Neighborhood pesticide exposure linked to prostate cancer
Researchers at the University of Southern California have found that men exposed to certain pesticides in Central Valley neighborhoods are at increased risk for prostate cancer, according to a study in the American Journal of Epidemiology.
The authors recruited 173 men, ages 60 to 74, from 670 diagnosed with prostate cancer in Tulare, Fresno and Kern counties, according to the state's cancer registry. They used Medicare and tax records to find 162 men ages 65 and older without prostate cancer to use as a control group.
They traced where the men lived and worked from 1974 to 1999, and compared those locations with state records of pesticide use. They found prostate cancer more prevalent among men who lived near areas sprayed with methyl bromide, captan and eight organochlorine pesticides.
Greenspace spoke with one of the study's authors, Myles Cockburn, an associate professor of preventive medicine at the University of Southern California's Keck School of Medicine, about the implications.
Q: Why did you look at these particular pesticides?
A: We had to isolate particular pesticides to determine causality. So we started with things that we had some lab-based evidence that they might lead to prostate cancer. There had to be biological plausibility.
Q: How could you be sure these people were exposed in their neighborhoods, not agricultural or other jobs?
A: We asked them if they worked in farming occupations, and our control group was a random selection from the [Central] Valley, and only about 3% worked in agriculture.
A: Other on-the-job pesticide studies assume your exposure is according to your job. But those applying pesticide often wear protective gear. Residents do not.
We often have that problem where we’re not looking at the right group of people. California's Central Valley has by far the largest use of pesticides and the largest population potentially exposed to them in the United States. In Iowa and parts of Utah, even in Washington state, there’s plenty of agriculture and pesticides, but hardly anyone lives there. In California, you have the pesticides and the people. And California also has a very good cancer registry.
Q: How could you be sure these men lived near places where these particular pesticides were used?
A: We used the state pesticide use registry and land surveys of about a mile square to estimate how much they were exposed to at their residences over time.
Q: How accurate are those records -- for instance, with the pesticide use registry, are there ways to double check where particular pesticides were used?
A: There is fairly extensive testing of how much of each pesticide is sold. There is some underreporting, but not much. There was a study few years ago that found people we think are highly exposed [according to the pesticide use registry] have high levels [of those pesticides] in their bloodstream.
The land use data also helped. For instance, one of the organocholine pesticides we looked at goes with almonds, and we could check California land use data to see where almond fields are and accurately predict where pesticides were used.
Q: How could you be sure these men were at increased risk for prostate cancer because they were exposed to pesticides, and not because of their genetics, behaviors or other variables?
A: Well, that might be true with other cancers -- lung cancer, for instance, you have a lot of other variables. But not with prostate cancer. There are not that many risk factors. We know it has something to do with hormones, but we don’t know what. There isn’t really anything you need to adjust for. We adjusted for body weight and the ways they produce endrogen. Some genes are related to over- and under-producing hormones, but those are extremely uncommon.
Q: Have you looked at the connection between pesticide exposure and other diseases?
A: We have used the same approach, the same methodology, in Parkinson’s and got good results, that is, the connection between Parkinson’s and pesticide exposures in California, in the same general area.
Q: Why do you think other researchers have not found the connections you did between these pesticides and prostate cancer?
A: We think it’s because they’ve been looking in the wrong place. We’ve been doing it with the wrong methods and missing it because either, one: we are doing occupational studies, or two: we study younger people on the job, not those who are older and sicker and no longer working. It's a “selection bias,” a “healthy worker effect.”
Q: How far away did these men live from where pesticides were applied?
A: Within 500 meters.
Q: So what does that tell us?
A: It was badly applied. They must have been drifted on, otherwise they wouldn’t have been exposed.
This is some evidence that we’re doing a very bad job of controlling how we apply pesticides, indirect evidence of pesticide drift.
Q: So what can be done?
A: There are groups that decide whether chemicals are carcinogenic. Most of these (pesticides) are considered “possible” because there is animal evidence but human evidence, mostly from occupational studies, is inconclusive.
This adds to the evidence that these things are carcinogenic. And if we think so, we should consider how we’re using them.
Q: But at least one of the pesticides, methyl bromide, has already been phased out by a lot of California farmers because it damages the ozone layer, right?
A: But that doesn’t mean we don’t still have 20 years' worth of people exposed to it. We should probably make sure they have access to cancer screening. We also need to look at alternative pesticides.
Q: What do you plan to study next?
A: We looked at these pesticides. Now we’re going to look at what else is there, other pesticides. Methyl iodide might be among them.
-- Molly Hennessy-Fiske
Photo: Myles Cockburn is an associate professor of preventive medicine at the University of Southern California's Keck School of Medicine. He recently completed a study that found neighborhood pesticide exposure was linked to prostate cancer in Central Valley men. Credit: USC.