Abstract
Background: In the developed world, occupational exposures are a leading cause of bladder cancer. A few studies have suggested a link between pesticide exposures among agricultural populations and bladder cancer.
Methods: We used data from the Agricultural Health Study, a prospective cohort study which includes 57 310 pesticide applicators with detailed information on pesticide use, to evaluate the association between pesticides and bladder cancer. We used Poisson regression to calculate rate ratios (RRs) and 95% confidence intervals (CIs) to estimate the association between each of 65 pesticides and 321 incident bladder cancer cases which accrued over the course of follow-up (1993–2011), adjusting for lifestyle and demographic and non-pesticide farm-related exposures, including those previously linked to bladder cancer. We conducted additional analyses stratified by smoking status (never, former, current).
Results: We observed associations with bladder cancer risk for two imidazolinone herbicides, imazethapyr and imazaquin, which are aromatic amines. Ever use of imazaquin (RR = 1.54, 95% CI: 1.05, 2.26) was associated with increased risk whereas the excess risk among users of imazethapyr was evident among never smokers (RR in highest quartile vs non-exposed = 3.03, 95% CI: 1.46, 6.29, P -interaction = 0.005). We also observed increased risks overall and among never smokers for use of several chlorinated pesticides including chlorophenoxy herbicides and organochlorine insecticides.
Conclusions: Several associations between specific pesticides and bladder cancer risk were observed, many of which were stronger among never smokers, suggesting that possible risk factors for bladder cancer may be more readily detectable in those unexposed to potent risk factors like tobacco smoke.
Keywords: Pesticides, bladder cancer, epidemiology
Key Messages
Occupational exposures are a leading cause of bladder cancer, but occupational pesticide exposure has been little explored as a possible risk factor.
We observed increased risks for two aromatic amine herbicides, chlorophenoxy herbicides and organochlorine insecticides.
Several associations were more apparent among never smokers, suggesting that pesticide exposure may be an overlooked exposure in bladder carcinogenesis.
Our results highlight the difficulty in trying to understand the impact of other exposures on smoking-related cancers.
Introduction
In the developed world, bladder cancer is the fourth and twelfth most common cancer in men and women, respectively. 1 The leading risk factors are cigarette smoking and occupational exposures. 2 Aromatic amines, including 2-naphthylamine, 4-aminobiphenyl, benzidine, ortho-toluidine and others, are established bladder carcinogens that have been described in the occupational setting. 3 Agricultural populations have a lower prevalence of smoking than the general population, 4–6 which may explain why several studies have found either no association or a decreased risk of bladder cancer in this occupational group. 7–13 On the other hand, two studies have shown a link between farming and bladder cancer among non-smokers, 14,15 which suggests a complexity in interpreting the effect of other exposures in the presence of smoking, the primary risk factor for bladder cancer. In addition, some studies have suggested a link between farming, herbicide exposure or specific agricultural settings and risk of bladder cancer. 14–22 Bladder cancer risk might be explained by the urogenous contact hypothesis which proposes that active carcinogens dissolved in urine come into contact with and transform cells of the bladder epithelium. 23 Many pesticides and their metabolites are readily excreted from the body via the urine. Thus, the potential exists for pesticides to adversely affect the bladder. We previously reported an increased risk of bladder cancer 24 in a cohort of farmers occupationally exposed to the aromatic amine herbicide, imazethapyr. Other specific pesticides, however, have been little explored as possible risk factors for bladder cancer. Thus, we used data from the Agricultural Health Study (AHS), a large prospective cohort study of pesticide applicators with detailed pesticide use data, to evaluate the association between several specific pesticides and bladder cancer risk.
Methods
Study population
The AHS is a prospective cohort study that includes 52 394 licensed private pesticide applicators in Iowa and North Carolina and 4916 licensed commercial applicators in Iowa. The cohort has been described in detail. 6,24,25 Briefly, individuals seeking licenses for restricted-use pesticides were recruited from December 1993 through December 1997 (82% of the target population enrolled). The protocol was approved by all relevant institutional review boards. We obtained cancer incidence information by regular linkage to cancer registry files in Iowa and North Carolina. In addition, the cohort is matched to state mortality registries and the National Death Index to identify vital status, and to home address records of the Internal Revenue Service, motor vehicle registration files and pesticide license registries of state agricultural departments to determine residence in Iowa or North Carolina. The current analysis included all incident bladder cancers (invasive and in situ ) diagnosed from enrolment (1993–97) through 31 December 2010 in North Carolina and 31 December 2011 in Iowa. We censored follow-up at the date of cancer diagnosis, time of death, movement out of state or at the end of the current follow-up time. Because there was only one case of bladder cancer diagnosed among female applicators, we excluded women from the analysis ( n = 1562), as well as 1071 individuals with prevalent cancer at enrolment and 333 with no follow-up information, leaving 54 344 men for analysis among whom a total of 321 incident bladder cancers were diagnosed.
Exposure assessment
Information on use of individual pesticides was captured in two self-administered questionnaires [ http://www.aghealth.nih.gov/collaboration/questionnaires.html ] completed during cohort enrolment. All applicators completed the first enrolment questionnaire, which enquired about ever/never use of 50 pesticides, as well as duration (years) and frequency (average days/year) of use for a subset of 22 pesticides. In addition, 44.1% of the applicators returned the second (take-home) enrolment questionnaire, which enquired about duration and frequency of use for the remaining 28 additional pesticides and ever/never use of additional pesticides. A follow-up questionnaire, which ascertained pesticide use since enrolment and last year applied, was administered 5 years after enrolment and completed by 36 342 (63%) of the original participants. For participants who did not complete a follow-up questionnaire (20 968 applicators, 37%), a data-driven multiple imputation procedure was used to impute use of specific pesticides at follow-up. A detailed description of the imputation process and validation is described by Heltshe et al . 26 Enrolment and follow-up information were combined to generate cumulative lifetime days of use and intensity-weighted lifetime days of use.
We restricted analyses to those pesticides with 10 or more exposed cases ( n = 65). Among these, 44 had detailed data to explore associations between cumulative exposure and bladder cancer risk, using two exposure metrics: (i) lifetime days of pesticide use, that is the product of years of use of a specific pesticide and the number of days used per year; and (ii intensity-weighted lifetime days of use, which is the product of lifetime days of use and a measure of exposure intensity. Intensity was derived from an algorithm using questionnaire data on mixing status, application method, equipment repair and use of personal protective equipment. 27 We also used 15-year lagged cumulative exposure, discounting the most recent 15 years of use. Supplementary Table 1 (available as Supplementary data at IJE online) provides the complete list of pesticides evaluated and their prevalence of use. Data were obtained from Agricultural Health Study data release versions P1REL201209.00 and P2REL201209.00.
Statistical analyses
For each pesticide, we categorized exposure based on the distribution of use among exposed cases. Depending on the prevalence of exposure, we created categories based on the median exposure, tertiles or quartiles. We used Poisson regression to calculate rate ratios (RRs) and 95% confidence intervals (CIs) and used the MIANALYZE procedure in SAS, version 9.3 (SAS Institute, Inc., Cary, NC, USA) to obtain the appropriate variance for the imputed data. Analyses were conducted using ever/never use, the lifetime days, intensity-weighted lifetime days and the 15-year lagged metrics. We evaluated several lifestyle, demographic and non-pesticide farm-related exposures, including those previously linked to bladder cancer (diesel exhaust exposure, welding, painting, grinding metal) as possible confounders of the relationship between pesticides and bladder cancer, and ultimately included the following variables which were independently related to bladder cancer in our population for adjustment of all models: attained age (10-year intervals), race (White, other), cigarette smoking (status, pack-years among former and current smokers) and pipe smoking (ever/never). Smoking status [never, former (smoked at least 100 cigarettes in the past], current) was ascertained at enrolment and subsequently upon cohort follow-up. Duration (years) and intensity (cigarettes/day) of smoking were assessed at enrolment. To fully explore possible confounding due to smoking, we explored adjusting for smoking in two ways: (i) status (never, former, current) and pack-years smoked; and (ii) status and duration (years) of smoking. We also conducted analyses stratified by smoking status (never, former, current). We also explored adjustment for ever use of pesticides most highly associated with a given individual pesticide in multivariate models, as well as mutual adjustment for pesticides that were associated with bladder cancer risk. Likelihood ratio tests were used to assess differences between strata ( P -interaction). All tests were two-sided and conducted at the α = 0.05 level. Tests for trend used the midpoint value of each exposure category in regression models.
