HOW REPELLENTS WORK
Repellents work by evaporation creating a shield a few inches above the area of application. The presence of the repellent vapor confuses insects so they can’t locate a target host. In most cases it usually requires less than 1% of the repellent to form a protective barrier. It is the combination of this "evaporation delivery system", and the base repellent you choose that determines how much of a particular repellent you must begin with. Different repellents will require different levels of initial application and re-application while consideration must also be given to type of insect you are trying to avoid and the risk of disease attributed. In the US repellents against mosquitoes are commonly considered as outside comfort products with little regard to disease issues. Some disease carrying mosquitoes outside the United States are extremely aggressive and only dissuaded by the most effective repellents used with diligence.
Essential Oils: Many compounds that occur in nature provide a brief period of repellency against certain insects. There are well over 150 natural repellents while the most common are: Citronella, Eucalyptus, Lemon Leaves, Peppermint, Lavender, Cedar Oil, Canola, Rosemary, Pennyroyal, and Cajeput. Persons concerned about exposure to deet use essential oils or ones who prefer a natural solution. Generally the EPA considers these oil safe to use in low dosage but overall their effectiveness is limited to less than 30 minutes.
CDC has indicated that oil of lemon eucalyptus (P-menthane diol; PMD) also is registered with the Environmental Protection Agency and is comparable in its duration of effectiveness to lower concentrations of DEET. Earlier studies also indicate that 2% soybean oil has similar levels of effectiveness. The range of DEET concentrations that have been shown to be similar in duration of action to these other products generally are in the 6.65% to 15% range. The mechanisms of action for oil of lemon eucalyptus and for soybean oil have not been determined.
Deet (N,N-diethyl-m-toluamide): Deet is by far the most commonly used insect repellent worldwide. This is because it is the most effective repellent against mosquitoes, ticks and other biting insects. After researching hundreds of compounds, deet was selected by the USDA and the US Military as the safest and most economical. See additional information on deet at the end of this report.
R-326 (Di-n-propyl Isocinchomeronate): R-326 is the most effective insect repellent against flies, gnats, no-see-ums, and similar pesky insects. R-326 is far more effective than deet against these insects and R-326 only needs to be present in small quantities.
MGK-264 (N-octyl bicycloheptene dicarboximide): MGK 264 is both a mosquito repellent and a synergist. As a synergist, MGK 264 both repels mosquitoes and helps the deet to do an even better job of repelling mosquitoes than it would by itself. The MGK 264 molecule is much larger than the deet molecule in size and thus not absorbed well by the skin.
PICARIDIN: In April 05, the Centers for Disease Control and Prevention (CDC) revised its recommendations for mosquito control to include compounds that contain picaridin (1-methylpropyl 2-(2-hydroxyethyl)-1-piperidinecarboxylate, also known as KBR 3023).
The repellent has been in use in Europe, Australia, Latin America and Asia for years and originally was registered by the Food and Drug Administration in 2000 but has just been recommended by CDC in terms of efficacy and safety when used as directed. The mechanism of action appears to be the same as for DEET. Picaridin currently is available in 7%.
Permethrin: Permethrin although known as a repellent is actually a contact insecticide. That is, it kills ticks or other insects that come in contact with it. Because it is deactivated by contact with skin within fifteen minutes it is used for treatment of clothing and gear. When applied to clothing, tents, sleeping bags, or bed nets permethrin is very effective at reducing the mosquito population in your campsite or sleeping quarters. Where ticks are a concern permethrin on clothing or gear will kill ticks who travel across as little as 10” of treated fabric. Spray applications of permethrin remain effective up several weeks and through weekly washings. Dip applications can remain effective even longer. Permethrin is harmless to skin and is used extensively in other formulas for treatment of head lice.
Delivery Systems: If you choose deet or essential oils the delivery system is through evaporation. If you choose permethrin the delivery system is through contact of treated clothing or gear.
Deet is used in numerous formulations that combine it at different strengths and carriers such as alcohol, polymers or others. Deet is also blended with other repellents noted earlier . . . the options are numerous. Deet has been found to limit incremental protection when concentrations go beyond 35%.
