3 central California counties
(Fresno, Tulare, or Kern county) and had lived in California
for at least 5 years prior to diagnosis were recruited into our
study within 3 years of diagnosis. Altogether, 28 (90%) of
the 31 practicing local neurologists who provided care for
PD patients assisted in recruiting cases for this study. We
solicited collaboration from Kaiser Permanente Medical
Center (Fresno, California), Kern Medical Center (Bakersﬁeld,
California), and Visalia Medical Clinic (Visalia, California)
and from the Veterans Administration, PD support groups,
local newspapers, and local radio stations that broadcast
public service announcements.
Of the 1,167 PD cases who were initially invited, 604
were not eligible: For 397, the case’s diagnosis date fell
outside the 3-year range prior to contact, 51 denied having
received a PD diagnosis, 134 lived outside the tricounty
area, and 22 were too ill to participate. Of the 563 eligible
cases, 473 (84%) were examined by a University of Cali-
fornia, Los Angeles, movement disorder specialist at least
once and were conﬁrmed to have clinically ‘‘probable’’ or
‘‘possible’’ PD; the remaining 90 potential cases could not
be examined or interviewed (54% withdrew, 32% were too
ill or died, and 14% moved out of the area prior to the
examination or did not honor a scheduled appointment).
We examined but excluded another 96 patients because they
had other causes of Parkinsonism. This left us with 377
cases; of these, 368 provided all information needed for
Controls aged 65 years or older were identiﬁed from
Medicare lists in 2001, but because of implementation of
the Health Insurance Portability and Accountability Act,
which prohibits the use of Medicare enrollees, 70% of our
controls were recruited from randomly selected tax assessor
residential units (parcels) in each of the 3 counties. We
mailed letters of invitation to a random selection of residen-
tial living units and also attempted to identify head-of-
household names and telephone numbers for these parcels,
using the services of marketing companies and Internet
We contacted 1,212 potential population controls by mail
and/or telephone for eligibility screening. Eligibility criteria
were: 1) not having PD, 2) being at least 35 years of age,
3) currently residing primarily in 1 of the 3 designated coun-
ties, and 4) having lived in California for at least 5 years
prior to the screening. Only 1 person per household was
allowed to enroll. Of the potential controls contacted, 457
were ineligible: 409 were too young, 44 were terminally ill,
and 4 resided primarily outside of the study area. Of the 755
eligible controls, 409 (54%) declined participation, were too
ill to honor an appointment, or moved out of the area prior to
interview; 346 (46%) were enrolled, and 341 provided all
information needed for analyses.
Assessment of environmental pesticide exposure
We conducted telephone interviews to obtain demo-
graphic and exposure information. Detailed residential his-
tory forms were mailed to subjects in advance of their
interview and were reviewed in person or over the phone.
We estimated pesticide exposures in the residential environ-
ment from applications to agricultural crops employing
a validated GIS-based system, which combined PUR data
and land-use maps (16, 18), to produce estimates of residen-
tial ambient pesticide applications within a set distance of
subjects’ homes. We recorded and geocoded lifetime resi-
dential histories and estimated ambient exposures for all
historical addresses at which participants had resided be-
tween 1974 and 1999, the period covered by the PUR data.
A technical discussion of our GIS-based approach is pro-
vided elsewhere (16); here we brieﬂy summarize the data
sources and the exposure modeling process.
