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Discussion: Permethrin and Lyme's Disease

in: Orienteering; Gear & Toys

Jul 11, 2012 6:15 PM # 
Lyme Disease Prevention:

This line gave me pause...
Spray (permethrin on) the outside of shoes and cuffs of socks, but not the bottoms of socks or clothes that can stick to skin when wet with perspiration, or permethrin could be absorbed through the skin.

Don't spray on sweaty clothes?!?!?!

I thought spraying my clothes with permethrin provided the solution to prevent Lyme's Disease. Now I read that I should not do that because I sweat. On what planet do people stay dry when they go into the woods?

Please advise.
Jul 11, 2012 8:37 PM # 
I think this report is the basis for the military use of permethrin, where soldiers might get a lifetime exposure from wearing permethrin-impregnated clothing daily for ten years.

It is way over my head. But that didn't stop me from reading the sixteen-page summary. Because I couldn't find an easy link to it, I copied some (way too much) of the report summary below.

My understanding is that there is much more proof that permethrin is safe than there is that DEET is safe. There are very few reported issues with DEET and hundreds of millions of DEET uses every year.

Also, the standard military application methods (which are available online with open distribution) include spraying with a 2-gallon sprayer or putting the clothing item inside a plastic bag and getting the item completely wet. They do seem to be very serious about not breathing the wet spray---they wear respirators for the spray application and minimize particulates from the plastic bag method.

This is the first time I recall seeing how quickly permethrin washes out---After 50 washings, it is at 26% of the original concentration. (Although I hear hot water washes it out much faster.) Most sprays say reapply every 2 weeks, which seems ludicrious---unless they know something they're not saying.

I'm really interested in what other people reading this who do have the right background think of it.

I have read most of the report I've linked to, and I use permethrin in my orienteering clothes and DEET on my skin. I don't apply permethrin to my clothes when I'm wearing them. I spray the clothes (and shoes) on the ground (outside) and hang them up to dry.

Excerpts from the summary of "Health Effects of Permethrin-Impregnated Army Battle-Dress Uniforms" (1994) (I took out a lot.):

Efficacy tests conducted by the U.S. Department of Agriculture and the U.S. Department of Defense show that the wearing of permethrin-impregnated BDUs in conjunction with application of DEET to areas of skin not covered by BDUs provides nearly 100% protection against bites from most insect vectors. (BDUs, made from either 100% cotton fabric or 50% nylon and 50% cotton fabric, are used to camouflage soldiers.)

Before introducing permethrin-impregnated BDUs for military personnel, the U.S. Army wanted a thorough and independent evaluation of the safety of wearing them or working with permethrin-impregnated fabric (as do garment workers) for long periods. Therefore, the Army requested that the National Research Council (NRC) review the toxicological and exposure data on permethrin to determine whether wearing BDUs impregnated with permethrin (at a concentration of 0.125 mg/cm2of fabric) 18 hr per day, 7 days per week, for up to 10 years is safe for soldiers, and whether handling permethrin-impregnated fabric is safe for garment workers. The Army also asked the NRC to identify gaps in the permethrin toxicity data and make recommendations for future research.

The subcommittee considered the dermal route to be the only significant route of exposure for soldiers wearing permethrin-impregnated BDUs. Because permethrin is solid at room temperature and has a relatively low vapor pressure, the subcommittee concluded that the inhalation route is probably insignificant and need not be considered. At present, there is no information to indicate that significant exposure to permethrin will occur by any route other than dermal absorption in soldiers wearing permethrin-impregnated BDUs.

Several conversion factors were used to translate the proposed fabric-impregnation concentration, 0.125 mg/cm2, to an estimated internal dose for military personnel through dermal absorption. These factors were the time-weighted-average percentage of permethrin remaining in fabric through 50 washings (26%), percentage of permethrin migration from fabric to skin (0.49%/day), body-contact area (1.5 m2), dermal absorption rate (2%/day), and adult body weight (70 kg).