Results
In all, 321 cases of bladder cancer were diagnosed among male applicators through the current follow-up period. Of these, 96% ( n = 307) were urothelial carcinomas and the majority of these were localized tumours ( n = 272) (data not shown); 83 cancers were diagnosed among never smokers, 161 among former smokers and 69 among current smokers ( Table 1 ); 13% of cases also reported a history of pipe use ( Table 1 ); and all of these men were former cigarette smokers at enrolment.
Table 1.
Characteristics of incident bladder cancer cases among men in the Agricultural Health Study
| Characteristic | Cohort Person-years (total = 802,905.7) | Total Bladder Cancer n = 321 n (%) a |
|---|---|---|
| Age at the end of current follow-up | ||
| <60 | 402510.437 (50.1) | 57 (17.8) |
| 60–69 | 203258.327 (25.3) | 100 (31.2) |
| 70–79 | 138180.408 (17.2) | 114 (35.5) |
| 80+ | 58956.5777 (7.3) | 50 (15.6) |
| Mean (SD) | 69.6 (10.4) | |
| State | ||
| Iowa | 534349.517 (66.6) | 185 (57.6) |
| North Carolina | 268556.233 (33.4) | 136 (42.4) |
| Applicator Type | ||
| Private/farmer | 729393.3 (91.0) | 300 (93.5) |
| Commercial | 70440.4 (8.8) | 21 (6.5) |
| Exposed to engine exhaust | ||
| No | 268975.2 (33.5) | 123 (38.3) |
| Yes | 80786.8 (10.1) | 50 (15.6) |
| Missing | 450071.6 (56.1) | 148 (46.1) |
| Paint at least once a year | ||
| No | 257887.4 (32.2) | 153 (47.7) |
| Yes | 541946.2 (67.5) | 168 (52.3) |
| Missing | ||
| Grind metal in summer and/or winter | ||
| Monthly | 93414.5 (11.6) | 57 (17.8) |
| Weekly | 145398.4 (18.2) | 63 (19.6) |
| Other | 68232.9 (8.5) | 36 (11.1) |
| Missing | 490545.0 (61.1) | 165 (51.4) |
| Race | ||
| White | 767652.107 (95.6) | 317 (98.8) |
| Black/Other | 35253.6427 (4.4) | 4 (1.2) |
| Smoking Status b | ||
| Never | 416616.101 (51.9) | 83 (25.9) |
| Former | 231281.971 (28.8) | 161 (50.2) |
| Current | 130657.717 (16.3) | 69 (21.5) |
| Missing | 24349.9603 (3.0) | 8 (2.5) |
| Pipe Smoker | ||
| Never | 764677.153 (95.2) | 278 (86.6) |
| Ever | 38228.5969 (4.8) | 43 (13.4) |
a Percents may not sum to 100 due to rounding.
b Assessed at enrolment and follow-up.
Table 2 shows the rate ratios of bladder cancer associated with ever use of specific herbicides, insecticides, fumigants and fungicides. Increased risks of bladder cancer were observed among ever users of the herbicides bentazon (RR = 1.55, 95% CI: 1.10, 2.19), bromoxynil (RR = 1.51, 95% CI: 1.04, 2.20), chloramben (RR = 1.56, 95% CI: 1.10, 2.22), diclofop-methyl (RR = 1.85, 95% CI: 1.01, 3.42) and imazaquin (RR = 1.54, 95% CI: 1.05, 2.26). Additional associations were observed between ever use of 2,4-D (RR = 1.46, 95% CI: 0.98, 2.18) and ever use of sethoxydim (RR = 0.65, 95% CI: 0.43, 1.00), with a positive and an inverse association observed, respectively. The organochlorine insecticides dichlorodiphenyltrichloroethane (DDT) and heptachlor were positively associated with bladder cancer risk (RR = 1.40, 95% CI: 1.10, 1.80 and RR = 1.30, 95% CI: 0.98, 1.74, respectively).
Table 2.
Ever use of pesticides and risk of bladder cancer in the Agricultural Health Study
| Pesticide | Exposed Cases | RR a (95% CI) |
|---|---|---|
| Herbicides | ||
| 2,4,5-T b | 91 | 1.15 (0.84, 1.59) |
| 2,4,5-TP b,c | 40 | 1.07 (0.74, 1.56) |
| 2,4-D | 245 | 1.46 (0.98, 2.18) |
| Acifluorfen, sodium salt c | 28 | 1.21 (0.79, 1.85) |
| Alachlor | 158 | 1.15 (0.86, 1.52) |
| Atrazine | 220 | 1.22 (0.88, 1.69) |
| Bentazon c | 67 | 1.55 (1.10, 2.19) |
| Bromoxynil c | 51 | 1.51 (1.04, 2.20) |
| Butylate | 86 | 0.86 (0.63, 1.19) |
| Chloramben b,c | 46 | 1.56 (1.10, 2.22) |
| Chlorimuron-ethyl | 91 | 0.85 (0.62, 1.17) |
| Clomazone c | 24 | 0.99 (0.64, 1.54) |
| Cyanazine | 101 | 0.90 (0.67, 1.21) |
| Dicamba | 125 | 0.84 (0.62, 1.14) |
| Diclofop-methyl c | 11 | 1.85 (1.01, 3.42) |
| EPTC | 49 | 0.98 (0.70, 1.37) |
| Ethalfluralin c | 10 | 0.77 (0.40, 1.45) |
| Fluazifop-butyl b,c | 26 | 1.06 (0.68, 1.64) |
| Glyphosate | 248 | 1.17 (0.78, 1.77) |
| Imazaquin c | 38 | 1.54 (1.05, 2.26) |
| Imazethapyr | 104 | 1.03 (0.76, 1.40) |
| Linuron c | 21 | 0.97 (0.60, 1.55) |
| Metolachlor | 113 | 0.86 (0.65, 1.13) |
| Metribuzin | 107 | 0.75 (0.54, 1.04) |
| Propachlor b,c | 27 | 1.20 (0.78, 1.83) |
| Paraquat | 71 | 0.86 (0.61, 1.20) |
| Pendimethalin | 113 | 0.75 (0.55, 1.02) |
| Petroleum Oil/Petroleum Distillates | 130 | 0.88 (0.65, 1.21) |
| Sethoxydim c | 28 | 0.65 (0.43, 1.00) |
| Simazine b,c | 16 | 1.04 (0.61, 1.77) |
| Thifensulfuron-methyl c | 14 | 1.04 (0.59, 1.82) |
| Trifluralin | 139 | 1.08 (0.80, 1.45) |
| Insecticides | ||
| Acephate c | 21 | 0.91 (0.55, 1.50) |
| Aldicarb | 35 | 0.88 (0.59, 1.32) |
| Aldrin b | 88 | 1.20 (0.92, 1.57) |
| Carbaryl | 192 | 1.04 (0.70, 1.54) |
| Carbofuran | 67 | 0.86 (0.63, 1.16) |
| Chlordane b | 97 | 0.95 (0.74, 1.22) |
| Chlorpyrifos | 108 | 0.88 (0.67, 1.14) |
| Coumaphos | 19 | 0.95 (0.59, 1.54) |
| DDT b | 136 | 1.40 (1.10, 1.80) |
| DDVP b | 25 | 1.01 (0.65, 1.55) |
| Diazinon | 98 | 0.74 (0.54, 1.02) |
| Dieldrin b,c | 32 | 1.19 (0.82, 1.72) |
| Disulfoton b,c | 15 | 0.94 (0.54, 1.65) |
| Ethoprop c | 11 | 0.73 (0.39, 1.37) |
| Fonofos b | 53 | 1.09 (0.78, 1.52) |
| Heptachlor b | 72 | 1.30 (0.98, 1.74) |
| Lindane b | 69 | 1.08 (0.82, 1.42) |
| Malathion | 223 | 1.01 (0.65, 1.58) |
| Methomyl c | 13 | 1.17 (0.64, 2.12) |
| Parathion b | 62 | 1.14 (0.81, 1.61) |
| Permethrin | 44 | 0.75 (0.53, 1.07) |
| Phorate | 96 | 0.99 (0.72, 1.37) |
| Terbufos | 92 | 1.05 (0.79, 1.41) |
| Toxaphene b | 56 | 0.96 (0.72, 1.30) |
| Fumigants | ||
| Aluminum Phosphide | 20 | 1.13 (0.70, 1.83) |
| Carbon Tetrachloride/Carbon Disulfide b | 32 | 1.39 (0.93, 2.09) |
| Ethylene Dibromide b,c | 17 | 0.86 (0.51, 1.46) |
| Methyl Bromide | 48 | 0.86 (0.60, 1.23) |
| Fungicides | ||
| Benomyl b | 42 | 1.09 (0.74, 1.60) |
| Captan | 32 | 1.19 (0.81, 1.74) |
| Chlorothalonil | 27 | 1.09 (0.71, 1.66) |
| Maneb/Mancozeb | 35 | 0.86 (0.57, 1.29) |
| Metalaxyl | 65 | 0.66 (0.47, 0.94) |
a Model adjusted for age, race, state, pack-years of cigarettes and pipe smoking.
