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I purchased a spray-bottle of Sawyer 0.5% permethrin to treat some of my clothing in preparation for a backpacking trip. The spray nozzle turns out to have been quite a bit more generous than I anticipated - a fact that I realized only after using roughly twice the recommended amount on my first outfit. In all other respects I have followed the directions on the packaging to the letter.

Is the overtreated outfit unsafe to wear? Would it be an overreaction to wash it once the treatment dries?

EDIT Just returned from our trip. Wore the aforementioned outfit without washing for a couple days on the trails. 100% anecdotal, but no tick bites, few mosquito bites, and most importantly none of the adverse symptoms associated with permethrin.

Great weather, too!

Conduit
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    According to the FAQ, dry cleaning will remove the permethrin. So if you want to be totally safe, that would be the way to go. If you want to remove a completely unknown amount, wash it in a washer with plenty of detergent on a non-gentle cycle (b/c the FAQ says to wash on gentle to minimize the amount you remove). Probably hot water will remove more than cold, though I didn't find a source for that. https://sawyer.com/maximize-effectiveness-permethrin-fabric-treatment/ – csk Jun 12 '21 at 03:07
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    If you have a cat, keep your treated gear away from them. See https://outdoors.stackexchange.com/questions/15740/is-permethrin-safe-for-cats-after-it-dries – Ivana Jun 14 '21 at 08:55
  • Why are you Asking anyone other than the maker of the outfit or the maker of permethrin, please? – Robbie Goodwin Jun 15 '21 at 22:41
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    @RobbieGoodwin same reason I smell my milk to find out if it's bad instead of throwing it out the second it expires: the manufacturer would probably give the advice their legal team suggests regardless of what's true, especially if it results in my purchasing another bottle. I'll stick to studies, facts, and MSDS, thanks – Conduit Jun 18 '21 at 16:54

2 Answers2

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I am almost certain that you will be safe.

Permethrin is not particularly toxic to humans, and exposures of notably higher concentrations than what you used for the clothing treatment have been found to not be a significant issue. In fact, there are treatments for head lice using 1% Permethrin, applied directly to the scalp, so I wouldn't worry too much about using a bit too much on the clothes, particularly since when it dries, the exposure risk lowers.

According to the fact sheet, permethrin is "efficiently metabolized" by the liver and has a very high lethal dose (though that doesn't mean you should drink the stuff). High enough that there is a very generous safety margin in products for use to treat clothing, where the quantity suggested to use is considered safe for pregnant women (and is even recommended by the CDC for such use). I don't think you could have applied enough to be an issue.

If you are truly worried, you can wash the clothes to remove some of the permethrin. The more vigorous the wash cycle, the more will be removed, as agitation is the primary mechanism of removal for the permethrin.

fyrepenguin
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drugs.com notes the Usual Adult Dose for Scabies is a Permethrin topical 5% cream, applied by thorough massage into the skin. 30 grams. That's essentially 1,5 grams pure permethrin applied to the skin directly.

I don't recommend doing so unless it's required, but if your spray is 0,5% and you've applied twice what is recommended, it would seem you're still way within safety margins. You're not massaging it into your skin so the effect per dosage on you will be much lower than the cream would have.

Worst case effects if you'd do this every so often (or are overly sensitive to it) would be pruritus, an itch. Washing it out would in my opinion be an overreaction indeed.

Mast
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    I tell you, massaging permethrin cream into your whole skin will make you sick. It kills insects by stimulating their nervous system to death, and even though humans are mostly resistant to it when I used it for scabies treatment it was like five cups of caffeine if caffeine just made you feel like you were going to vibrate yourself to death. Stick to ivermectin, sulfur, and heat treating bedding and clothes. – amara Jun 14 '21 at 00:51