What You Should Know About Insect Repellents
Using Insect Repellents Safely
Clark E. Julius, M.D.
Professor of Medicine
University of Tennessee Graduate School of Medicine
Mosquitoes, biting flies, and ticks can be annoying and sometimes pose a serious risk to one’s health. Mosquitoes can transmit diseases like equine and St. Louis encephalitis. Biting flies can inflict a painful bite that can persist for days, swell, and become infected. Ticks can transmit serious diseases such as Lyme disease and Rocky Mountain spotted fever. When properly used, insect repellents can discourage biting insects from landing on treated skin or clothing.
Mosquitoes are attracted by the moisture, warmth, carbon dioxide, odor, and estrogen that surrounds human skin. Repellents that are applied to the skin or clothing vaporize and produce a repellent barrier which extends to 4 cm from the skin when freshly applied. Mosquitoes turn away only when they approach the repellent barrier. Rubbing of clothing, sweating, washing with water, warm temperatures, or high winds will significantly decrease the duration of the effectiveness of repellent agents.
Oil of Citronella
Oil of citronella is an essential oil that is extracted from the long narrow leaves of a perennial grass native to tropical Asia; the grass is now grown in tropical America. Before the 1940s, oil of citronella was the most popular repellent and was reported to be effective for 4 hours or even overnight. A recent report in the New England Journal of Medicine indicated oil of citronella was affective for about 30 minutes. Citronella products are widely used, but scientific results have been disappointing. It is available as candles for indoor and outdoor use as well as topical skin products. Other gimmicks, such as wristbands or devices that emit sounds that are purportedly noxious to mosquitoes, do not work either.
The one exception to the trend was Bite Blocker for Kids– a repellent made with soybean oil. This product made by Consep Inc., kept the mosquitoes away for an average of 95 minutes.
DEET
Most early research on insect repellents has been done by the U.S. Armed Forces in conjunction with the Department of Agriculture because military personnel in World War II, the Korean War, the Vietnam conflict, and Operation Desert Storm faced exposure to multiple arthropods that caused local and systemic reactions and were vectors for many diseases. Clinical trials began in 1942, with compounds selected at random from various commercial, governmental, or college laboratory shelves; other repellents were synthesized in programs that were established to develop better products. Those having valid repellent activity were then tested for skin irritation or other toxic side effects. This is how deet, the active ingredient in most insect repellents on the market today, was discovered. Since it was first marketed in 1956, deet has remained the best repellent in the 40 years of comparative testing on tens of thousands of other compounds.
Important Information on Using Deet
Despite some concerns about nervous system toxicity, DEET (diethyltoluamide) is generally regarded as safe in children if used in concentrations below 10%. Infants under two years of age should not use DEET.
Keep out of the reach of small children, because like many chemicals, DEET can be toxic if ingested.
Do not allow children to apply the product themselves.
Do not apply to the hands of small children, since they frequently put their hands in their mouth.
When applying, avoid wounds, scratches, and the area around the eyes and mouth.
Avoid over-saturation. It is not necessary for adequate protection. Remember the repellent barrier extends from 2-4 centimeters.
Do not use combination sunscreen/repellent products. Deet is usually the repellent ingredient: it degrades the sunscreen by 30%. Also, sunscreens should be reapplied every 1-2 hours. DEET should only be applied once or twice a day.
Wash off insect repellents with soap and water after returning indoors.
Increased content of DEET in not necessarily better. Products containing 50% DEET provided a 95% level of protection for more than 4 hours, whereas a concentration of 100% provided less than 1 additional hour of the same level of protection. In adults, a maximum of 30% is usually recommended.
Health effects of DEET
Rash, blisters, skin and mucous membrane irritation and numb or burning lips have occurred among people who applied products containing a high concentration (e.g., 50% or 75%), or among those excessively exposed. Toxic encephalopathy and seizures have been associated with use in children specifically in children with ornithine transcarbamylase (OTC) enzyme deficiency. Subtle neurotoxicity in adults (e.g., insomnia, mood disturbances, impaired cognitive function) has been associated with extensive applications.
Deet poses no significant health risks when used properly.
Permethrin
Unlike DEET, which is used primarily on the skin, permethrin is applied to fabric, such as clothing or mosquito net. Permethrin is a repellent and a powerful, rapidly acting contact insecticide that knocks down or kills insects which come in contact with it. Permethrin is the best repellent against ticks.
● Permethrin kills or stuns insects touching treated fabric.
● Permethrin adheres tightly to fabric and will last through multiple washings.
● Unlike DEET, permethrin will not soften plastic or synthetic materials.
● Permethrin is biodegradable and does not accumulate in the environment.
● Permethrin is highly toxic to insects, but is not hazardous to mammals; skin absorption of the chemical is extremely low, and any absorbed permethrin is rapidly metabolized.
Protective clothing
Clothing provides a physical barrier to biting insects, provided it is sufficiently thick or tightly woven.
Summary
In summary, the use of avoidance measures and protective clothing in tandem with DEET and permethrin can achieve almost 100% protection from biting insects.
Plant Derived Insect Repellents
Product Brand Name Forms Active ingredient
Skin-So-Soft Moisturizing Lotion Citronella oil, 0.05%
Skin-So-Soft Bug Guard Pump spray Citronella oil, 0.10%
Bite Blocker Lotion, oil, pump spray Soybean oil, 2%
Buzz Away Towelette, pump spray Citronella oil, 5%
Natrapel Lotion, pump spray Citronella oil, 10%
Repellents that contain Deet
Product Brand Name Available Forms % DEET
OFF! Skintastic for Kids Pump spray 5.0
Cutter Just for Kids Pump spray 5.0
Skedaddle Insect Protection for Children Lotion 6.5
OFF! Skintastic Unscented Lotion 7.5
Skedaddle 4-Hour Insect Protection Lotion 10.0
OFF! Unscented Aerosol spray 15.0
Sawyer Deet Plus Spray aerosol 17.5
Sawyer Extended Release Lotion 20.0
Ben’s Backyard Lotion, pump spray 24.0
Ben’s Wilderness Pump spray 27.0
Sawyer 30 Lotion 30.0
Deep Woods OFF! Unscented Aerosol spray 30.0
Deep Woods OFF! For Sportsman Aerosol spray 30.0
Sawyer Deet Plus Aerosol spray 38.0
DEET is scientifically known as N,N-diethyl-3-methylbenzamide or N,N-diethyltoluamide.
Permethrin Sprays and Solution
Products Brand Name Forms %
Duranon (Coulston) Aerosol spray, pump spray 0.5
Outdoorsman Gear Guard (Cutter) Aerosol spray 0.5
Permanone (Wisconson Pharm) Aerosol spray 0.5
Permethrin Arthropod Repellent Aerosol spray, pump spray 0.5
Permethrin Spray (Sawyer) Aerosol spray, pump spray 0.5
Permakill (Coulston Products) Solution 13.3
