Nearly two dozen pesticides are associated with an increased risk of prostate cancer in the United States, researchers report November 4 in Cancer. Four of those, the study finds, are also linked to prostate cancer deaths.

The findings can’t say for certain that these pesticides caused prostate cancer, says John Leppert, a urologist at Stanford University School of Medicine. It’s unknown whether the people who were diagnosed with prostate cancer in Leppert’s data were exposed to the pesticides.

“This study is really best at finding the potential pesticides that may be linked with prostate cancer,” Leppert says, “so that we could narrow down the list of things that need to be further studied.”

Despite being the second most common cancer in the United States, some risk factors of prostate cancer remain elusive (SN: 10/16/19). “Your risk of developing prostate cancer or other cancers varies in the United States depending on where you live, and we don’t have good explanations for that geographic variation yet,” Leppert says.

A few pesticides are thought to increase prostate cancer risk, but prior studies have been patchy: They’ve focused on small geographic areas or just a few pesticides. So Leppert and colleagues looked at data on prostate cancer incidence and the use of nearly 300 pesticides in more than 3,100 U.S. counties.

Counties with a higher use of 22 particular pesticides, after adjustment for characteristics such as age distribution, were more likely to have more prostate cancer cases or deaths several years after the pesticides were used.

The team analyzed pesticide use and cancer outcomes in two different time periods. The first period focused on pesticide use from 1997 to 2001 and cancer outcomes from 2011 to 2015. The second period looked at pesticide use from 2002 to 2006 and cancer incidence from 2016 to 2020.

The long lag between pesticide use and cancer discovery exists because prostate cancer takes time to progress, Leppert says. The 22 pesticides showed links to prostate cancer incidence in both time periods. This included commonly known pesticides such as 2,4-D, an herbicide frequently used to treat weeds.

“As a clinician, I hope that as we understand [environmental exposures] better, then we can be better doctors to our patients,” says Leppert. “Hopefully, our understanding of a patient’s environment will help us to catch prostate cancer early and, if needed, to treat it better.”