The chemical DEET was developed for the U.S. Army in 1946 to provide protection for soldiers engaged in jungle warfare. Now the most common ingredient in commercial insect repellents, DEET has been used by the general public since 1957. That’s pretty amazing considering the fact that scientists don’t know exactly why the stuff sends insects packing. It’s probably the smell, most conclude. Or possibly it blocks an insect’s ability to smell human sweat and breath.

Whatever the case, the stuff works, as just about anyone who’s used it will tell you. The problem, for some, is that DEET may be dangerous to health.

But is it?

“Researchers have not found any evidence that DEET causes cancer in animals or humans,” according to the National Pesticide Information Center. “DEET has been classified by the Unites States Environmental Protection Agency as ‘not classifiable as a human carcinogen,’ which means that there is not enough evidence to say that it does or does not cause cancer.”

To some that’s reassuring. To some it is not reassuring enough.

The EPA, in 1998, conducted a definitive assessment of the chemical. The agency turned up 46 seizures and four deaths that were potentially linked to DEET exposure, according to the report. The EPA estimated that since 1960, the incidence of seizures with a potential link to DEET exposure was one per 100 million uses. Most of those reported cases involved a misuse of DEET products.

“After completing a comprehensive re-assessment of DEET, we concluded that insect repellents containing DEET do not present a health concern,” according to the EPA fact sheet. “Consumers are advised to read and follow label directions when using any pesticide product, including insect repellents. Based on extensive toxicity testing, we believe that the normal use of DEET does not present a health concern to the general population, including children.”

The long and short of it seems to be: Go ahead and use DEET products to keep the bugs away. Just do so with care.

It’s much the same with permethrin: There is danger (particularly for cats), but if used properly, most believe it’s next to harmless.

“Permethrin is an insecticide in the pyrethroid family,” according to the NPIC. “Pyrethroids are synthetic chemicals that act like natural extracts from the chrysanthemum flower. Permethrin can affect insects if they eat it or touch it. Permethrin affects the nervous system in insects, causing muscle spasms, paralysis and death. Permethrin is more toxic to insects than it is to people and dogs. This is because insects can’t break it down as quickly as people and dogs. Cats are more sensitive to permethrin than dogs or people because it takes their bodies a long time to break it down.”

On your hat, shirt or pants, permethrin probably won’t harm you at all. But don’t make a meal of the stuff.

“Permethrin was classified by the International Agency for Research on Cancer as ‘not classifiable as to its carcinogenicity to humans’ in 1991. This means that IARC could not decide whether or not permethrin can cause cancer. The EPA decided that permethrin was ‘likely to be carcinogenic to humans’ if it was eaten.”