An insect repellent  is a substance applied to skin, clothing or other surfaces that discourages insects from landing or climbing on that surface. While there are products that emit ultrasound to repel insects, this column is devoted to repellents that are applied directly to the skin, which are generally more effective. Insect repellents help to control the outbreak of diseases such as malaria, Lyme disease, Dengue fever, bubonic plague and West Nile virus. Insects that cause these diseases include fleas, flies, mosquitoes and ticks. Usually repellents work by masking human scent or by using a scent that insects avoid.  A good repellent should also be non-irritating and non-toxic to the skin, be non-corrosive and have no deleterious effect on clothing.
   
DEET (N,N-Diethyl-m-toluamide)  C12H17NO is by far the most used insect repellent in the world, because it is the most effective repellent against mosquitoes and ticks. The U.S. Department of Agriculture discovered it in the 1930s after researching hundreds of compounds.  It was patented by the U.S. Army in 1946, and released to the general public in 1957. Besides the insects mentioned above, it also targets midges, gnats and chiggers.

How DEET Works

DEET works by masking carbon dioxide and lactic acid odors that are emitted by the human body. The amount required is approximately 0.05 grams of DEET for 72.1 square inches of skin to repel the typical mosquito. About 0.1 grams are needed for ticks and more aggressive mosquitos. For a high density of bugs, 0.15 grams are used. It is claimed that alcohol-based sprays can increase the absorption of DEET by up to 67% when compared to undiluted DEET.
   
At my local drug store, I conducted some market research on the use of DEET by examining S.C. John- son’s Off! insect repellent sprays. Its Family Care spray had 5% DEET, the regular Off! spray had 15%, while Off! Deep Woods spray (also available in towelette) had 25% DEET. All of them claimed to “repel mosquitoes that may carry West Nile Virus.”
   
At the high end of the scale, PIC, a company based in Linden, NJ, had a spray with 98.11% DEET which claimed to repel insects for up to 10 hours. Below is a simple gelled DEET formula from RITA Corp.

Insect Repellent

Ingredients:                               %Wt

DEET                                        10.0
SD alcohol 40 (190 proof)    40.0
Carbomer                                  0.5
Water                                        48.5
Triethanolamine 50%              1.0

Procedure:

Add carbomer to the first two ingredients and mix for 15 minutes.  Slowly add the water and mix for 45 minutes. Neutralize with TEA.

Another effective synthetic is Picardin which has a lighter odor than DEET and a light feel too. It works the same way that DEET does by masking the human body odor. It has been used in Europe for many years and is now being sold in the U.S. by Spectrum Brands under the Cutter Advanced brand name. Permethrin (pyrethrin) is a contact insecticide, not a repellent. It kills insects when it comes in contact with them. It is used in shampoos to kill lice, and can be used on clothing. However, if applied to the skin, it becomes deactivated within 15 minutes. The ideal combination would be DEET on skin and permethrin on clothing. The best natural repellent seems to be oil of lemon eucalyptus and its active ingredient p-menthane-3,8-diol. 

The U.S. Department of Agriculture is still developing a more effective insect repellent. USDA researchers at the University of Florida have isolated seven compounds based on a type of chemical known as N-acylpiperdines. Tests were done by treating cloths with chemicals and placing them on volunteers’ arms. Tests are underway to check safety when used directly on the skin. Early data showed that some chemicals were repelling mosquitos for as long as 73 days, with many working for 40 to 50 days. DEET is effective for an average of 17.5 days.

Harvey Fishman has a consulting firm at 34 Chicasaw Drive, Oakland, NJ 07436, [email protected], specializing in cosmetic formulations and new product ideas, offering tested finished products. He has more than 30 years of experience and has been director of research at Bonat, Nestlé LeMur and Turner Hall. He welcomes descriptive literature from suppliers and bench chemists and others in the field.