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How to Repel Insects That Feed on Humans

By Dr. Peter W. Kujtan, B.Sc., M.D., Ph.D.

Article printed on page 26 in the August 1, 2012 issue of
The Mississauga News under the feature: Health & Beauty, Medicine Matters.
Portrait of Dr. Peter W. Kujtan, supplied 2005
Dr. Peter W. Kujtan

With the worldwide malaria problem and the advent of West Nile Virus has come a new resurgence of interest in mosquito repellents. It has been amusing to watch all of the hype and false information spread about the best way to avoid mosquitoes.

DEET (N,N-diethyl-3-methylbenzamide) is the commonest and most effective agent known to repel insects that feed on humans. It is the active ingredient in most lotions and spray-on repellents.

Initially, it was developed by the U.S. Army at the end of World War II, after the Pacific campaign became bogged down with troops affected by yellow fever, malaria and dengue.

DEET is effective in concentrations as low as 5%. Mosquitoes and ticks localize their prey with the help of special receptors on their antennae. DEET is thought to scramble this signal.

Effectiveness increases with concentration with the maximum effect observed at just above the 50% mark. Most agents that I see in local stores carry a 5-10% strength, and require re-application every couple of hours.

It is important to remember that some of what you put on your skin eventually does trickle into your system. Blood concentrations of DEET resulting from topical application are rarely significant. There have been reports of seizures occurring largely when DEET solutions have been ingested intentionally in large amounts or otherwise.

These actions result in blood concentrations hundreds of times higher, which are quite toxic. The evidence for 5% topical DEET application causing seizures in children is poor, but none the less has caused a great deal of concern for parents.

Children are not more sensitive to the effects of DEET than adults. Similarly, there is little evidence to show that DEET can damage the unborn fetus, suggesting a margin of safety in pregnancy as well. Some people develop rashes from topical DEET application, and most campers learn the hard way of its ability to irritate the eyes.

Common sense is perhaps the best approach. Applications should be limited to exposed areas only, and restricted to the early morning or sunset times of the day when mosquitoes feed. DEET containing products are relatively safe in concentrations less than 30%.

Used in moderation, it is a sensible way to prevent many insect transmitted diseases both here in Canada and when traveling to the tropics. Hesitation to use DEET could cause bigger problems than you know.

Related resources:

DEET from Wikipedia.

How to choose the best bug repellent by Lisa Bendall, Best Health Magazine.

Insect repellents for children from Caring for Kids, Information for Parents from Canada's Paediatricians.

DEET General Fact Sheet from National Pesticide Information Center.

West Nile Virus Resource Guide from National Pesticide Information Center.

Insect Repellents by Mark S Fradin, MD; Chief Editor: Dirk M Elston, MD, from eMedicine.

Using Insect Repellents Safely from United States Environmental Protection Agency (EPA).

Health Advisory: Tick and Insect Repellents from Department of Health, New York.

Follow safety precautions when using DEET on children from the American Academy of Pediatrics (AAP) Committee on Environmental Health. "DEET should not be used in a product that combines the repellent with a sunscreen . . . Other precautions: Apply DEET sparingly on exposed skin; do not use under clothing. Do not use DEET on the hands of young children; avoid applying to areas around the eyes and mouth. Do not use DEET over cuts, wounds or irritated skin. Wash treated skin with soap and water after returning indoors; wash treated clothing. Avoid spraying in enclosed areas; do not use DEET near food."

Sunscreen and Bug Spray by Guest Author: Paula Petrie, from BellaOnline: The Voice of Women. "The EPA no longer allows safety claims to be made for any level of DEET used on children, as evidence does not support a claim of safety."

Mosquito Repellent and DEET from Fight the Bite Colorado.

To DEET Or Not To DEET by David Shaw, CEO of Quantum, Inc.

West Nile virus from Wikipedia.

West Nile Virus: FAQ - Insect Repellent Use and Safety from Centers for Disease Control and Prevention (CDC).

Safe Use of DEET Insect Repellent from Arboviruses. Precautions While Using DEET Insect Repellents. "Do not apply to infants under 2 months of age ... Do not use a product containing more than 30% DEET for children ... Apply repellents only to exposed skin and/or clothing (as directed on the product label). Do not use under clothing. Never use repellents over cuts, wounds, or irritated skin. Do not apply to eyes and mouth, and apply sparingly around ears ..."

DEET Chemistry: What You Need to Know from About.com.

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