To adjust for actual exposure conditions, it was assumed that military personnel would wear the permethrin-treated BDUs 18 hr per day for 10 years during a 75-year lifetime. Adjusting for the proportion of lifetime exposure resulted in a calculated average daily lifetime dose of 6.8 × 10−5 mg/kg per day.

Following absorption, permethrin is extensively and rapidly metabolized. The two major pathways for metabolism are hydrolysis, which essentially splits the permethrin molecule in two, and oxidation, which occurs at a number of carbon atoms throughout the molecule. Both of these metabolic processes make the resulting permethrin metabolite more water soluble and more likely to be excreted in the urine. Thus, metabolism can be viewed as an important detoxification pathway for permethrin, because only the parent chemical exerts toxic effects.

Experiments with laboratory animals have shown that, upon absorption, permethrin is distributed throughout the body but appears to concentrate predominantly in fat. Solubility in fat might explain its high concentrations in brain and nervous tissue in comparison with other body organs.

Because dermal penetration of many chemicals is enhanced by DEET, use of DEET in combination with permethrin might also facilitate dermal absorption of permethrin. Research specifically on the interaction of DEET and permethrin has not been conducted.

Although permethrin is highly toxic to insects and other arthropods, it is one of the least toxic insecticides to mammals. Its acute toxicity has been studied in several animal species and has been found to be more toxic by the oral route than by the dermal or inhalation routes. The oral LD50 (acute oral lethal dose for 50% of the subjects) of technical-grade permethrin in experimental animals is in the range of 0.5-5 g/kg of body weight. Aqueous suspensions of permethrin usually produced the least toxicity, with LD50 values ranging from 3 to 4 g/kg of body weight. Permethrin in corn oil suspensions yielded LD50 values of approximately 0.5 g/kg in most of the studies involving oral administration to rats and mice. The cis/trans isomer ratio also affects the toxicity, the cis isomer being more toxic than the trans isomer. Permethrin in BDU fabric would contain 60% cis isomer and 40% trans isomer.

The clinical signs of acute poisoning become evident within 2 hr of exposure to permethrin and are targeted to the central nervous system; symptoms are uncoordination, ataxia, hyperactivity, convulsions, and, finally, prostration, paralysis, and death.

The dermal toxicity of permethrin has been studied in animals and humans. Single dermal application of permethrin failed to produce skin irritation in rabbits. Repeated dermal exposure to permethrin in rabbits has been shown to produce slight erythema. When cotton cloth impregnated with permethrin was applied to the clipped skin of rabbits for 21 days to mimic occupational exposure, no adverse effects were reported. Experiments with guinea pigs showed that permethrin might be a skin sensitizer at high doses. In photochemical irritation studies, permethrin did not cause phototoxicity in experimental animals.

In a study with 184 human subjects, a 21-day repeat patch test with a 40% permethrin solution did not cause any skin sensitization. However, several subjects described a transient burning, stinging, or itching sensation (subjective irritation).

Permethrin is neurotoxic at high doses. It produces a variety of neurotoxic effects in animals. Some of these effects are tremors, salivation, paresthesia, splayed gait, depressed reflexes, and tiptoe gait; reversible axonal injury occurs at very high doses.

In one study, rats fed permethrin in diet at 6,000 mg/kg for 14 days showed fragmented and swollen sciatic nerve axons and myelin degeneration. In another study, rats fed permethrin at up to 9,000 mg/kg developed severe trembling but exhibited no consistent histological effects in nerve tissues.

The estimated no-observed-adverse-effect level (NOAEL) for neurotoxicity by the dermal route in rats is 200 mg/kg. Based on that NOAEL from available neurotoxicity data, the MOS {Margin of Safety?} associated with daily human exposure from permethrin-treated BDUs at a level of 6.8 × 10−5 mg/kg per day is approximately 3 million.
Therefore, neurotoxicity from wearing permethrin-impregnated BDUs or working with permethrin-treated fabric should not be a concern.