b No longer registered for use in the USA.
c Results available on ever use only.
Table 3 shows the associations between cumulative intensity-weighted lifetime days of herbicide use and risk of bladder cancer overall and stratified by smoking status. We observed positive trends for 2,4,5-T [RR in tertile 3 (T3) vs non-exposed = 2.64, 95% CI: 1.23, 5.68, P -trend = 0.02], 2,4-D [RR in quartile 4 (Q4) vs non-exposed = 1.88, 95% CI: 0.94, 3.77, P -trend = 0.02], glyphosate (RR in Q4 vs non-exposed = 1.93, 95% CI: 0.95, 3.91, P -trend = 0.03), and imazethapyr (RR in Q4 vs. non-exposed = 3.03, 95% CI: 1.46, 6.29, P -trend = 0.004) among never smokers. There was evidence of effect modification by smoking on the relationship between cumulative intensity-weighted days of imazethapyr and bladder cancer ( P -interaction = 0.005). An inverse trend with 2,4,5-T among former smokers, and a borderline inverse trend with dicamba among current smokers, were also observed.
Table 3.
Cumulative intensity-weighted days for herbicide use and risk of bladder cancer, overall and stratified by smoking status
| Pesticide |
OVERALL
|
NEVER
|
FORMER
|
CURRENT
|
|||||
|---|---|---|---|---|---|---|---|---|---|
|
n
= 321 cancers
|
n
= 83 cancers
|
n
= 161 cancers
|
n
= 69 cancers
|
||||||
| Cases | RR a (95% CI) | Cases | RR b (95% CI) | Cases | RR c (95% CI) | Cases | RR b (95% CI) | p-interaction | |
| 2,4,5-T d | |||||||||
| Non-exposed | 122 | Ref | 28 | Ref | 70 | Ref | 22 | Ref | |
| T1 | 14 | 1.35 (0.77, 2.36) | 4 | 1.73 (0.60, 4.99) | 8 | 1.16 (0.56, 2.43) | 1 | ** | |
| T2 | 14 | 0.99 (0.56, 1.73) | 2 | 0.63 (0.15, 2.66) | 9 | 1.00 (0.50, 2.02) | 3 | 1.54 (0.46, 5.23) | |
| T3 | 15 | 0.83 (0.48, 1.42) | 9 | 2.64 (1.23, 5.68) | 3 | 0.25 (0.08, 0.81) | 3 | 1.12 (0.33, 3.77) | |
| p-trend | 0.45 | 0.02 | 0.02 | 0.82 | 0.02 | ||||
| 2,4-D | |||||||||
| Non-exposed | 61 | Ref | 13 | Ref | 31 | Ref | 17 | Ref | |
| Q1 | 60 | 1.25 (0.86, 1.82) | 13 | 0.99 (0.44, 2.25) | 34 | 1.26 (0.74, 2.14) | 13 | 1.41 (0.67, 2.94) | |
| Q2 | 61 | 1.01 (0.70, 1.47) | 18 | 1.19 (0.58, 2.44) | 30 | 0.87 (0.51, 1.48) | 13 | 1.16 (0.54, 2.48) | |
| Q3 | 61 | 0.89 (0.61, 1.30) | 16 | 0.90 (0.42, 1.90) | 30 | 0.75 (0.43, 1.31) | 15 | 1.30 (0.63, 2.69) | |
| Q4 | 62 | 1.25 (0.87, 1.81) | 23 | 1.88 (0.94, 3.77) | 31 | 1.12 (0.66, 1.91) | 8 | 0.83 (0.33, 2.04) | |
| p-trend | 0.31 | 0.02 | 0.69 | 0.45 | 0.65 | ||||
| Alachlor | |||||||||
| Non-exposed | 126 | Ref | 33 | Ref | 61 | Ref | 32 | Ref | |
| Q1 | 37 | 1.10 (0.75, 1.60) | 10 | 1.10 (0.54, 2.25) | 22 | 1.25 (0.76, 2.07) | 5 | 0.71 (0.26, 1.91) | |
| Q2 | 39 | 0.90 (0.63, 1.30) | 12 | 1.06 (0.54, 2.06) | 18 | 0.83 (0.49, 1.41) | 9 | 0.94 (0.44, 2.03) | |
| Q3 | 38 | 1.23 (0.85, 1.77) | 11 | 1.33 (0.67, 2.63) | 21 | 1.41 (0.85, 2.32) | 6 | 0.82 (0.34, 1.97) | |
| Q4 | 39 | 1.00 (0.70, 1.43) | 14 | 1.43 (0.77, 2.68) | 18 | 0.99 (0.59, 1.68) | 7 | 0.67 (0.29, 1.51) | |
| p-trend | 0.94 | 0.25 | 0.99 | 0.37 | 0.84 | ||||
| Atrazine | |||||||||
| Non-exposed | 89 | Ref | 23 | Ref | 52 | Ref | 14 | Ref | |
| Q1 | 53 | 1.30 (0.91, 1.86) | 23 | 1.04 (0.51, 2.11) | 29 | 1.10 (0.68, 1.76) | 11 | 2.39 (1.09, 5.27) | |
| Q2 | 55 | 0.94 (0.65, 1.36) | 22 | 0.63 (0.29, 1.36) | 23 | 0.67 (0.40, 1.12) | 21 | 2.72 (1.32, 5.62) | |
| Q3 | 56 | 0.98 (0.69, 1.39) | 26 | 0.95 (0.5,0 1.83) | 28 | 0.78 (0.48, 1.27) | 12 | 1.67 (0.77, 3.62) | |
| Q4 | 55 | 0.95 (0.67, 1.34) | 28 | 1.03 (0.54, 1.96) | 27 | 0.80 (0.50, 1.29) | 10 | 1.28 (0.56, 2.89) | |
| p-trend | 0.46 | 0.69 | 0.43 | 0.52 | 0.13 | ||||
| Butylate d | |||||||||
| Non-exposed | 115 | Ref | 35 | Ref | 58 | Ref | 19 | Ref | |
| Q1 | 16 | 1.29 (0.76, 2.19) | 3 | 0.65 (0.20, 2.13) | 11 | 1.81 (0.94, 3.49) | 2 | 1.13 (0.26, 4.92) | |
| Q2 | 15 | 1.44 (0.84, 2.49) | 3 | 0.87 (0.26, 2.84) | 10 | 1.84 (0.93, 3.64) | 2 | 1.39 (0.32, 6.04) | |
| Q3 | 16 | 0.98 (0.58, 1.66) | 3 | 0.57 (0.18, 1.88) | 10 | 1.38 (0.70, 2.73) | 3 | 0.96 (0.28, 3.29) | |
| p-trend | 0.98 | 0.36 | 0.32 | 0.98 | 0.64 | ||||
| Chlorimuron-ethyl d | |||||||||
| Non-exposed | 121 | Ref | 27 | Ref | 71 | Ref | 20 | Ref | |
| T1 | 15 | 1.07 (0.62, 1.83) | 6 | 1.66 (0.68, 4.07) | 6 | 0.75 (0.32, 1.73) | 3 | 1.30 (0.38, 4.40) | |
| T2 | 15 | 0.88 (0.51, 1.54) | 3 | 0.76 (0.23, 2.52) | 7 | 0.82 (0.37, 1.79) | 5 | 1.31 (0.44, 3.89) | |
| T3 | 17 | 0.79 (0.47, 1.31) | 8 | 1.75 (0.79, 3.88) | 6 | 0.54 (0.23, 1.24) | 3 | 0.62 (0.18, 2.09) | |
| p-trend | 0.33 | 0.21 | 0.15 | 0.43 | 0.34 | ||||
| Cyanazine | |||||||||
| Non-exposed | 175 | Ref | 48 | Ref | 87 | Ref | 40 | Ref | |
| Q1 | 25 | 0.71 (0.46, 1.10) | 6 | 0.59 (0.24, 1.46) | 17 | 0.88 (0.51, 1.51) | 2 | 0.33 (0.08, 1.40) | |
| Q2 | 25 | 0.66 (0.42, 1.03) | 9 | 0.90 (0.43, 1.89) | 10 | 0.46 (0.23, 0.94) | 6 | 0.87 (0.36, 2.09) | |
