DEET, one of the most widely used ingredients in insect repellent, has been found to actually lose its effectiveness against mosquitos, BBC News reported.

At first, mosquitos are repelled by their initial encounter with the substance, but they then get used to the smell and later ignore it, according to scientists from the London School of Hygiene and Tropical Medicine.

The researchers noted more research needed to be done on DEET, as well as repellent alternatives.

“The more we can understand about how repellents work and how mosquitoes detect them, the better we can work out ways to get around the problem when they do become resistant to repellents,” Dr. James Logan, from the London School of Hygiene and Tropical Medicine told BBC News.

DEET, chemical name N,N-Diethyl-meta-toluamide, was created by the U.S. military in 1946 and has been determined not to present any health concerns to the general population.  The oil generally works by creating an unpleasant smell for insects, repelling them rather than killing them.

To determine DEET’s effectiveness, the researchers from the London School tempted some mosquitos in the lab with an arm covered in DEET.  At first the mosquitos were repelled, but a few hours later, the mosquitos were tested again – and the scientists found the DEET was less effective in deterring the insects.

After placing electrodes on the mosquitos’ antennae, the researchers were able to measure what was going on.  Ultimately, the mosquitos were growing less sensitive to the chemical.

“There is something about being exposed to the chemical the first time that changes their olfactory system - changes their sense of smell - and their ability to smell DEET, which makes it less effective,” Logan told BBC News.

The study was published in the journal PLOS ONE.

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