Residential addresses. Addresses were automatically
geocoded to TigerLine ﬁles (NAVTEQ (Chicago, Illinois),
unpublished data, 2006), and discrepancies were then man-
ually resolved in a multistep process similar to that de-
scribed by McElroy et al. (19). Resulting locations were
recorded, along with the relevant year range of residence,
so they could be matched to the appropriate year-speciﬁc
PUR and land-use data (below). For our GIS model, we
relied on addresses in Fresno, Kern, and Tulare counties
(the tricounty area) at which participants had resided be-
tween 1974 and 1999. Out of 9,568 total residential years
contributed by cases (26 years 3368 cases), 7,593 years
(79%) were spent at addresses within the tricounty area as
compared with 6,757 (76%) of 8,866 years contributed by
controls (26 years 3341 controls). We geocoded these
tricounty residential addresses for the period 1974–1999
with similar precision for cases and controls; that is, both
had spent 88% of their respective residential years at ad-
dresses we considered to have been mapped with high pre-
cision (i.e., at the level of a residential parcel, street
address, or street intersection rather than a zip code or city
920 Costello et al.
Am J Epidemiol 2009;169:919–926
Pesticide use reporting. PUR data are recorded by the
California Department of Pesticide Regulation for any com-
mercial application of restricted-use pesticides (deﬁned as
agents with harmful environmental or toxicologic effects
(20)) and, since 1990, for all commercial uses of pesticides
regardless of toxicologic proﬁle. The location of each PUR
record is referenced to the Public Land Survey System
(PLSS), a nationwide grid that parcels land into sections
at varying resolutions. Each PUR record includes the name
of the pesticide’s active ingredient, the poundage applied,
the crop and acreage of the ﬁeld, the application method,
and the date of application.
Land-use maps. Because the PUR records link an agri-
cultural pesticide application only to a whole PLSS grid
section, we added information from land-use maps to more
precisely locate the pesticide application, as described in
detail elsewhere (18). The California Department of Water
Resources periodically (every 7–10 years) performs county-
wide large-scale surveys of land use and crop cover, which
allowed us to identify the locations of speciﬁc crops within
each PLSS grid section. Digital maps from more recent
(1996–1999) surveys are available (21), and paper maps
were manually digitized for earlier periods (1977–1995).
The 1977 land-use survey was conducted closest in time
to 1974, when PUR data became available. We constructed
historical electronic maps of land use and crop type, and
using the PLSS grid section and the crop type reported in the
PUR record, we allocated pesticide applications to an agri-
cultural site to which we assigned a GIS-based location.
Deriving estimates of residential pesticide exposure. The
time-speciﬁc total exposure at each location, by pesticide,
was derived through summation of exposures over a ﬁxed
500-m radius (suggested in previous literature (13, 15, 19))
around the home for the relevant years of residence. The
numbers of pounds of pesticide applied annually per acre
were summed for each residential buffer and weighted by
the proportion of treated acreage in each buffer, resulting in
pesticide application rates that could be averaged over spe-
ciﬁc calendar periods of each subject’s lifetime.
We estimated residential exposures to maneb and para-
quat, alone and in combination, for the following time win-
dows: 1) 1974–1999, 2) 1974–1989, and 3) 1990–1999, to
assess the possibility of an extensive induction period prior
to PD onset and the inﬂuence of age at exposure. We strat-
iﬁed models by sex and age (60 years, >60 years) and, in
additional sensitivity analyses, controlled for exposure to
some groups of pesticides suspected to increase PD risk.
We controlled for occupational exposure to pesticides
among subjects who had held jobs in the agricultural sector,
assigning them to categories of ‘‘likely exposed to pesti-
cides’’ when they reported pesticide handling and applica-
tions or ﬁeldwork and ‘‘possibly exposed to pesticides’’
when they reported managerial, produce processing, and
other nonﬁeld farm work; all other subjects were considered
‘‘not occupationally exposed to pesticides’’ (22). In some
models, we also adjusted for residential exposures to groups
of other pesticides that some studies have found to be linked
to dopaminergic cell damage or possibly PD (organochlo-
rines, organophosphates, and dithiocarbamates (23) and pro-
teasome inhibitors (24)).
We considered the following demographic variables as
potential confounders in all analyses: age (age at diagnosis
for cases and age at interview for controls), sex, race (white,
nonwhite), education (<12 years, 12 years, >12 years), and
cigarette smoking (current, former, never). We used SAS 9.1
(SAS Institute Inc., Cary, North Carolina) to perform un-
conditional logistic regression analyses.