| Q3 | 24 | 1.25 (0.80, 1.95) | 5 | 0.90 (0.35, 2.31) | 12 | 1.22 (0.65, 2.30) | 7 | 1.90 (0.82, 4.40) | |
| Q4 | 26 | 0.81 (0.53, 1.24) | 9 | 1.03 (0.49, 2.15) | 14 | 0.89 (0.49, 1.59) | 3 | 0.42 (0.13, 1.37) | |
| p-trend | 0.59 | 0.76 | 0.94 | 0.31 | 0.27 | ||||
| Dicamba | |||||||||
| Non-exposed | 150 | Ref | 30 | Ref | 74 | Ref | 37 | Ref | |
| Q1 | 31 | 0.92 (0.61, 1.38) | 9 | 0.83 (0.38, 1.78) | 15 | 0.85 (0.47, 1.54) | 7 | 1.14 (0.48, 2.74) | |
| Q2 | 32 | 0.70 (0.45, 1.08) | 7 | 0.56 (0.23, 1.34) | 20 | 0.85 (0.49, 1.47) | 5 | 0.54 (0.19, 1.58) | |
| Q3 | 32 | 0.81 (0.54, 1.22) | 9 | 0.84 (0.39, 1.83) | 15 | 0.70 (0.39, 1.28) | 8 | 1.05 (0.45, 2.42) | |
| Q4 | 32 | 0.77 (0.51, 1.16) | 13 | 1.12 (0.56, 2.27) | 17 | 0.84 (0.48, 1.49) | 2 | 0.23 (0.05, 0.98) | |
| p-trend | 0.31 | 0.50 | 0.62 | 0.05 | 0.32 | ||||
| EPTC | |||||||||
| Non-exposed | 226 | Ref | 66 | Ref | 116 | Ref | 44 | Ref | |
| T1 | 15 | 0.72 (0.42, 1.23) | 3 | 0.50 (0.15, 1.60) | 8 | 0.68 (0.33, 1.4) | 4 | 1.29 (0.45, 3.70) | |
| T2 | 15 | 1.33 (0.79, 2.27) | 3 | 0.83 (0.26, 2.67) | 5 | 0.86 (0.35, 2.13) | 7 | 3.75 (1.64, 8.58) | |
| T3 | 17 | 0.96 (0.58, 1.58) | 5 | 1.02 (0.41, 2.55) | 11 | 1.23 (0.65, 2.30) | 1 | ** | |
| p-trend | 0.94 | 0.93 | 0.49 | 0.44 | 0.09 | ||||
| Glyphosate | |||||||||
| Non-exposed | 60 | Ref | 14 | Ref | 31 | Ref | 15 | Ref | |
| Q1 | 62 | 1.28 (0.86, 1.89) | 19 | 1.64 (0.75, 3.58) | 31 | 1.22 (0.72, 2.08) | 12 | 1.00 (0.46, 2.13) | |
| Q2 | 62 | 0.96 (0.65, 1.41) | 11 | 0.79 (0.35, 1.77) | 36 | 1.07 (0.64, 1.78) | 15 | 0.88 (0.41, 1.87) | |
| Q3 | 62 | 0.85 (0.58, 1.26) | 14 | 0.85 (0.37, 1.95) | 30 | 0.83 (0.49, 1.39) | 16 | 0.86 (0.40, 1.82) | |
| Q4 | 62 | 1.07 (0.73, 1.56) | 23 | 1.93 (0.95, 3.91) | 29 | 1.00 (0.58, 1.72) | 10 | 0.58 (0.25, 1.34) | |
| p-trend | 0.99 | 0.03 | 0.67 | 0.17 | 0.19 | ||||
| Imazethapyr | |||||||||
| Non-exposed | 167 | Ref | 41 | Ref | 87 | Ref | 39 | Ref | |
| Q1 | 24 | 0.82 (0.51, 1.31) | 7 | 1.00 (0.41, 2.27) | 12 | 0.77 (0.40, 1.47) | 5 | 0.79 (0.27, 2.32) | |
| Q2 | 26 | 0.96 (0.61, 1.49) | 13 | 1.88 (0.96, 3.71) | 10 | 0.71 (0.35, 1.42) | 3 | 0.51 (0.15, 1.74) | |
| Q3 | 23 | 0.92 (0.58, 1.46) | 3 | 0.46 (0.14, 1.53) | 16 | 1.27 (0.72, 2.26) | 4 | 0.70 (0.24, 2.05) | |
| Q4 bottom | 14 | 2.08 (1.18, 3.66) | 4 | 2.12 (0.74, 6.10) | 6 | 1.83 (0.78, 4.28) | 4 | 0.76 (0.26, 2.23) | |
| Q4 top | 13 | 0.94 (0.52, 1.68) | 10 | 3.03 (1.46, 6.29) | 3 | 0.47 (0.15, 1.53) | 0 | ** | |
| p-trend | 0.63 | 0.004 | 0.61 | 0.20 | 0.005 | ||||
| Metolachlor | |||||||||
| Non-exposed | 168 | Ref | 40 | Ref | 86 | Ref | 42 | Ref | |
| Q1 | 27 | 0.88 (0.58, 1.34) | 8 | 0.99 (0.44, 2.20) | 17 | 1.09 (0.63, 1.86) | 2 | 0.28 (0.07, 1.17) | |
| Q2 | 27 | 0.74 (0.49, 1.12) | 6 | 0.69 (0.29, 1.64) | 13 | 0.69 (0.38, 1.28) | 8 | 0.92 (0.43, 1.99) | |
| Q3 | 28 | 0.66 (0.44, 0.99) | 14 | 1.29 (0.69, 2.42) | 14 | 0.65 (0.36, 1.17) | 0 | ** | |
| Q4 | 28 | 0.95 (0.63, 1.44) | 10 | 1.50 (0.74, 3.01) | 14 | 0.97 (0.54, 1.75) | 4 | 0.47 (0.15, 1.46) | |
| p-trend | 0.73 | 0.18 | 0.78 | 0.12 | 0.01 | ||||
| Metribuzin d | |||||||||
| Non-exposed | 108 | Ref | 29 | Ref | 63 | Ref | 15 | Ref | |
| Q1 | 12 | 1.09 (0.59, 2.01) | 3 | 0.88 (0.26, 2.94) | 5 | 0.72 (0.29, 1.83) | 4 | 3.14 (1.00, 9.86) | |
| Q2 | 15 | 0.85 (0.49, 1.48) | 3 | 0.56 (0.16, 1.89) | 7 | 0.64 (0.29, 1.43) | 5 | 2.37 (0.82, 6.87) | |
| Q3 | 10 | 0.89 (0.46, 1.72) | 3 | 0.86 (0.26, 2.88) | 6 | 0.89 (0.38, 2.09) | 1 | ** | |
| Q4 | 17 | 0.72 (0.43, 1.22) | 6 | 0.89 (0.37, 2.19) | 8 | 0.56 (0.27, 1.20) | 2 | 0.73 (0.16, 3.32) | |
| p-trend | 0.21 | 0.86 | 0.17 | 0.48 | 0.44 | ||||
| Paraquat d | |||||||||
| Non-exposed | 130 | Ref | 33 | Ref | 70 | Ref | 24 | Ref | |
| T1 | 10 | 0.96 (0.49, 1.89) | 3 | 1.30 (0.39, 4.26) | 4 | 0.63 (0.20, 2.03) | 3 | 1.66 (0.49, 5.67) | |
| T2 | 13 | 1.64 (0.91, 2.96) | 5 | 2.97 (1.10, 8.03) | 8 | 1.96 (0.92, 4.19) | 0 | ** | |
| T3 | 12 | 1.29 (0.69, 2.40) | 3 | 2.20 (0.71, 6.87) | 7 | 1.45 (0.64, 3.28) | 2 | 0.45 (0.06, 3.48) | 0.08 |
| p-trend | 0.65 | 0.54 | 0.45 | 0.57 | |||||
| Pendimethalin d | |||||||||
| Non-exposed | 106 | Ref | 26 | Ref | 61 | Ref | 17 | Ref | |
| T1 | 19 | 1.00 (0.60, 1.67) | 3 | 0.59 (0.18, 1.96) | 12 | 1.13 (0.58, 2.20) | 3 | 0.97 (0.28, 3.35) | |
| T2 | 22 | 0.62 (0.39, 0.99) | 5 | 0.67 (0.25, 1.82) | 12 | 0.58 (0.31, 1.09) | 5 | 0.73 (0.25, 2.10) | |
| T3 | 23 | 1.11 (0.67, 1.84) | 10 | 2.08 (0.91, 4.75) | 9 | 0.89 (0.42, 1.86) | 4 | 0.92 (0.30, 2.82) | |
| p-trend | 0.67 | 0.11 | 0.80 | 0.93 | 0.49 | ||||
| Petroleum Oil/Petroleum Distillates d | |||||||||
| Non-exposed | 132 | Ref | 36 | Ref | 73 | Ref | 20 | Ref | |
| T1 | 10 | 0.90 (0.46, 1.77) | 2 | 0.68 (0.16, 2.84) | 5 | 0.71 (0.26, 1.95) | 3 | 2.17 (0.64, 7.33) | |
| T2 | 10 | 0.70 (0.37, 1.34) | 1 | ** | 6 | 0.78 (0.34, 1.80) | 3 | 1.34 (0.39, 4.58) | |
