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.
KNOWN ABSORPTION STUDIES OF DEET
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.