Study participants were predominantly Caucasian, over
the age of 60, and without a family history of PD (Table 1).
Cases were slightly older than controls, were more often
male, and had completed fewer years of education. They
were also more likely to have been occupationally exposed
to pesticides and to be never or former smokers.
We did not ﬁnd increased risks of PD among subjects
exposed to paraquat alone during the years 1974–1999
(Table 2). While the rarity of sole maneb exposure (4 subjects)
precluded any meaningful interpretation of the maneb-only
results, combined exposure to both maneb and paraquat in-
creased the risk of PD by 75% (odds ratio (OR) ¼1.75,
95% conﬁdence interval (CI): 1.13, 2.73), an effect estimate
which was essentially unchanged after adjustment for
occupational pesticide exposure (OR ¼1.74, 95% CI:
When we examined 2 separate exposure time windows,
the years 1974–1989 and 1990–1999, the risk increase ob-
served for the whole period was found to be mainly attribut-
able to exposures incurred during the earlier window
(OR ¼2.14, 95% CI: 1.24, 3.68), while being exposed
during the later window did not seem to increase PD risk
(Table 2). Furthermore, for younger (60 years) subjects,
exposure to both maneb and paraquat in both windows in-
creased PD risk as much as 4- to 6-fold (Table 3). Exposure
to either maneb or paraquat alone during 1974–1989 also
increased risk of PD in younger subjects (OR ¼2.27, 95%
CI: 0.91, 5.70). When we examined exposure windows
among our older subjects (>60 years), combined exposure
to both pesticides in the earlier window only (1974–1989)
was also associated with a 2-fold increase in PD risk
(OR ¼2.15, 95% CI: 1.15, 4.02), but no increase was found
for either the later window (1990–1999) or the combined
exposure periods (Table 3). Stratiﬁcation by sex suggested
no differences in estimates between males and females.
In this population-based case-control study, agricultural
application of both maneb and paraquat within 500 m of
a residence during the period 1974–1999 greatly increased
the risk of developing PD, especially when exposure oc-
curred between 1974 and 1989 or when PD was diagnosed
at a younger age (60 years). Exposure to both pesticides
during the earlier time window (1974–1989) also doubled
the risk for older cases. Associations were particularly
PD and Residential Maneb and Paraquat Exposure 921
Am J Epidemiol 2009;169:919–926
strong for younger-onset patients (60 years), who would
have been children, teenagers, and young adults during the
exposure period: Among those exposed in the earlier time
window, risk was increased more than 4-fold with exposure
to both pesticides and more than 2-fold with exposure to just
1 of the pesticides. Consistent with some theories regarding
the progression of PD pathology (25), these data suggest
that the critical window of exposure to toxicants may be
years before the onset of motor symptoms which lead to
Pesticide and herbicide exposures have previously been
implicated in idiopathic PD. Paraquat is structurally similar
to the toxic metabolite (MPPþ) of the 1-methyl-4-phenyl-
pyridinium ion (a metabolite of 1-methyl-4-phenyl-1,2,3,6-
tetrahydropyridine), an agent known to induce Parkinsonian
symptoms in humans that has been widely used to study
Table 1. Odds Ratio for Parkinson’s Disease According to Various Sociodemographic
Characteristics, Central Valley of California, 1998–2008
No. or Mean % No. or Mean %
68.1 (34–88) 67.6 (34–92) 1.00 0.99, 1.02
Age group, years
41–50 25 7 26 8
51–60 47 13 55 16
61–70 111 30 95 28
71–80 145 39 121 35
>80 33 9 38 11
Female sex 161 44 165 48 0.83 0.62, 1.12
First-degree relative with
55 15 37 11 1.44 0.93, 2.25
White 296 80 279 82 1 Reference
72 20 62 18 1.09 0.75, 1.60
Asian 4 1 8 2
Black 3 1 13 4
Latino 49 13 31 9
Native American 16 4 10 3
<12 68 18 38 11 1.15 0.69, 1.90
12 100 27 64 19 1 Reference
>12 200 54 239 70 0.54 0.37, 0.77
Job exposure matrix
exposed to pesticides
232 63 240 70 1 Reference
exposed to pesticides
26 7 26 8 1.03 0.58, 1.83
exposed to pesticides
110 30 75 22 1.52 1.08, 2.14
Cigarette smoking status
Never smoker 195 53 146 43 1 Reference
Former smoker 151 41 161 47 0.70 0.52, 0.96
Current smoker 22 6 34 10 0.48 0.27, 0.86
0 195 53 146 43 1 Reference
>0–19 96 26 89 26 0.81 0.56, 1.16
>19 77 21 106 31 0.54 0.38, 0.78
The odds ratio was calculated for all nonwhites versus whites.