| T3 | 11 | 1.10 (0.59, 2.04) | 3 | 1.17 (0.36, 3.80) | 6 | 1.09 (0.47, 2.51) | 2 | 1.40 (0.32, 6.03) | |
| p-trend | 0.78 | 0.82 | 0.83 | 0.70 | 0.63 | ||||
| Trifluralin | |||||||||
| Non-exposed | 133 | Ref | 36 | Ref | 71 | Ref | 26 | Ref | |
| Q1 | 34 | 1.23 (0.83, 1.81) | 13 | 1.39 (0.68, 2.82) | 14 | 1.02 (0.57, 1.84) | 7 | 1.48 (0.60, 3.64) | |
| Q2 | 33 | 0.76 (0.50, 1.17) | 9 | 0.76 (0.34, 1.68) | 16 | 0.64 (0.36, 1.15) | 8 | 1.10 (0.49, 2.49) | |
| Q3 | 35 | 0.89 (0.61, 1.30) | 7 | 0.63 (0.28, 1.43) | 21 | 0.95 (0.57, 1.58) | 7 | 1.17 (0.50, 2.76) | |
| Q4 | 34 | 0.86 (0.58, 1.27) | 12 | 1.14 (0.59, 2.23) | 15 | 0.72 (0.41, 1.29) | 7 | 0.92 (0.37, 2.25) | |
| p-trend | 0.39 | 0.86 | 0.35 | 0.75 | 0.80 | ||||
a Model adjusted for age, race, state, pack-years of cigarettes and pipe smoking.
b Model adjusted for age, race, state.
c Model adjusted for age, race, state, pipe smoking.
d Detailed information for these chemicals was collected on the take-home questionnaire at enrolment.
Table 4 shows the associations between cumulative intensity-weighted lifetime days of insecticide use and risk of bladder cancer overall and stratified by smoking status. Overall, there were no positive trends in risk with increasing levels of insecticide use. Among never smokers, positive gradients in risk were observed with increasing use of two carbamate insecticides, aldicarb [RR high (M2) vs non-exposed = 4.04, 95% CI: 1.20, 13.57, P -trend = 0.03] and carbofuran (RR in T2 vs non-exposed = 1.99, 95% CI: 1.06, 3.75, P -trend = 0.03), two organochlorine insecticides, chlordane (RR T3 vs non-exposed = 2.83. 95% CI: 1.16, 6.90, P -trend = 0.02) and toxaphene (RR high vs non-exposed = 3.75, 95% CI: 1.57, 8.97, P -trend = 0.003), one organophosphate insecticide, fonofos (RR T3 vs non-exposed = 2.01, 95% CI: 1.01, 4.00, P -trend = 0.05) and one pyrethroid insecticide, permethrin use (RR high vs non-exposed = 2.28, 95% CI: 1.08, 4.82, P -trend = 0.04). No trends were observed between bladder cancer and pesticides among former or current smokers. The interaction between exposure and smoking was only evident for carbofuran ( P -interaction = 0.04) and chlorpyrifos ( P -interaction = 0.01).
Table 4.
Cumulative intensity-weighted days for insecticide use and risk of bladder cancer, overall and stratified by smoking status
| Pesticide |
OVERALL
|
NEVER
|
FORMER
|
CURRENT
|
|||||
|---|---|---|---|---|---|---|---|---|---|
|
n
= 321 cancers
|
n
= 83 cancers
|
n
= 161 cancers
|
n
= 69 cancers
|
||||||
| Cases | RR a (95% CI) | Cases | RR b (95% CI) | Cases | RR c (95% CI) | Cases | RR b (95% CI) | p-interaction | |
| Aldicarb d,h | |||||||||
| Non-exposed | 153 | Ref | 39 | Ref | 85 | Ref | 26 | Ref | |
| M1 | 8 | 1.18 (0.56, 2.48) | 2 | 1.75 (0.39, 7.94) | 3 | 0.73 (0.22, 2.39) | 2 | 1.42 (0.30, 6.65) | |
| M2 | 8 | 1.25 (0.56, 2.79) | 4 | 4.04 (1.20, 13.57) | 2 | 0.71 (0.17, 2.98) | 2 | 0.81 (0.09, 6.88) | |
| p-trend | 0.58 | 0.03 | 0.61 | 0.84 | 0.23 | ||||
| Aldrin e,h | |||||||||
| Non-exposed | 113 | Ref | 30 | Ref | 59 | Ref | 21 | Ref | |
| T1 | 15 | 0.88 (0.50, 1.53) | 6 | 1.38 (0.55, 3.48) | 9 | 0.94 (0.46, 1.94) | 0 | ** | |
| T2 | 18 | 1.61 (0.96, 2.68) | 1 | ** | 11 | 1.75 (0.90, 3.40) | 6 | 2.98 (1.15, 7.71) | |
| T3 | 17 | 1.51 (0.89, 2.55) | 6 | 2.30 (0.92, 5.75) | 9 | 1.44 (0.71, 2.96) | 2 | 1.01 (0.23, 4.40) | |
| p-trend | 0.08 | 0.12 | 0.21 | 0.57 | 0.05 | ||||
| Carbaryl d,h | |||||||||
| Non-exposed | 73 | Ref | 23 | Ref | 34 | Ref | 14 | Ref | |
| Q1 | 25 | 1.10 (0.68, 1.78) | 6 | 0.82 (0.31, 2.17) | 15 | 1.25 (0.66, 2.38) | 4 | 1.25 (0.41, 3.82) | |
| Q2 | 28 | 1.93 (1.21, 3.09) | 5 | 1.06 (0.36, 3.12) | 16 | 2.35 (1.25, 4.41) | 7 | 2.77 (1.10, 7.00) | |
| Q3 | 26 | 1.49 (0.92, 2.41) | 6 | 1.50 (0.57, 3.91) | 13 | 1.38 (0.68, 2.81) | 6 | 1.94 (0.69, 5.42) | |
| Q4 | 27 | 0.91 (0.55, 1.50) | 6 | 0.90 (0.32, 2.53) | 18 | 1.19 (0.60, 2.34) | 2 | 0.34 (0.07, 1.61) | |
| p-trend | 0.29 | 0.84 | 0.90 | 0.08 | 0.45 | ||||
| Carbofuran d | |||||||||
| Non-exposed | 206 | Ref | 50 | Ref | 110 | Ref | 46 | Ref | |
| T1 | 21 | 0.52 (0.33, 0.82) | 4 | 0.39 (0.14, 1.09) | 13 | 0.55 (0.31, 0.97) | 4 | 0.62 (0.22, 1.73) | |
| T2 | 23 | 0.98 (0.64, 1.51) | 12 | 1.99 (1.06, 3.75) | 8 | 0.65 (0.32, 1.33) | 3 | 0.60 (0.19, 1.92) | |
| T3 | 22 | 0.90 (0.58, 1.40) | 11 | 1.81 (0.94, 3.50) | 7 | 0.55 (0.26, 1.19) | 4 | 0.73 (0.26, 2.05) | |
| p-trend | 0.77 | 0.03 | 0.12 | 0.51 | 0.04 | ||||
| Chlordane e,h | |||||||||
| Non-exposed | 120 | Ref | 33 | Ref | 60 | Ref | 24 | Ref | |
| T1 | 14 | 1.21 (0.69, 2.12) | 1 | 0.35 (0.05, 2.56) | 12 | 1.75 (0.94, 3.26) | 1 | ** | |
| T2 | 15 | 0.78 (0.45, 1.34) | 3 | 0.62 (0.19, 2.03) | 10 | 0.93 (0.47, 1.82) | 2 | 0.66 (0.16, 2.83) | |
| T3 | 15 | 1.46 (0.85, 2.52) | 6 | 2.83 (1.16, 6.90) | 8 | 1.34 (0.64, 2.84) | 1 | ** | |