Extensive medical investigations of workers exposed to permethrin have not revealed any clinical chemistry changes that would suggest liver toxicity.

[Using] the daily exposure to permethrin at a level of 6.8 × 10−5 mg/kg per day [for soldiers] from wearing treated BDUs provide a MOS of approximately 150,000 for liver toxicity.
The MOS for garment workers is approximately 340,000. Therefore, liver toxicity from wearing permethrin-impregnated BDUs or working with treated fabric should not be a concern.


Data on reproductive and developmental toxicity of orally administered permethrin suggest that there are few toxic effects, and those tend to be limited to high doses. No reproductive or developmental toxicity data are available from dermal exposure studies, but dermal absorption is poor, and oral dosing would be expected to maximize any effects. Some studies involving oral exposures have reported reproductive or developmental toxicity effects, but the effects have not been confirmed in other similar studies. Also, there is disagreement among the studies regarding the doses at which such toxicity occurs.

Given the lack of effects in most of the reproductive and developmental toxicity studies on permethrin and a MOS of approximately 44,000 from the most sensitive end point (decreased testicular weight), the possibility of male reproductive effects or other reproductive and developmental effects occurring from wearing permethrin-impregnated BDUs or working with permethrin-treated fabric is remote.

Studies conducted to determine the potential of permethrin to produce gene mutations were all negative.
Studies conducted to determine the potential of permethrin to produce chromosomal damage provided an array of results. Some were positive, some negative, and others deficient in information needed to draw a definitive conclusion.
The subcommittee believes that the weight of evidence suggests that permethrin does not produce gene mutations but is a potential clastogen in certain in vitro systems.

There is no information in the literature on carcinogenic effects of permethrin in humans. Evidence of permethrin's possible carcinogenicity in humans is derived from bioassays in rodents. Permethrin has been tested in seven chronic exposure studies in which permethrin was administered in the diet to rats in three studies and to mice in four studies.

The three rat studies were negative for carcinogenicity; however, permethrin concentrations were not high enough to adequately assess the oncogenic potential of permethrin.

An upper bound on the lifetime carcinogenic risk was estimated by multiplying the carcinogenic potency factor by the estimated average daily lifetime dose. For military personnel wearing permethrin-impregnated BDUs, the upper bound on lifetime carcinogenic risk is estimated to be 1.6 × 10−6. That same value applies to nonfield and field personnel and assumes that topically applied DEET does not enhance dermal absorption of permethrin.

The carcinogenic risk to field or nonfield military personnel or to garment workers from exposure to permethrin-impregnated fabric is very small—of the order of 10−6 or less. Therefore, the subcommittee concludes that premethrin-impregnation of BDUs is not a serious carcinogenic risk to field or nonfield military personnel or to garment workers.

The subcommittee analyzed the risk of adverse health effects to soldiers who wear permethrin-impregnated BDUs and the risk to garment workers who handle permethrin-treated fabric. Based on the review of the toxicity data on permethrin, the subcommittee concludes that soldiers who wear permethrin-impregnated BDUs are unlikely to experience adverse health effects at the suggested permethrin exposure levels (fabric impregnation concentration of 0.125 mg/cm2). The risk of adverse health effects in garment workers who handle permethrin-impregnated fabric is even smaller because their exposure to permethrin is estimated to be less than that of soldiers.
Jul 11, 2012 8:44 PM # 
Also, I learned from Attackpoint that permethrin is very toxic to cats.
Jul 12, 2012 3:37 AM # 
"On what planet do people stay dry when they go into the woods?"