Controlled Release: (Sub-Micron Encapsulation): Introduced in 1998, is by far the most advanced and effective delivery system available. The active ingredient deet, is encapsulated (surrounded) at a 20% concentration within a skin nourishing protein just the way air is captured within a ping pong ball. An application of Controlled Release contains many of these protein ping-pong balls that are suspended in either a water-based lotion, or water for spray application. After contact with skin the protein balls begin to breakdown releasing the captured deet. The process continues as each microscopic ball is depleted then replaced by a new ball that contacts the skin, releases its deet and so on. The process takes up to 24 hours for one application.
Because protein's adherence to the skin is so effective, these formulas are very resistant to perspiration (sweat-off), and water. When applied they are dry and comfortable with no greasiness. This system results in very effective protection and is the safest repellent by far. However, it is only effective when used on skin because clothing does not have the capability to release the proteins.
Entrapment: This system uses a polymer to encase the repellent (deet), which slows down the early evaporation leaving more deet available for later evaporation. This system can often increase a repellent's length of effectiveness by 25% to 50% over comparable non-entrapped deet products. The negatives of this system are that these formulas are often greasy because of the presence of the polymer, and they’re less effective than the Controlled Release system.
Composites: Composites use the presence of a synergist, or potentiator, to keep stimulating the evaporation of the remaining deet and fly repellents (R-326). In non-synergist products only 25% to 30% of the total deet applied evaporates, but the synergist can increase this rate and improve the overall effectiveness of a formula by 50% or more.
Deet + R-326 + MGK 264 = Composite
Add these three active ingredients together and you get what we call a COMPOSITE insect repellent. You get the effective mosquito and tick repellency of deet and MGK 264, but you also get effective repellency of flies, gnats, no-see-ums, because of the R-326. By working together they each become more effective, last longer, and absorb less into the skin than straight deet formulas. A simple rule is to look for three names you cannot pronounce on the label and not just one.
Lotions: The active ingredients in a repellent formula suspended in a lotion can be gentler to the skin, and up to 50% more effective than comparable sprays. The downside to lotions is they can not be applied to clothing and as such can not protect against bite through of clothing at contact points eg: ankles, thigh, shoulders and back.
Sprays: Due to convenience most repellents are applied in a spray format. Sprays can be applied to clothing. (always read the warning labels since some repellents can damage specific materials) Sprays used to use an alcohol base to carry and disperse the deet; however, alcohol promotes premature evaporation and in turn shortens the effective period of the protection. Alcohol can also open skin pores and promote deet absorption. Water based and low percentage alcohol formulas are better.
We recommend that your choice of deet products be kept below 35%. Those who recommend more may not be informed regarding the limited advantage and increased risk to self and gear associated with higher concentrations.
To date only two studies have been conducted regarding the skin absorption rate of deet:
The first was a series of studies conducted by the industry in Belgium in the mid-1980's. In all, eight human subjects were observed. It is from these studies that the generally accepted 4% to 8% absorption rate, which is often used by the medical community, was obtained. Due to the complexity of the study not all deet applied could be accounted for, but based on a long history of animal studies conducted by manufacturers during the EPA approval process, it was concluded that none of the deet remains in the human body after 72 hours.
The second study was conducted in California in 1995 where deet was observed as part of a blend with R-11, a repellent similar to R-326, and MGK-264. In this study four human subjects were observed. The conclusion of the study was that when combined with these two larger molecules, the absorption rate of deet was reduced to a range of 3% to 6%. This represented a reduction in absorption of 25%. This blend is used in the Sawyer composites.
The American Academy of Pediatrics published information on using deet with infants an excerpt follows: “Pediatricians and other pediatric health care providers should help their patients understand the risk of WNV infection and methods to prevent infection. Children and pregnant women should be encouraged to apply insect repellent to skin and clothing when exposed to mosquitoes. The most effective repellents contain DEET, which can be used by children and pregnant women without adverse effects.” Please visit the AAP website at http://www.aap.org/ and enter a search of “deet infants”. It’s an excellent site with a wealth of important information.