Cancer, Herbicides, and Pesticides: What’s the Link?

Written by: Lukas Harnisch-Weidauer
Medically Reviewed By: Timothy Richard Rebbeck, PhD

While pesticides and herbicides — substances used to control pests and weeds, respectively — can be toxic to humans when ingested, researchers are not sure if exposure to these compounds are linked to a higher risk of cancer. Timothy Rebbeck, PhD, a cancer epidemiologist at Dana-Farber Cancer Institute and the Harvard T.H. Chan School of Public Health, says this is because observational research has produced mixed results.

It’s incredibly difficult to measure the effect of environmental exposures, particularly herbicides and pesticides, on cancer. Large-scale cohort studies that follow exposed individuals over a long period of time are usually required to answer these questions. While some such studies have been done, Rebbeck says they still have not resolved whether there are links to many herbicides and pesticides.

“Not only is it difficult to study large cohorts of subjects, but it requires asking people to remember their exposures. Furthermore, the effects of these exposures on cancer risk may be small, may take decades to appear, and there are often other factors that can impact an individual’s cancer risk,” he says. “The question of whether cancer is caused by historical exposures to herbicides and pesticides is very difficult to conclusively resolve.”

What U.S. regulatory organizations say

Concerns about these chemicals come from a valid place: The historical misuse of pesticides like dichlorodiphenyltrichloroethane (DDT), an insecticide, has led to distrust of these substances. Although the EPA ordered that DDT use be discontinued in 1972, and its concentration in the environment has decreased since then, its effects continue to persist.

A 2015 study tracked nearly 15,000 mothers, daughters, and granddaughters living in the San Francisco Bay area. Researchers measured DDT levels in the mothers of 118 women who were diagnosed with breast cancer by age 52, and in the mothers of 354 women without breast cancer. They found that daughters of women with the highest DDT exposures were 3.7 times more likely to develop breast cancer than daughters of women with lower exposures.

Under the Food Quality Protection Act of 1996, the Environmental Protection Agency (EPA) is able to ensure that all pesticides used on food in the United States meet stringent safety standards. The agency evaluates new and existing pesticides to ensure they can be used with a reasonable certainty of safety to infants and adults.

Each new pesticide and herbicide registered used on food in the United States is evaluated for its carcinogenicity (or potential to cause cancer) through studies on laboratory animals and humans.

For example, in its review of the widely used-herbicide glyphosate (an active ingredient in the popular herbicide Round-Up), the EPA “found that there are no risks of concern to human health when glyphosate is used in accordance with its current label.”

However, in 2015, the World Health Organization’s International Agency for Research on Cancer (IARC) said that glyphosate can potentially cause cancer. However, this assertion was based on what they called “limited evidence of cancer in humans” and “sufficient evidence of cancer in study animals.”

Rebbeck says that these different conclusions may stem from differing interpretation of data by experts in different disciplines.

“Environmental toxicologists, like those at IARC, will look at data and conclude that a compound like glyphosate can have effects that are carcinogenic,” says Rebbeck. “But if you ask an epidemiologist whether their exposure to glyphosate in the environment increases their risk of getting cancer in humans, they may say that there isn’t enough consistent evidence to make such a conclusion. Both perspectives are correct in a sense. Yes, it is a carcinogen on a chemical level. But, no, we don’t have enough evidence to say it causes cancer in humans.”

Should I worry about pesticides and herbicides in my food?

The American Cancer Society notes that “pesticides and herbicides can be toxic when used improperly in industrial, farming, or other workplace settings. Although vegetables and fruits sometimes contain low levels of these chemicals, overwhelming scientific evidence supports the overall health benefits and cancer-protective effects of eating vegetables and fruits. At this time there is no evidence that residues of pesticides and herbicides at the low doses found in foods increase the risk of cancer. Still, fruits and vegetables should be washed thoroughly before eating, not only to lower exposure to these compounds but also to limit the risk of health effects from germs.”

What to do if you are concerned

To relieve anxieties over the relationship between cancer and herbicides and pesticides, simply avoid the substances as much as possible. Don’t use herbicides or pesticides on your property.

If you are concerned about ingesting these chemicals through fruits and vegetables, seek out U.S. Department of Agriculture-certified organic produce in grocery stores. USDA certification means that the produce been grown without the use of synthetic pesticides.