The problem with pesticides is that they can be dangerous to more than the pests they're targeting.

You've likely heard of pyrethrins and pyrethroids. They are types of pesticides specifically used to wipe out insects.

Pyrethrins are botanical insecticides derived from chrysanthemum flowers typically found in Australia and Africa. Pyrethroids are synthetic versions of those insecticides that mimic the same chemical structure. They both work by affecting nerve function, paralyzing insects and eventually killing them, according to the U.S. Environmental Protection Agency (EPA).

Pyrethrins and pyrethroids are found in more than 3,500 registered products, according to the EPA, many of which are commonly used in the home and around pets, as well as in agriculture and in mosquito control. You might find them in flea collars, insect repellents, treated clothing and lice-killing shampoos.

They have become more popular over the past decade with the decreasing popularity of organophosphate pesticides, which are more toxic to birds and mammals, reports the EPA.

And although that's good news for those creatures, there are other concerns. Residential and agricultural use can result in runoff into streams and rivers, exposing aquatic life to potentially harmful levels of the insecticide.

Recently, the insecticides were associated with an increased risk of death, according to a new study in the journal JAMA Internal Medicine.

Researchers followed 2,116 adults for an average of 14 years and found that those with higher exposure to pyrethroid insecticides were slightly more likely to die from all causes than those with the least amount of exposure. They were about three times more likely to die specifically of heart disease.

The scientists took into account other factors that might have influenced the results, including diet, level of physical activity, smoking status, alcohol intake, education and income. But they were unable to control for all possibilities.

The results suggest that pyrethroids might be linked to an increased risk of cardiovascular disease, says 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. However, Lehmler tells Consumer Reports, "one study doesn't mean that you've answered all the questions," saying more research needs to be done.

Earlier research

dog wearing flea collar The insecticides can be found in some flea collars and shampoos. (Photo: alexei_tm/Shutterstock)

This isn't the first time these insecticides have come under scrutiny. More than a decade ago, the Center for Public Integrity did a report on the insecticides. They found:

A Center analysis of EPA data shows that the number of reported human health problems, including severe reactions, attributed to pyrethrins and pyrethroids increased by about 300 percent over the past decade. A Center review of the past 10 years' worth of more than 90,000 adverse-reaction reports, filed with the EPA by pesticide manufacturers, found that pyrethrins and pyrethroids together accounted for more than 26 percent of all fatal, "major," and "moderate" human incidents in the United States in 2007, up from 15 percent in 1998.

According to the report, people have died from exposure to these chemicals, including one child in 2000 who died after her mom washed the child's hair with lice treatment shampoo that contained pyrethrins.

Even slight contact with these insecticides can cause some short-term side effects. They can cause itching, burning and irritation, according to the Department of Health and Human Services Agency for Toxic Substances and Disease Registry. Prolonged exposure to larger amounts can lead to dizziness, nausea, headache, vomiting and loss of consciousness.

Scientists know less about the long-term effects of the insecticides.

"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," 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, tells Consumer Reports. The pesticides, he points out, are an important tool for protecting people against diseases spread by mosquitoes and ticks.

Editor's note: This story has been updated since it was written in September 2009.

Mary Jo DiLonardo covers a wide range of topics focused on nature, health, science and anything that helps make the world a better place.

How much should we worry about pyrethrins and pyrethroids?
Flea collars, lawn-care products, insect repellant and de-licing products could be harming more than just pests.