922 Costello et al.
Am J Epidemiol 2009;169:919–926
Parkinsonism in animal models (26). MPPþis believed to
cause cell death by interfering with mitochondrial respira-
tion (27), because it concentrates in mitochondria and in-
hibits complex I of the electron transport chain (28). Many
lines of evidence point to possible mitochondrial dysfunc-
tion in PD. Several genes have been identiﬁed in familial
forms of PD that are linked to mitochondrial function
(PINK1 and DJ1), and in sporadic cases of PD, pathologic
free radical reactions that damage mitochondria and de-
crease electron transport activity have been described (29).
Impaired electron transport hampers adenosine triphosphate
production and leads to the diversion of electrons from their
normal electron transport recipients and, thus, further for-
mation of damaging free radicals (29).
Although paraquat is also used to induce Parkinsonism in
some animal models, the mechanism by which it produces
symptoms is not yet understood (30). Recent mammalian
and yeast-cell experiments suggest that mitochondria take
up paraquat actively across their membranes, where com-
plex I reduces it to the paraquat radical cation that subse-
quently produces mitochondria-damaging superoxide (31).
It has also been suggested that maneb may inhibit the ubiq-
uitin proteasome system, thereby damaging the dopaminer-
gic neuron (24, 32). Additionally, maneb has been linked to
Parkinsonism in mice also exposed to paraquat. In 3 recent
studies, investigators reported that only when mice were
exposed to a combination of the fungicide maneb and the
Table 2. Odds Ratio for Parkinson’s Disease According to
Residential Ambient Exposure to Maneb and/or Paraquat, Central
Valley of California, 1974–1999
No. % No. %
Missing data 13 4 13 4
No exposure 115 31 126 37 1 Reference
Paraquat only 149 40 152 45 1.01 0.71, 1.43
Maneb only 3 1 1 0 3.04 0.30, 30.86
88 24 49 14 1.75 1.13, 2.73
Missing data 53 14 52 15
No exposure 93 25 113 33 1 Reference
148 40 137 40 1.25 0.85, 1.85
74 20 39 11 2.14 1.24, 3.68
Missing data 15 4 15 4
No exposure 215 58 213 62 1 Reference
113 31 95 28 0.96 0.64, 1.43
25 7 18 5 0.93 0.45, 1.94
Odds ratios were adjusted for age, sex, nonwhite race, education,
and smoking status. Results were mutually adjusted for exposure in
each time window.
Table 3. Odds Ratio for Parkinson’s Disease According to
Residential Ambient Exposure to Maneb and/or Paraquat, by Time
Window of Exposure and Age Group, Central Valley of California,
Cases Controls Odds
No. % No. %
1974–1999 Time Window
Missing data 2 3 4 5
No exposure 18 23 34 39 1 Reference
38 48 42 48 1.77 0.84, 3.75
21 27 7 8 5.07 1.75, 14.71
Missing data 11 4 9 4
No exposure 97 34 92 36 1 Reference
114 39 111 44 0.90 0.60, 1.34
67 23 42 17 1.36 0.83, 2.23
1974–1989 Time Window
Missing data 16 20 20 23
No exposure 13 16 27 31 1 Reference
36 46 34 39 2.27 0.91, 5.70
14 18 6 7 4.17 1.15, 15.16
Missing data 37 13 32 13
No exposure 80 28 86 34 1 Reference
112 39 103 41 1.18 0.75, 1.84
60 21 33 13 2.15 1.15, 4.02
1990–1999 Time Window
Missing data 2 3 5 6
No exposure 43 54 58 67 1 Reference
27 34 22 25 2.00 0.84, 4.74
7 9 2 2 5.74 0.55, 59.62
Missing data 13 4 10 4
No exposure 172 60 155 61 1 Reference
86 30 73 29 0.78 0.49, 1.24
18 6 16 6 0.66 0.29, 1.50
Age-stratiﬁed models with adjustment for sex, nonwhite race,
education, and smoking status. Results were mutually adjusted for
exposure in each time window.