| p-trend | 0.24 | 0.02 | 0.55 | ** | 0.27 | ||||
| Chlorpyrifos f | |||||||||
| Non-exposed | 200 | Ref | 45 | Ref | 117 | Ref | 38 | Ref | |
| Q1 | 22 | 0.67 (0.43, 1.05) | 8 | 1.02 (0.47, 2.21) | 7 | 0.34 (0.16, 0.73) | 7 | 1.34 (0.60, 3.00) | |
| Q2 | 23 | 0.84 (0.54, 1.31) | 6 | 0.86 (0.37, 2.01) | 7 | 0.43 (0.18, 0.99) | 10 | 2.08 (1.03, 4.17) | |
| Q3 | 23 | 0.99 (0.64, 1.54) | 11 | 1.86 (0.96, 3.61) | 10 | 0.74 (0.37, 1.46) | 2 | 0.55 (0.13, 2.31) | |
| Q4 | 23 | 0.69 (0.45, 1.06) | 10 | 1.23 (0.62, 2.44) | 9 | 0.50 (0.25, 0.98) | 4 | 0.54 (0.19, 1.53) | |
| p-trend | 0.14 | 0.42 | 0.06 | 0.19 | 0.01 | ||||
| Coumaphos f | |||||||||
| Non-exposed | 245 | Ref | 74 | Ref | 121 | Ref | 50 | Ref | |
| M1 | 8 | 0.49 (0.24, 0.99) | 2 | 0.36 (0.09, 1.49) | 4 | 0.46 (0.17, 1.25) | 2 | 0.78 (0.19, 3.20) | |
| M2 | 11 | 1.79 (0.98, 3.27) | 0 | ** | 9 | 2.91 (1.48, 5.73) | 2 | 1.66 (0.40, 6.86) | |
| p-trend | 0.09 | ** | 0.003 | 0.50 | 0.07 | ||||
| Diazinon f,h | |||||||||
| Non-exposed | 133 | Ref | 39 | Ref | 70 | Ref | 22 | Ref | |
| T1 | 11 | 0.76 (0.41, 1.40) | 1 | ** | 8 | 0.99 (0.47, 2.06) | 2 | 0.97 (0.23, 4.11) | |
| T2 | 10 | 0.52 (0.26, 1.04) | 1 | ** | 6 | 0.40 (0.14, 1.15) | 3 | 1.56 (0.47, 5.21) | |
| T3 | 13 | 1.03 (0.56, 1.90) | 2 | 0.78 (0.18, 3.35) | 7 | 1.06 (0.47, 2.37) | 3 | 1.07 (0.24, 4.66) | |
| p-trend | 0.96 | ** | 0.95 | 0.86 | 0.34 | ||||
| DDT e,h | |||||||||
| Non-exposed | 102 | Ref | 31 | Ref | 48 | Ref | 21 | Ref | |
| Q1 | 15 | 0.96 (0.55, 1.66) | 4 | 0.98 (0.34, 2.86) | 11 | 1.19 (0.61, 2.32) | 0 | ** | |
| Q2 | 16 | 1.43 (0.84, 2.44) | 1 | ** | 13 | 1.97 (1.05, 3.67) | 2 | 1.25 (0.29, 5.41) | |
| Q3 | 15 | 0.76 (0.43, 1.32) | 4 | 0.80 (0.27, 2.34) | 6 | 0.56 (0.24, 1.33) | 4 | 1.24 (0.41, 3.72) | |
| Q4 | 16 | 1.11 (0.64, 1.90) | 4 | 1.29 (0.44, 3.79) | 11 | 1.40 (0.71, 2.73) | 1 | ** | |
| p-trend | 0.78 | 0.59 | 0.48 | 0.34 | 0.18 | ||||
| DDVP f | |||||||||
| Non-exposed | 253 | Ref | 69 | Ref | 129 | Ref | 55 | Ref | |
| M1 | 12 | 0.85 (0.47, 1.54) | 3 | 0.65 (0.20, 2.08) | 8 | 1.04 (0.51, 2.15) | 1 | ** | |
| M2 | 12 | 0.93 (0.52, 1.67) | 4 | 1.05 (0.38, 2.89) | 7 | 0.97 (0.45, 2.09) | 1 | ** | |
| p-trend | 0.82 | 0.92 | 0.94 | ** | 0.77 | ||||
| Fonofos f | |||||||||
| Non-exposed | 220 | Ref | 57 | Ref | 116 | Ref | 47 | Ref | |
| T1 | 15 | 0.72 (0.42, 1.22) | 5 | 0.88 (0.35, 2.23) | 7 | 0.57 (0.26, 1.24) | 3 | 0.93 (0.29, 3.05) | |
| T2 | 17 | 0.92 (0.56, 1.53) | 5 | 1.01 (0.40, 2.57) | 9 | 0.86 (0.43, 1.71) | 3 | 0.92 (0.28, 2.99) | |
| T3 | 18 | 0.92 (0.57, 1.50) | 10 | 2.01 (1.01, 4.00) | 7 | 0.64 (0.30, 1.39) | 1 | ** | |
| p-trend | 0.78 | 0.05 | 0.28 | 0.20 | 0.37 | ||||
| Heptachlor e,h | |||||||||
| Non-exposed | 139 | Ref | 34 | Ref | 76 | Ref | 26 | Ref | |
| M1 | 14 | 0.82 (0.46, 1.44) | 4 | 0.91 (0.31, 2.66) | 7 | 0.65 (0.30, 1.44) | 3 | 1.49 (0.44, 5.11) | |
| M2 | 14 | 1.10 (0.63, 1.93) | 6 | 1.91 (0.78, 4.70) | 8 | 1.06 (0.51, 2.23) | 0 | ** | |
| p-trend | 0.75 | 0.15 | 0.89 | ** | 0.21 | ||||
| Lindane e | |||||||||
| Non-exposed | 139 | Ref | 36 | Ref | 77 | Ref | 23 | Ref | |
| M1 | 12 | 0.77 (0.43, 1.37) | 4 | 0.82 (0.29, 2.32) | 5 | 0.56 (0.22, 1.39) | 3 | 1.49 (0.44, 5.03) | |
| M2 | 12 | 1.43 (0.78, 2.62) | 4 | 2.00 (0.71, 5.63) | 6 | 1.21 (0.53, 2.81) | 2 | 1.62 (0.38, 6.97) | |
| p-trend | 0.27 | 0.20 | 0.72 | 0.45 | 0.54 | ||||
| Malathion f,h | |||||||||
| Non-exposed | 49 | Ref | 17 | Ref | 24 | Ref | 7 | Ref | |
| Q1 | 28 | 1.00 (0.62, 1.59) | 4 | 0.35 (0.11, 1.11) | 17 | 1.16 (0.62, 2.17) | 6 | 1.88 (0.62, 5.67) | |
| Q2 | 27 | 1.15 (0.71, 1.86) | 9 | 1.09 (0.49, 2.43) | 13 | 1.03 (0.52, 2.04) | 5 | 1.80 (0.57, 5.72) | |
| Q3 | 29 | 1.14 (0.71, 1.83) | 9 | 1.05 (0.45, 2.44) | 15 | 1.13 (0.59, 2.15) | 4 | 1.26 (0.33, 4.90) | |
| Q4 | 29 | 0.95 (0.60, 1.52) | 6 | 0.66 (0.26, 1.71) | 19 | 1.11 (0.60, 2.04) | 4 | 1.17 (0.34, 4.01) | |
| p-trend | 0.73 | 0.63 | 0.85 | 0.82 | 0.44 | ||||
| Parathion f,h | |||||||||
| Non-exposed | 148 | Ref | 41 | Ref | 77 | Ref | 27 | Ref | |
| M1 | 7 | 1.05 (0.49, 2.26) | 2 | 1.09 (0.26, 4.60) | 5 | 1.28 (0.51, 3.19) | 0 | ** | |
| M2 | 8 | 1.13 (0.55, 2.36) | 1 | ** | 5 | 1.39 (0.54, 3.54) | 2 | 1.54 (0.35, 6.84) | |
| p-trend | 0.74 | ** | 0.90 | ** | 0.62 | ||||
| Permethrin g | |||||||||
| Non-exposed | 239 | Ref | 64 | Ref | 123 | Ref | 52 | Ref | |
| T1 | 13 | 0.92 (0.52, 1.61) | 4 | 0.96 (0.36, 2.65) | 7 | 0.90 (0.42, 1.93) | 2 | 0.79 (0.19, 3.26) | |
| T2 | 13 | 0.45 (0.25, 0.81) | 4 | 0.46 (0.17, 1.28) | 5 | 0.33 (0.13, 0.81) | 4 | 0.75 (0.25, 2.25) | |
| T3 | 15 | 1.11 (0.65, 1.87) | 8 | 2.28 (1.08, 4.82) | 5 | 0.72 (0.30, 1.77) | 2 | 0.62 (0.15, 2.58) | |
| p-trend | 0.93 | 0.04 | 0.31 | 0.49 | 0.44 | ||||
| Phorate f,h | |||||||||
| Non-exposed | 115 | Ref | 30 | Ref | 62 | Ref | 21 | Ref | |
