For decades people have relied on various forms of insect repellent to keep clear of bugs and enjoy the outdoors unbitten. From citronella candles to lotions to aerosol sprays, bug repellents come in many different forms and are designed to keep all types of species at bay. One of the more controversial insect repellents of the last 40 years is DEET, a colorless, oily concoction used to keep bugs away.

DEET has been around since the 1950s and scientists continue to question whether or not it’s safe for human use. While the EPA’s official stance on DEET is that it’s fine when used in moderation they remain concerned about traces of the chemical found in some water sources as DEET can be harmful to fish and wildlife. One thing no one questions, however, is DEET’s place as the most effective insect repellent on the market. That is, until now.

Scientists at the London School of Hygiene and Tropical Medicine recently ran a series of tests that showed some surprising results when it comes to mosquitos and DEET. It seems the insects were at first repelled by the chemical as thought but within less than 3 hours lost their sensitivity to DEET and bit the human subject anyway. DEET is not a pesticide designed to harm insects but rather works as it gives off an offensive odor that keeps bugs away.

The results seem to indicate that DEET is losing (or loses) its effectiveness when a user stays in one place for more than a couple of hours – curious and worrying to researchers. Mosquitoes transmit some of the deadliest diseases on the planet including malaria which kills 1.2 million people a year.

If you’re planning on spending time outdoors this spring and summer it’s recommended you continue to use your typical method of insect protection. Citronella candles and high-tech electronic field generating repellents work well for stationary situations. Bug sprays and lotions can be purchased anywhere that sells sunscreen and should be chosen by their “use time,” that is, how long the specific brand lasts on the skin.

If you are bitten by a mosquito you’ll likely notice swelling and itching but not much more. If you have more severe symptoms like nausea, headache, or aching be sure to contact a doctor to ensure you haven’t contracted the West Nile Virus which has experienced a resurgence in the US over the last few years.