Permethrin is a common ingredient in many insect repellents and treatments for lice and scabies. It works by damaging the nervous systems of insects and arthropods when they come into contact with it, causing paralysis and death. Permethrin binds strongly to fabrics and surfaces, providing lasting protection against pests even through washing. However, some people wonder if permethrin can wash or sweat off the skin and clothing, reducing its effectiveness over time. Here is a detailed look at how permethrin works, how long it lasts, and whether sweating or getting wet can remove permethrin from treated surfaces and skin.
How Does Permethrin Work?
Permethrin belongs to a class of chemicals called pyrethroids, which are synthetic versions of naturally occurring compounds with insecticidal properties called pyrethrins. Pyrethroids work by disrupting the function of sodium channels in insect nerve cells. This causes the nerves to fire erratically, leading to paralysis and death.[1]
When permethrin is applied to clothing, bed nets, or other surfaces, it binds very tightly to fabric fibers through hydrophobic interactions. This means it is not easily dissolved in water or washed out with sweat or moisture. The strong binding allows it to remain effective after repeated washings and provide long-lasting repellency and insecticidal activity.[2]
On human skin, permethrin does not bind as tightly due to its poor solubility in fats/oils. However, it can persist on the surface of the skin, continuing to provide some repellent activity. The repellent effect diminishes over time as the permethrin gets absorbed into the skin.[3]
How Long Does Permethrin Last?
When applied to clothing and fabrics, permethrin is extremely stable and persists through many wash cycles. Here is an overview of how long permethrin lasts in different applications:
- Treated clothing: Permethrin bonds strongly to fabric fibers like cotton and remains effective even after 6+ washings. Insecticidal activity can persist for up to 2 weeks with a single application.[4]
- Mosquito nets: When used to treat mosquito nets and fabrics, a single application of permethrin can repel insects and kill mosquitoes that come in contact for 3-6 months.[5]
- Outdoor gear: Permethrin treatment on outdoor gear like tents and sleeping bags can remain effective for up to 70 washes. It helps repel insects like mosquitoes, ticks, and flies.[6]
- Human skin: When applied directly to the skin, permethrin can be effective for 3-6 days. However, it diminishes each time the skin is washed.[7]
So while permethrin breaks down faster on skin, it is extremely long-lasting on clothing, fabrics, and surfaces due to its strong binding to fibers and low solubility in water.
Can Sweating or Getting Wet Remove Permethrin?
Since permethrin adheres so tightly to fabrics, normal sweating and moisture is unlikely to significantly impact its effectiveness. Here is some research on permethrin durability:
- One study tested permethrin-treated uniforms that soldiers wore continuously for 50 days. There was no major impact from sweating or washing once per week.[8]
- Another test mimicked hot and humid jungle conditions by placing treated fabric in a 100% humidity chamber at 98°F. The permethrin remained stable and active.[9]
- Multiple washes with detergent only reduced the concentration of permethrin on fabrics by about 5-25% each wash. It took many cycles before the concentration declined substantially.[10]
The sweat and sebum on human skin may impact permethrin more than moisture on fabrics. One study tested repellent activity after individuals exercised vigorously for 30 minutes. For some individuals, sweat decreased protection time by up to 43%, but for others there was no major impact.[11]
So while soaking wet clothing or extensive sweating may slightly reduce permethrin levels, its insecticidal activity remains relatively stable. Unless you are completely submerging treated clothing in water, sweat and moisture should not significantly impact permethrin’s ability to repel and kill insects.
Tips to Maintain Permethrin Effectiveness
You can take some basic precautions to help your permethrin-treated clothing, gear, and fabrics maintain effectiveness:
- Reapply permethrin every 2-6 weeks depending on your activity level and how heavily you sweat.
- Wash clothing only when necessary in cold to lukewarm water.
- Line dry instead of machine drying if possible.
- Re-treat clothing after a few washes or once protection seems to be decreasing.
With proper care, most permethrin treatments can remain effective for many weeks or months, even with normal sweating and washing.
Is Permethrin Safe?
When used as directed, permethrin is very safe for humans. The US Environmental Protection Agency has reviewed many studies and concluded that permethrin does not pose risks of concern for humans, even with long-term exposure.[12]
While it is designed to disrupt insect nervous systems, permethrin has very low toxicity in humans and mammals. Here are some key points about permethrin safety:
- Does not easily absorb through human skin or get metabolized.
- Low potential for irritation or sensitization.
- Not linked to carcinogenic, mutagenic, or reproductive effects.
- Safe for pregnant women when following product instructions.
Of course, you should always carefully follow instructions when applying permethrin and take care not to ingest it or spray it directly on skin. Let sprays fully dry before using clothing and gear. With proper usage, permethrin-treated fabrics are very safe and do not pose risks from sweating or water exposure.
Frequently Asked Questions
How long does permethrin last on clothes?
Permethrin bonds strongly to fabric fibers and remains effective for up to 6 weeks and through several wash cycles when applied to clothing and outdoor gear. It may begin breaking down faster with heavy wear and sweating over time.
Should I reapply permethrin after getting rained on?
You do not necessarily need to reapply after getting wet from rain. Reapplication every 2-6 weeks is reasonable for heavy use. If rain is prolonged or heavy enough to completely soak clothing, reapplying may be beneficial.
Can I treat cotton, wool, and synthetic fabrics?
Yes, permethrin adheres well to most types of natural and synthetic fabric fibers. It can be used to treat cotton, polyester, rayon, wool, and more.
Does sun exposure break down permethrin?
Sunlight can gradually degrade permethrin, but it is stable enough to persist through normal outdoor use for weeks or months. Store treated fabrics out of direct sunlight when not in use.
Should I wash permethrin-treated clothes before wearing?
Washing is not necessary after proper drying time. Some people wash before first use as an extra precaution if skin sensitivities. Washing once before wearing is reasonable but not strictly required.
Is permethrin harmful to humans?
No, permethrin has very low toxicity in humans and mammals when used as directed. The EPA considers permethrin-treated clothing safe for long-term use. Take precautions to avoid ingesting it or getting it in eyes.
Conclusion
Permethrin binds extremely well to fabrics, making it resistant to normal sweating, washing, and water exposure. While heavy sweating or soaking may gradually reduce permethrin levels, its insecticidal activity remains relatively stable through hot and humid use. With proper application and occasional re-treatment, permethrin-impregnated fabrics provide safe and long-lasting protection against insects, ticks, and mosquitoes.