PD and Residential Maneb and Paraquat Exposure 923
Am J Epidemiol 2009;169:919–926
herbicide paraquat (paraquat þmaneb), not to either pesti-
cide alone, did they exhibit increased neuronal pathology
(7), age-dependent motor degeneration and progressive re-
ductions in dopamine metabolites and dopamine turnover
(8), and reduced tyrosine hydroxylase and dopamine trans-
porter immunoreactivity (9).
The fungicide maneb and the herbicide paraquat are both
used in the Central Valley of California and are often used
on the same crops, including potatoes, dry beans, and toma-
toes. The average amount of maneb applied near the homes
of these study subjects was relatively stable throughout both
time windows; however, annual paraquat exposure in-
creased during the later (1990–1999) time window. Persons
living near ﬁelds sprayed with maneb and paraquat may also
be exposed to a host of other agricultural chemicals. When
we controlled for the inﬂuence of other groups of pesticides
suspected a priori to be risk factors for PD in our study, the
odds ratios for combined maneb and paraquat exposure and
PD in the younger subjects were still in the 3- to 6-fold range
and statistically signiﬁcant; however, our precision de-
creased, probably because of correlated exposures. Correla-
tion between pesticides is an inherent problem when
assessing the effects of human exposure. However, since
adjustment for other pesticides did not remove the associa-
tion for maneb and paraquat, our data provide compelling
evidence that these 2 pesticides may in fact affect PD risk in
humans, as has been suggested by animal experiments.
Paraquat and maneb are applied by ground, aerial, and
backpack methods; however, paraquat has a much longer
ﬁeld half-life of 1,000 days (33), as compared with only
12–36 days for maneb (34). Both chemicals bind strongly
to soil, though, and are not thought to be a threat to ground-
water (35, 36). Such strong binding could result in contam-
inated soil getting blown or tracked into homes by wind,
pets, and shoes, thereby increasing exposure for persons
who live closer to agricultural application sites (3, 37, 38).
In a previous validation study, our prediction model for a
serum measure of dichlorodiphenyldichloroethylene (DDE)
explained 47% of the biomarker’s variance (39). Addition-
ally, our GIS-derived measure of organochlorine exposure
identiﬁed persons with high serum DDE levels reasonably
well (speciﬁcity of 87%) (39).
Although our GIS model allowed us to calculate the num-
ber of pounds of each active ingredient applied per acre
within a 500-m buffer, these quantities are not comparable
across pesticides. That is, a pound of active ingredient does
not represent the same human neurotoxicity across pesti-
cides, and no information currently exists that would allow
us to standardize these measures. Thus, while we believe
that our model provided us with an accurate indicator of any
pesticide exposure from applications close to a residence,
our exposure measure cannot be considered quantitative
beyond a crude rank ordering of low/medium likelihood
of exposure and high likelihood of exposure. Since we hy-
pothesized that coexposure to 2 pesticides, maneb and para-
quat, would increase the risk of PD, we also lacked the
statistical power to perform extensive categorical analyses
(note that only 3 cases and 1 control were exposed solely to
maneb). We conducted additional analyses after dichoto-
mizing pounds per acre at their median and mean levels
and found that exposure to both pesticides at the highest
level was associated with PD, especially in persons aged
60 years; however, wide conﬁdence intervals surrounding
our point estimates rendered these results generally uninfor-
mative (results not shown).
