How persistant is permethrin?

Malaria and other mosquito-borne infections are a significant cause of morbidity and mortality for many countries, and visitors to those countries are certainly not immune. While travelers certainly do not want to experience illness during or after a trip, the military has an even more vested interest in keeping soldiers healthy. Considering that  more troop casualties result from these infections than there are from direct combat, it is easy to understand why this is such an important area of study for the military.

Applying repellent to clothing is appealing to everyone, as it does not need frequent re-application, often has less of an odor, and has less skin irritation. A side benefit of clothing based repellent is that it is usually advertised as lasting through “many” washes, thus allowing one to apply the product before a trip and possibly not need to carry it with them. But like many commercial products, the validity of those claims is not easy to find out, and often you have to take the company at their word.

The authors of this study set out to determine exactly how effective permethrin-impregnated army uniform cloth was at repelling or killing mosquitoes after multiple washings.

Surprisingly, permethrin stays on the fabric through a large number of washes. Even more surprising, it remains somewhat effective, even after 55 washings. Of course, they weren’t using commercial washing machines, instead agitating with glass rods in beakers, so external validity may vary.

However, how effective it is depends on what you’re asking for. For knockdown (ie, a mosquito landing on the fabric falls off due to toxicity), it goes from 98.3% to 23.3% after 20 washings. 24 hour mortality, on the other hand, remained 100% after 24 hours even after 55 washes. One hour mortality drops after only 10 washes. For repellency, the rate of mosquitoes landing on the sleeves increased from 32% at 0 washes to 51% at 55 washes, both of which were below the 86-87% landing rate of the controls.

Then they go into scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis of the cloth that basically shows that there are measurable permethrin levels on the cloth after 55 washes. This leads you to the logical assumption that the effect is still from the permethrin.

So there you go. Permethrin works, even after washing. You can save some money by not needing to reapply it after every wash, and maybe only every 10 or so (depending on how you’re washing it). Don’t use your dryer though, as you have to air dry the product based on this study. Also make sure not to use fabric softener. This does not preclude use of topical agents on exposed skin, but the combined use can significantly reduce your exposure to mosquito-borne illnesses (or simply itchy bites). 

Knockdown and repellent effect of permethrin-impregnated army uniform cloth against Aedes aegypti after different cycles of washings.

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