A truck sprays anti-mosquito pyrethroid pesticides.

Pyrethroids, a type of pesticide commonly used in consumer products such as lice treatments and on conventionally grown crops, are associated with an increased risk of death, according to a new study in the journal JAMA Internal Medicine.

In the study, researchers followed people for an average of 14 years and found that those with more exposure to these chemicals were somewhat more likely to die from any cause than those with the lowest exposure. And they were three times more likely to die from heart disease in particular.

Pyrethroids, which currently account for about 30 percent of the insecticides used worldwide, may also be used in pet flea medications, and to protect against ants, roaches, mosquitoes, and ticks. The category includes permethrin, cypermethrin, deltamethrin, and cyfluthrin. (Deet and other insect repellents that are typically applied directly to skin are not pyrethroids and were not included in this study.)

more on ticks and mosquitoes

The strong link between exposure to pyrethroids and death from heart disease surprised Hans-Joachim Lehmler, Ph.D., professor of environmental health at the University of Iowa College of Public Health in Iowa City, and one of the study’s authors. He says little past evidence has hinted at any potential heart harms from these pesticides.

One small study conducted in China in 2017 did find a link between heart disease and pyrethroid exposure, but the new JAMA study is the first to examine deaths associated with these chemicals.

While the new findings may sound alarming, Lehmler says that a study like this one can’t prove cause and effect and that a lot more research is needed to confirm, or refute, these results. Here is what you need to know about pyrethroid pesticides now.

What Are Pyrethroids?

Pyrethroids are the synthetic version of pyrethrin, an insecticide derived from the chrysanthemum flower. They have been chemically altered to be more stable in sunlight, according to the Environmental Protection Agency

Their use has increased in popularity in recent years because they’re thought to be less harmful to birds and mammals than some other types of pesticides, such as organophosphates (which include malathion, used in some prescription head lice treatments and for controlling insects such as mosquitoes).

Permethrin is known for its ability to kill mosquitoes and ticks. In fact, the EPA says it’s the chemical most widely used for mosquito spraying in the U.S.—covering 9 to 10 million acres annually

And treating clothing with permethrin is one of several strategies the Centers for Disease Control and Prevention recommends that consumers employ to protect themselves from tick bites. 

Pyrethroids have some well-known short-term side effects. They can cause itchiness and irritation if they come into contact with your skin. And although ingesting or inhaling a large amount of one of these pesticides is unlikely, a significant dose could cause headache, nausea, vomiting, dizziness, muscle twitching, or loss of consciousness, according to the CDC’s Agency for Toxic Substances and Disease Registry [PDF].

But much less is known about the effects of long-term, lower-level exposure to these chemicals. The current study adds new information about those long-term effects.

What the Study Found

In the study, scientists at the University of Iowa looked at information on 2,116 adults with an average age of about 43. All had provided a urine sample to the U.S. National Health and Nutrition Examination Survey between 1999 and 2002.

The scientists tested the samples for a variety of substances, including 3-phenoxybenzoic acid (3PBA), a marker that indicates previous exposure to pyrethroids.

The scientists also looked at data on the participants’ overall health and socioeconomic status, whether they died during the study period, and, if so, their cause of death. People with the highest concentrations of 3PBA in their urine were about three times more likely to die from cardiovascular disease than those who had the lowest concentrations of the chemical marker.

The researchers took a variety of factors that may have affected their results into consideration, including quality of diet, smoking status, and income and education levels. 

But they might not have been able to control for all possibilities, so the study does not prove that pyrethroids caused the added deaths. It could be that people with the higher levels of the chemical marker in their urine were also subject to additional risk factors for heart disease and death that the research wasn’t able to measure. In addition, the researchers say the results should be interpreted with caution because the number of people who died from cardiovascular disease during the study was relatively small—just 41 out of 246 total deaths over the study period. 

What the results amount to are a suggestion that pyrethroids might be linked to an increased risk of cardiovascular disease, says Lehmler, one of the study’s authors. However, he says, “one study doesn’t mean that you’ve answered all the questions,” and more research needs to be done.

Steven Stellman, Ph.D., M.P.H., a professor of clinical epidemiology at the Mailman School of Public Health at Columbia University in New York City, who wrote a commentary that accompanied the new study, agrees. “There’s a lot of hurdles to overcome before you bring down the gavel and indict pyrethroids as a family, or even individually, as increasing the risk of total mortality or cardiovascular mortality,” Stellman says.

These pesticides, he notes, are one of our most important tools for protecting people against diseases spread by mosquitoes and ticks.

Safely Protect Yourself From Biting Bugs

It’s too early to tell whether you should stop using pyrethroids altogether. But you can take a number of measures to protect yourself from mosquito and tick bites without using any pesticides. Wearing long sleeves and pants when you might be exposed to these bugs is a good start. 

For mosquitoes, cleaning up any standing water around your home and yard helps to keep them from breeding near where you live. To make your yard less friendly to ticks, keep it mowed and free of brush and leaf litter.

And you can opt for other chemicals for preventing bug bites. Consumer Reports’ tests of insect repellents have shown that products containing 25 to 30 percent deet provide consistently high protection against mosquitoes. Some products we’ve rated that contain 20 percent picaridin or 30 percent oil of lemon eucalyptus have also performed well in our tests. Members can view our full ratings for details.

If you do choose to use permethrin to protect against tick and mosquito bites, be sure to use it safely. Never apply it directly to your skin. If you’re treating your own clothing with permethrin, apply the chemical outdoors, wash your hands well afterward, and make sure the clothes dry completely before you wear them. You should also wash your hands after you apply tick or flea medication to your dog

Consumer Reports recommends skipping pyrethroid-based lice treatments altogether. While these products were once effective, studies have shown that lice have developed resistance to them. That means using one of these treatments exposes you to potential risks of the chemicals with little chance of any benefit.

Finally, consider buying organic groceries. This will help you avoid ingesting pyrethroids, which aren’t allowed in the production of organic crops.