In only 1 previous analysis, conducted within the Agri-
cultural Health Study cohort (40), did researchers assess the
effects of maneb and paraquat exposures. Statistical power
was limited by the small number (n¼78) of incident cases
identiﬁed during follow-up and the very small number (n¼
4–10) of cases exposed to maneb/mancozeb (OR ¼2.1) and
paraquat (OR ¼1.4). In a small Taiwanese study, the only
case-control study to date with sufﬁcient statistical power to
examine exposure to the herbicide paraquat, Liou et al. (41)
reported a 4- to 6-fold increase in PD risk among long-term
applicators. In a case-control study from the Mayo Clinic
(Rochester, Minnesota), Brighina et al. (42) presented asso-
ciations between self-reported pesticide exposure and PD in
subjects younger than 60 years only (for all pesticides,
OR ¼1.80, 95% CI: 1.12, 2.87; for herbicides, OR ¼2.46,
95% CI: 1.34, 4.52).
Our exposure estimates did not depend on the subject’s
recall of pesticide exposure and are therefore unlikely to
have been biased by differential exposure misclassiﬁcation.
Since all of our PD diagnoses were clinically conﬁrmed, we
expect disease misclassiﬁcation to have been minimal. Non-
differential exposure misclassiﬁcation is a possibility in our
study and may have attenuated our effect estimates.
Our results may be biased if cases and controls selected
themselves into our study according to their potential for
pesticide exposure, but our subjects were not asked to self-
report environmental exposures and probably were unaware
of their true historical exposures. There is no reason to
suspect that cases and controls would have chosen to par-
ticipate on the basis of their historical residence near certain
agricultural plots. We saw no difference in estimated effects
when we restricted analyses to only those subjects with
more (12 years) or less (<12 years) education. Similarly,
we saw no difference in our results when we restricted the
sample to persons whose addresses had been mapped with
high precision in the tricounty area during the period 1974–
1999 (363 cases, 336 controls).
Our analysis has conﬁrmed 2 previous observations from
animal studies: 1) exposure to multiple chemicals may po-
tentiate the effect of each chemical (of interest, since hu-
mans are often exposed to more than 1 pesticide in the
environment) and 2) the timing of exposure is important.
To our knowledge, this is the ﬁrst epidemiologic study to
provide strong evidence that 2 speciﬁc pesticides, suggested
by animal research as potentially acting synergistically to
become neurotoxic, strongly increase the risk of PD in
humans, especially given combined exposure and when
encountered earlier in life.
Author afﬁliations: Department of Environmental Health
Sciences, School of Public Health, University of California,
924 Costello et al.
Am J Epidemiol 2009;169:919–926
Berkeley, Berkeley, California (Sadie Costello); Department
of Preventive Medicine, Keck School of Medicine, Univer-
sity of Southern California, Los Angeles, California (Myles
Cockburn, Xinbo Zhang); Department of Geography, College
of Letters, Arts and Sciences, University of Southern
California, Los Angeles, California (Myles Cockburn,
Xinbo Zhang); Department of Neurology, School of Medicine,
University of California, Los Angeles, Los Angeles, Cali-
fornia (Jeff Bronstein); and Department of Epidemiology,
School of Public Health, University of California, Los
Angeles, Los Angeles, California (Beate Ritz).
This work was supported by the National Institute
of Environmental Health Sciences (grants ES10544,
U54ES12078, and 5P30 ES07048), the National Institute of
Neurological Disorders and Stroke (grant NS 038367), and
the Department of Defense Prostate Cancer Research Pro-
gram (grant 051037). In addition, initial pilot funding was
provided by the American Parkinson’s Disease Association.
The authors thank the participating neurologists and med-
ical centers in Fresno, Kern, and Tulare counties for their
Conﬂict of interest: none declared.
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