Common 'Safe' Pesticides May Kill More Than Insects

Exposure to certain insect-killing chemicals, such as mosquito repellent, was linked with poor health outcomes in an analysis of federal survey data.

Among participants in the National Health and Nutrition Examination Survey (NHANES), higher levels of exposure to commonly used household pyrethroid insecticides was associated with a three times higher risk for cardiovascular disease-related mortality (hazard ratio 3.00, 95% CI 1.02-8.80), reported Wei Bao, MD, PhD, of the University of Iowa in Iowa City, and colleagues.

Compared with people in the lowest tertile of exposure to these insecticides, people in the highest tertile also had a 56% increased risk for all-cause mortality (HR 1.56, 95% CI 1.08-2.26), they wrote in JAMA Internal Medicine.

However, pyrethroid insecticides were not associated with cancer-related mortality (HR 0.91, 95% CI 0.31-2.72), the authors noted.

These models were adjusted for race/ethnicity, sex, age, BMI, creatinine levels, dietary, lifestyle, and sociodemographic factors.

Approved for use by the U.S. Environmental Protection Agency, pyrethroid insecticides are most commonly used for mosquito repellents, head lice, pet shampoos and spray, and other indoor and outdoor pest controls, and have been considered relatively safe.

"Although more than 1,000 pyrethroids have been made, only about a dozen pyrethroid pesticides, such as permethrin, cypermethrin, deltamethrin, and cyfluthrin, are on the market in the United States," Bao's group explained, adding that use of pyrethroids "has increased drastically in recent decades, owing to the phase-out of organophosphates from residential use."

In an accompanying commentary, Steven Stellman, PhD, MPH, and Jeanne Mager Stellman, PhD, both of Columbia University in New York City, noted that pyrethroids "are the second most-used insecticides in the world, totaling thousands of kilograms and billions of dollars in U.S. sales."

Moreover, "[p]yrethroid pesticides are ubiquitous, and exposure is unavoidable," they wrote. And it's not just an issue for agricultural workers: "in New York City and elsewhere, aerial spraying for mosquito control to prevent West Nile virus and other vector-borne illnesses is largely based on pyrethroids," the Stellmans pointed out.

The study examined outcomes in more than 2,000 adult participants in the 1999-2000 iteration of NHANES, in which they underwent physical exams and blood sampling, as well as answering survey questions. Pyrethroid exposure was measured via urinary levels of 3-phenoxybenzoic acid -- a pyrethroid metabolite -- and participants were stratified into tertiles of exposure.

During a median 14-year follow-up period, 246 participants died: 52 from cancer and 41 linked with cardiovascular disease.

On average, non-Hispanic black individuals tended to have higher pyrethroid exposure than Hispanic and non-Hispanic white individuals. Those who fell into lower income levels, lower education levels, and who reported poorer quality of diet also tended to fall into the highest tertile of pyrethroid exposure.

Stellman and Stellman highlighted the "very short half-life" of the pyrethroid biomarker, averaging as low as 5.7 hours.

"The prevalence of detectable levels of a rapidly eliminated pyrethroid metabolite in a large, geographically diverse population is suggestive of chronic exposure, which also makes it important to identify specific environmental sources," they noted.

However, they also pointed out that because of the relatively young age range of the study's participants (20 to 59 years), it was difficult to fully assess the extent of cardiovascular mortality associations.

Regardless, the "unusually large hazard ratios" warrant additional research on these chemicals and their potential public health risks, Stellman and Stellman stated.

Another study limitation was that spot urine samples were used to measure pyrethroid metabolites, and those may not have captured temporal variability, leading to misclassification of habitual exposure to pyrethroid insecticides, the authors said.