| T1 | 16 | 0.74 (0.43, 1.27) | 4 | 0.61 (0.21, 1.76) | 8 | 0.66 (0.31, 1.42) | 4 | 1.24 (0.41, 3.73) | |
| T2 | 16 | 0.99 (0.58, 1.69) | 3 | 0.64 (0.19, 2.13) | 10 | 1.13 (0.57, 2.26) | 2 | 0.89 (0.21, 3.87) | |
| T3 | 17 | 0.98 (0.58, 1.64) | 7 | 1.42 (0.62, 3.28) | 8 | 0.89 (0.42, 1.88) | 2 | 0.71 (0.17, 3.07) | |
| p-trend | 0.96 | 0.36 | 0.90 | 0.62 | 0.76 | ||||
| Terbufos f | |||||||||
| Non-exposed | 182 | Ref | 47 | Ref | 96 | Ref | 39 | Ref | |
| T1 | 29 | 0.83 (0.56, 1.24) | 7 | 0.76 (0.34, 1.71) | 14 | 0.68 (0.38, 1.20) | 8 | 1.48 (0.68, 3.20) | |
| T2 | 30 | 0.93 (0.63, 1.38) | 16 | 1.77 (0.99, 3.15) | 10 | 0.59 (0.31, 1.14) | 4 | 0.69 (0.24, 1.94) | |
| T3 | 30 | 0.82 (0.55, 1.21) | 8 | 0.80 (0.38, 1.71) | 18 | 0.92 (0.55, 1.55) | 4 | 0.57 (0.20, 1.59) | |
| p-trend | 0.35 | 0.74 | 0.81 | 0.22 | 0.11 | ||||
| Toxaphene e,h | |||||||||
| Non-exposed | 135 | Ref | 30 | Ref | 77 | Ref | 25 | Ref | |
| M1 | 13 | 1.13 (0.64, 2.01) | 6 | 2.34 (0.97, 5.68) | 5 | 0.74 (0.30, 1.84) | 2 | 1.14 (0.27, 4.86) | |
| M2 | 16 | 1.40 (0.82, 2.39) | 7 | 3.75 (1.57, 8.97) | 8 | 1.10 (0.52, 2.33) | 1 | ** | |
| p-trend | 0.24 | 0.003 | 0.80 | ** | 0.09 | ||||
a Model adjusted for age, race, state, pack-years of cigarettes and pipe smoking.
b Model adjusted for age, race, state.
c Model adjusted for age, race, state, pipe smoking.
d Carbamate insecticide.
e Organochlorine insecticide.
f Organophosphate insecticide.
g Pyrethroid insecticide.
h Detailed information for these chemicals was collected on the take-home questionnaire at enrolment.
There were no associations overall or among any of the smoking strata for use of any fumigants or fungicides evaluated (Supplementary Table 2 , available as Supplementary data at IJE online) and bladder cancer, with the exception of a positive association among smokers using carbon tetrachloride/carbon disulfide, which was based on only three exposed cases. In addition, Supplementary Table 3 (available as Supplementary data at IJE online) provides stratified risks of bladder cancer by smoking status for those pesticides with no cumulative use information. No notable differences in observed associations emerged from analyses of lifetime days or from lagged exposures and these are, therefore, not shown.
Discussion
In this analysis, we saw associations between two imidazolinone herbicides, imazethapyr and imazaquin which are aromatic amines, and bladder cancer risk. Ever use of other herbicides, including the general use pesticides bentazon and bromoxynil, the chlorophenoxy herbicide diclofop-methyl and another chlorinated herbicide chloramben, were also associated with bladder cancer. Increased risks of bladder cancer were also observed with regard to use of the chlorinated insecticide DDT; however, no consistent exposure-response relationship was observed in expanded analyses.
Imazethapyr is an imidazolinone herbicide used to control weeds in corn, soybean, dry bean, alfalfa and other crops. 28 Imazaquin is a general-use pesticide used to control grasses and broadleaf weeds. 29 In a previous analysis in the AHS focusing on risk of all cancer in a subcohort of applicators that used imazethapyr, we reported a relationship between imazethapyr and bladder cancer based on 41 exposed cases. In this analysis, which includes 6–7 years of additional follow-up and an additional 100 exposed cases, we did not observe an overall association with imazethapyr. An exposure-response relationship, however, was observed ( P -trend = 0.004) among never smokers, with the highest category of exposure experiencing a 3-fold risk. We also observed that ever use of another imidazolinone herbicide, imazaquin, was associated with bladder cancer risk. Although neither herbicide has demonstrated evidence of carcinogenicity in mice or rats, there is some plausibility for a possible link between exposure to imazethapyr and imazaquin and risk of bladder cancer because these herbicides are aromatic amine compounds, a chemical class which has been linked to bladder cancer, and animal metabolism studies show that these pesticides are readily excreted in the urine predominantly as the parent aromatic compounds. 28,29 The risk associated with imazethapyr exposure, however, was predominantly observed only among a smaller group of never smokers and it was not possible to evaluate quantitative exposure for imazaquin, and thus findings are unclear. Neither imazethapyr nor imazaquin have undergone a complete evaluation for evidence of human carcinogenic potential by the USA Environmental Protection Agency (U.S. EPA) or the International Agency for Research on Cancer (IARC). We are unaware of any other epidemiological study outside the AHS that has evaluated exposure to these pesticides as possible risk factors for cancer.