May 29, 2018 3:04 AM # 
I ran across this today and thought I'd share on AP. Basically for $8-$10/item you can send in your O' clothes to be treated to provide protection from ticks and other insects. I have been treating my own gear (socks, O' pants, O' top) but the directions tell me to retreat every 6 washings. The Insect Shield treatment is suppose to last for 70. Haven't tried it yet but probably will.
May 29, 2018 12:52 PM # 
I think that Permethrin is still not approved for use as an insect repellent in Canada.
May 29, 2018 5:19 PM # 
No, I don't think it is approved in Canada yet. The difficulty in Canada is that the Pest Management Regulatory Agency will not assess and approve its use in Canada until a business applies for an approval. As we are a fairly small market, there may not be an incentive for companies to enter into business in Canada. Having said that, I contacted the makers of Insect Shield about 1.5 years ago, and they said they were in the process of applying for regulatory approval in Canada. Best bet is still to buy clothing while down south.
May 30, 2018 3:12 PM # 
Going back to toddp's original question, although the link referenced appears to be gone, any research I have done around permithrin does NOT include spraying it on clothes while you are wearing them - it should not be used as an insect repellent as you would use a Deet product. There are lots of sites explaining how to treat your own clothes with permithrin (most common being the "bag" method or spray) but all methods involve letting things dry completely so the chemical becomes embedded in the fabric. As others have noted the studies, the risk at this stage (even if you sweat or get wet, is very low. Deet products are repellents and will limit the likelihood of ticks, but they may also continue to crawl on your clothes (or under) until they come to a Deet free spot and then dig in. The permithrin treatment will actually kill the tick, substantially reducing the risk. So three layers of protection 1: treated clothes, 2: Deet spray and 3: really good tick check afterwards!

I did write a short article for our club a few years ago as the local risk increased some information on self-treating clothes). I have not updated it this year or checked the links but I have also seen a lot more published information from both government and other organizations as the risk here (Ottawa, Canada) increases.
Jun 24, 2018 3:31 AM # 
An update for those thinking about the Insect Shield Permethrin Option for tick protection.

I sent two packs of clothes to Insect Shield for treatment. I am using the "Easy Pack system" where you send in your own clothes for treatment using a 14"x17" USPS mailing envelope provided by Insect Shield. Info is here

The info says the envelope normally holds 8-10 items but O' clothes are thin and light so I was able to put 4-O'tops, -4 O' pants, 6 pr of socks, 1 thin casual shirt and 1 thin pair of casual pants---16 items, into one pack. The 2nd pack had 17 items. I purchased the 3 pack system ($175) so the cost per pack is $58.33 and the cost per item works out to about $3.65. The treatment is suppose to last beyond 70 washings.

I should add that I did verify with Insect Shield that more than 10 items per pack is okay and they said yes.
Jun 25, 2018 5:43 PM # 
Carl - I noticed a note on their site about shrinkage ... did anything you sent for treatment shrink significantly?
Jun 27, 2018 12:26 AM # 
I have only sent the stuff, haven't got it back yet. I'll let you know when we do but I am not anticipating a problem cause our O' clothes are somewhat loose and I didn't really notice shrinkage when we first washed them (that is the shrinkage they say to expect). However, I did send a pair of levi blue jeans that were a size too big so I am hoping they will come back a little shrunk.
Jul 1, 2018 9:26 PM # 
Was training in Denmark for JWOC 2019. After every training session I found 5-8 ticks on my person. After treating just my shoes with Permethrin I found only one during the last 4 training sessions.
Jul 5, 2018 6:01 PM # 
I have received back all the treated clothes and I did not notice any shrinkage in the Nylon material items (O' Pants, O' shirts, O' socks, casual shirts and pants). However there was some shrinkage in the cotton items including 2 pairs of athletic socks and one pair of levi jeans. The jeans were big so that's okay and the athletic socks will stretch.

What I was surprised about is that everything came backed marked. The socks all have an Insect Shield Stamp and the shirts and pants all have sewn tags. Not sure how long the sock stamp will be visible after orienteering but we'll see.
Jul 5, 2018 11:35 PM # 
Carl - thanks for the update! Definitely helpful.
Jul 9, 2018 7:46 PM # 
On NPR this morning:

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