WASHINGTON – Researchers have identified seven possibilities for the next generation of mosquito repellent, some of which may work several times longer than the current standard-bearer, DEET.

The next step: safety testing to make sure they’re not harmful.

While the new repellents aren’t likely to be available commercially for a few years, early tests on cloth were promising, with some chemicals repelling mosquitoes for as long as 73 days and many working for 40 to 50 days, compared with an average of 17.5 days with DEET, according to a study appearing in today’s edition of Proceedings of the National Academy of Sciences.

Biting insects such as mosquitoes and ticks can spread diseases such as encephalitis, Lyme disease, Rocky Mountain spotted fever, malaria and dengue.

Several of the new chemicals “were just phenomenal,” said Ulrich R. Bernier, a research chemist at the Agriculture Department’s mosquito and fly research unit in Gainesville, Fla. “I was so surprised.”

Bernier, a co-author of the study, said he regularly receives new repellents from people and he ends up writing them back to say they don’t work.

In this case, researchers funded by the Defense Department set out to determine what makes repellents work, and then to use that information in finding more effective ways to chase away disease-carrying insects, Bernier explained in a telephone interview.

“We thought, can we do a better job of designing repellants,” Bernier said.

Using USDA data on hundreds of chemicals collected over 50 years, researchers led by Alan R. Katritzky of the University of Florida rated chemicals from 1 to 5 on ability to repel insects, and then focused on what the most effective ones – the 5’s – had in common.

Focusing on a type of chemical known as N-acylpiperidines, they narrowed the study down to 34 molecules – 23 that had never been tested before and 11 that had – Bernier explained. .

From those, the 10 most effective were narrowed to seven, with eliminations based on concerns about toxicity and high cost to produce.

The tests were done on cloth treated with the chemicals and then placed on the arms of volunteers.

This summer, safety tests will begin on the seven, Bernier said, to make sure they are safe to use directly on skin.

While the military is paying for the research, any success is expected to benefit the general public too.

The current standard for repellents, DEET – N,N-diethyl-meta-toluamide, also was developed originally for military use in 1946 and was registered for use on civilians in 1957.

While DEET has a good safety record, some people dislike its odor and others worry about safety for some individuals, especially children and pregnant women.

The Environmental Protection Agency says DEET has been implicated in seizures among children, but there is not enough information to confirm it as the cause of the incidents.

EPA estimates that as much as one-third of the U.S. population uses products containing DEET every year to repel biting insects such as ticks and mosquitoes.