We also observed an increased risk of bladder cancer associated with ever use of the herbicides bentazon and bromoxynil. Bentazon and bromoxynil are used on a variety of food crops but are also used on lawns, turfs and golf courses. In our data, ever use of bentazon and bromoxynil were moderately correlated (r = 0.54). When we mutually adjusted models for these two herbicides, the results for both became non-significant. However, whereas the magnitude of the effect for bromoxynil diminished, the effect of bentazon was similar to that observed overall, and additional analyses stratified by smoking status also showed a strong association between bentazon and bladder cancer among never smokers (RR = 2.14, 95% CI: 1.09, 4.21, Supplementary Table 3 , available as Supplementary data at IJE online), suggesting the effect is unlikely to be due to smoking and that bentazon might be more important in driving the observed bladder cancer risk than bromoxynil. There are limited experimental data on bentazon as a bladder carcinogen. In a combined chronic toxicity-carcinogenicity study in rats, 30 bentazon was found to result in increases in urine volume along with reduced urinary specific gravity, which may be related to bladder cancer risk. 31 Although there are few other data to support our findings regarding bentazon and bromoxynil, the use of these pesticides in both agricultural and general-use purposes indicates additional evaluation is warranted. Bentazon has been classified as a Group E carcinogen, evidence of non-carcinogenicity to humans, by the U.S. EPA based on animal models 30 and bromoxynil has been classified as a Group C, possible human carcinogen, based on observed liver tumours in animals; 32 neither have been evaluated by IARC.
Several chlorinated pesticides were also shown to influence bladder cancer risk in our analyses. Chloramben is an herbicide used to control weeds on soybean and other crops. No information is available on the carcinogenic effects of chloramben in humans, although a US study reported that oral exposure to chloramben caused liver tumours in mice but not in rats. 33 We also found that ever use of the organochlorine insecticide DDT increased bladder cancer risk, but no trend in risk with increasing use was observed. This may be due, in part, to the lack of detailed information from more than half of those reporting being ever exposed to DDT (only 46% reported days and years of use). Two other organochlorine insecticides, chlordane and toxaphene, showed evidence of increased bladder cancer risk but only among never smokers. Organochlorine insecticides have been linked to several cancer sites, 34 but we are unaware of any studies suggesting a link with bladder cancer.
In subgroup analyses, we also observed some interesting associations between several herbicides and insecticides and bladder cancer among never smokers. Never smoking applicators with the highest use of the chlorophenoxy herbicides 2,4,5-T and 2,4-D had higher risk of bladder cancer, and heavy users of the herbicide glyphosate had increased risk as well. Recently, a cohort of chlorophenoxy herbicide manufacturing workers in The Netherlands was observed to have excess bladder cancer mortality, in particular among workers involved in the manufacture of 2,4,5-T. 35 Because the numbers of observed bladder cancer deaths in this and other manufacturing cohorts was small, 36,37 it is difficult to draw a definitive conclusion. Observational studies in dogs showed that exposure to herbicide-treated lawns, in particular those treated with phenoxy herbicides, was associated with higher bladder cancer risk. 38,39 Interestingly we also observed a positive association between another chlorophenoxy herbicide, diclofop-methyl, and bladder cancer, albeit among few exposed cases ( n = 11). Diclofop-methyl is classified as likely to be carcinogenic to humans by the U.S. EPA 40 and IARC ranks chlorophenoxy herbicides as possibly carcinogenic to humans (Group 2B). Taken together, these data suggest a possible link between chlorophenoxy herbicide exposure and bladder cancer. Several insecticides showed higher risk of bladder cancer among the never smokers as well, but power was limited to draw conclusions as the numbers of exposed cases were often small, given their lower prevalence of use.
An interesting element of this analysis is the observed differences in risk among never smokers for multiple chemicals. Since cigarette smoking is the major risk factor for bladder cancer, it is perhaps not surprising that smoking may obscure the effect of another exposure, particularly if that effect is weaker than the smoking effect. Recently, a study of agricultural workers in Egypt found that the associations between farming and bladder cancer were more evident among those who never smoked, and there are other historical examples of positive risks for bladder cancer in association with several factors among never smokers. 14,41–43 A common challenge in these studies, as in ours, is the low precision of estimated associations and lack of statistical interaction, given that the number of never smokers who develop bladder cancer is small. Thus, much larger studies will be needed to fully evaluate a relationship between pesticides, smoking and risk of bladder cancer. Along the same lines, studies have also suggested an interaction with smoking for some exposures, where risk can either be potentiated 42 or diminished 44 across smoking strata. These data and ours suggest that evaluating possible bladder cancer risk factors such as pesticides across strata of smoking may provide valuable insights into bladder cancer risk; however, large studies will be needed to be able to detect risks among specific subgroups and true interactions.
Our study had both strengths and limitations. Detailed self-reported pesticide use information, at two points in time, was used to evaluate cancer risk. Information on pesticide use provided by farmers in the AHS has been found to be accurate and reliable, 45,46 allowing for this exploration of the relationship between specific pesticide exposures and bladder cancer risk. Nonetheless, there is potential for exposure misclassification though it is probably non-differential and would bias relative risks toward the null, diminishing any real exposure-response gradients. 47 Smoking status information was collected at enrolment for use in analyses but also reconciled with data from two follow-up questionnaires that allowed us to carefully characterize this important bladder cancer risk factor. In addition, we performed several sensitivity analyses related to smoking, including exploring adjustment for status and intensity and status and duration, which provided comparable results. We also had information on the ever use of other tobacco products reported at enrolment. Using detailed questionnaire data, we were also able to control for several other suggested bladder cancer risk factors, including exposure to diesel exhaust 48 and grinding metal, 49 none of which changed the estimates between pesticide exposures and bladder cancer risk. In addition we were able to take into consideration the use of pesticides that were correlated with the pesticide of interest and, except for where stated (bentazon and bromoxynil), we found only weak correlation among pesticides, whcih did not influence the calculated risk estimates. Although we evaluated a large number of pesticides ( n = 65), we observed more positive associations than would have been expected by chance alone (6 observed less than P = 0.05 and 3 additional borderline positive associations, wheras 3.25 (or 5%) would have been expected by chance, Table 2 ). Still, we cannot rule out the possibility that some of our findings might be due to chance, in particular in some of the stratified analyses where the number of exposed cases is small. Thus, future follow-up in the AHS to further evaluate the relationship between pesticides and bladder cancer, and to evaluate whether smoking modifies this relationship, are anticipated.
In conclusion, we observed increased risk of bladder cancer with two aromatic amine herbicides, the imidazolinone herbicides imazethapyr and imazaquin. The relationship between bladder cancer and imazethapyr, as well as for several other agricultural and general use herbicides, was more apparent among never smokers and highlights the complexity of trying to understand the impact of other exposures on smoking-related cancers. Associations with bladder cancer incidence and use of several chlorinated pesticides, including chlorophenoxy herbicides and organochlorine insecticides, were observed for the first time. Because farmers generally have lower rates of bladder cancer compared with the general population, few studies have explored whether pesticides, which readily pass through the bladder, might be risk factors for bladder cancer. Collectively, our data suggest that pesticide exposure may be an overlooked exposure in bladder carcinogenesis. Future studies with detailed pesticide information on specific active ingredients and those that explore risks across smoking status are needed.
Funding
This work was supported by the Intramural Research Program of the National Institutes of Health, NCI, Division of Cancer Epidemiology and Genetics (Z01CP010119), NIEHS (Z01ES0490300), the Iowa Cancer Registry (HHSN261201300020I) and Iowa’s Holden Comprehensive Cancer Center (P30CA086862) as well as the NIEHS-funded Environmental Health Sciences Research Center at the University of Iowa (P30ES005605).
Acknowledgement
We thank the participants of the Agricultural Health Study.
Conflict of interest: None declared.
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