Review of Insect Repellents

Howdy All:

Divers often travel to tropical locations where they might be exposed to mosquitoes carrying malaria, West Nile Virus is spreading across N. America, noseeums are a constant irritant, and who likes ticks and fleas? Most divers will likely need an insect repellent at some point.

The Medical Letter* (http://www.medicalletter.org) just released their review of the literature for insect repellents in their May 26, 2003 issue. DEET (N, N-diethylmetatoluamide) remains the king of repellents. It is effective against most biting bugs. It comes in various concentrations, but generally the higher the concentration, the longer lasting the protection. (Protection nearly maxes out at 50% DEET with only modest increases in duration of action with higher concentrations.)

Most DEET comes in alcohol-based formulations. Products with less than 20% DEET can provide complete protection for 1-3 hours, and high concentration products can protect you from biting insects for as long as 12 hours in good conditions. There is now a long-lasting lower concentration polymer formulation originally developed for the US Army. It is available commercially in the US from 3M as Ultrathon. It contains 25% or 33% DEET and gives more than 95% protection for 6-12 hours (depending on environmental conditions and type of mosquito) which is similar to 75% DEET in an alcohol base.

DEET can damage synthetic fibers in clothes and plastics. DEET can also be irritating to some people's skin, but toxic or allergic reactions are uncommon and serious problems are rare when DEET is used as directed. Concentrations up to 50% are considered generally safe even in young children when used properly. The lower concentration but equally/more effective polymer based Ultrathon formulation looks attractive to me.

There's a new product called picaridin (Bayrepel and others) that will soon be available in the US and is currently available in Europe and Australia. It seems to be as effective as the polymer based DEET (19.2% picaridin gave >95% protection against mosquitoes for 8 hours) and it does not seem to irritate the skin or damage plastics/synthetics like DEET.

Permethrin (Repel and others), is a contact insecticide rather than a repellent, but it can decrease bites when applied to clothing, mosquito nets, tents, and sleeping bags. It is more effective against ticks than DEET. The Medical Letter believes that wearing permethrin-treated clothes and applying a DEET-containing repellent to the skin might be the most effective way to prevent insect bites. Permethrin is non-staining and can persist through 20 or more washings.

Other products designed to repel biting insects were less effective. Citronella based repellents (Natrapel and others) gave protection for less than 1 hour and probably aren't effective against ticks. Eucalyptus-oil-based repellents (Repel Lemon Eucalyptus, and others) give about 4 hours protection. In one lab study, products with citronella gave about 20 minutes protection, analine analog IR 3535 (eg Skin-So-Soft Bug Guard Plus) lasted 23 minutes, and a soybean oil product (Bite Blocker) worked for 95 minutes.

So DEET (especially the long lasting polymer formulation) is so far the clear champion for repelling insects in the US. Adding permethrin treated clothing to DEET may be the best protection against mosquitoes and ticks. And picaridin is a promising newcomer that might be as effective as DEET without damaging plastics/synthetics or causing skin irritation.

HTH,

Bill

*Sorry, The Medical Letter article doesn't seem to be available online.

Thank for the info Bill. As one who went through a bout of Lyme, I am ever cautious of little things that bite. Very timely as summer arrives, more people spending time in the great outdoors & this wet spring weather is sure to turn into mosquito heaven. I'll be checking up on the DEET content of my insect repellants.

I'll have to check the FOD (Fraction of Deet) in my "Deep Woods Off". I have to have my garage door open while filling tanks, and I get eaten alive without the "Off".

any idea what deet does to neoprene and the rubber bands of dive watches?

Deep Woods OFF FOD=25%.

Best regards.

DocVikingo

Stone asked...
I'll have to check the FOD (Fraction of Deet) in my "Deep Woods Off".

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Howdy Stone:

SC Johnson has at least 3 different products in the US that a consumer might call "Deep Woods Off". There's "OFF® Deep Woods Insect Repellent V" that is indeed 25% DEET as DocV said. This is the product in the green top can that most people would likely think of when they talk about "Deep Woods OFF".

But there's also "Deep Woods OFF® for Sportsmen Insect Repellent IV" that is 30% DEET, and "Deep Woods OFF® for Sportsmen Insect Repellent I Pump Spray" that is 100% DEET. I suppose it depends on what kind of "Deep Woods OFF" you have, so checking the container is probably a good idea.

docmartin asked...
any idea what deet does to neoprene and the rubber bands of dive watches?

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Howdy docmartin:

Good question. I once forgot to take off my watch when applying insect repellent and got a DEET containing spray on one of those black plastic Casio watch bands. It gave the band a "scuffed" look for about a year. I think it more likely that the rest of the band eventually got "scuffed" to match the damaged part rather than the damaged part eventually cleaning up. DEET will also damage plastic watch crystals. I was always careful to take off my watch when spraying after that.

I'm not sure about the neoprene. Most neoprene these days is lined inside and out with synthetic material and DEET could damage it depending on what it's made of. My best guess is that DEET could damage at least the lining of neoprene if applied directly, but that's just a guess. I would suspect that DEET product applied to the skin might damage the neoprene's lining or a lycra skin over time, but again that's a guess. Hopefully someone who knows more than I will reply.

HTH,

Bill

Are there any issues with long-term topical applications of these products? Can they be safely used for weeks/months?

Zept

Hi Zept:

DEET has been used as an insect repellent for about 40 years now and it has an excellent safety profile. Toxic and allergic reations are uncommon (and serious problems are rare) when DEET products are used as directed, but they do occur (especially when DEET is not used as directed) so you should always read the label.

I'm not aware of any directions limiting the length of use of DEET products, but some do warn that over-applying with too frequent applications is unnecessary. As with many things, it's probably wise to use the the lowest concentration and frequency that is effective. That's one reason I find 3M's Ultrathon attractive- effectiveness equal to high concentration alcohol-based DEET, but at a lower concentration. But if someone only needed an hour or two's protection, another product might be even better.

HTH,

Bill