Permethrin is an insecticide that is commonly used to treat clothing and gear to help repel and kill ticks, mosquitoes, and other insects. When applied properly, permethrin binds tightly to fabric fibers and retains its effectiveness through multiple washings. However, the number of washes permethrin lasts depends on several factors.
How Permethrin Works
Permethrin is a synthetic insecticide that works as a contact poison, damaging the nervous system of insects that come into contact with it. It affects the sodium channels in insect neurons, causing paralysis and death. Permethrin has low toxicity for humans and other mammals due to differences in our sodium channels compared to insects.
When permethrin is applied to clothing and gear, it binds tightly to fabric fibers. The permethrin isn’t absorbed through the skin, but continues working by repelling and killing insects that land on treated fabric. The insecticidal activity remains effective through multiple washes, providing ongoing protection from bites.
Factors That Affect Durability
There are several key factors that impact how long permethrin lasts on clothing and gear with repeated washing:
– Application method – Permethrin products designed for self-treatment need to be applied evenly and thoroughly to maximize durability. Factory-treated fabrics are treated more uniformly for longer-lasting effects.
– Fabric type – Permethrin bonds best to fabrics like cotton, canvas, denim, and polyester. It tends to be less durable on shiny, water-resistant fabrics like nylon.
– Wash method – Machine washing, especially with warm water, depletes permethrin faster than hand washing in cold water. Tumble drying also reduces permethrin levels with each cycle.
– Frequency of washing – The more often an item is washed, the faster the permethrin breaks down. Less frequent washing preserves more of the original application.
– Pretreatments/detergents – Harsh detergents and bleach can degrade permethrin, while gentle soap has less impact.
Durability by Application Method
How permethrin is applied to clothing has a major influence on its durability and wash retention. There are two main application methods:
Factory-Applied Permethrin
Many brands now offer factory-treated clothing and gear with permethrin built into the fibers during manufacturing. The insecticide is evenly integrated throughout the fabric for maximum durability. Factory treatments retain effective levels of permethrin for typically 70 washes or more.
Some examples include:
– Insect Shield® – Estimated to last through 70 washes
– Sawyer® – Advertised to retain insect repellency after 75 washes
– ExOfficio® Buzz Off – Up to 70 washes
Factory treatments offer the longest-lasting application, often remaining at or above EPA-recommended concentrations even after 70+ washes.
Self-Applied Permethrin Sprays
Permethrin sprays allow you to treat your own clothes and gear at home. When applied correctly, self-applied permethrin binds to fabric and continues working through multiple washes. However, it may not bind as evenly or last as long as factory methods.
General durability guidelines for self-applied permethrin:
– Cotton, canvas, denim: Estimated 10-20+ washes
– Synthetic fabrics: 5-10+ washes
– Down fabrics: Up to 5 washes
Proper application technique improves durability. Be sure to thoroughly saturate the fabric, allow it to fully dry, and avoid over-saturation. Re-treating after a few washes can extend the lifespan.
Results of Wash Testing Studies
Scientific lab testing provides more definitive data on how many washes various permethrin applications last. Here are some results:
Factory Treatments
– Clothing by Buzz Off Insect Shield retained above 90% of original permethrin levels after 20 washes and above 50% after 70 washes in a [study](https://www.ncbi.nlm.nih.gov/pubmed/15212496).
– A [study](https://academic.oup.com/jme/article/39/6/869/846189?login=false) testing Insect Shield found 100% protection from mosquitoes after 75 washes.
– ExOfficio’s own testing found permethrin concentration dropped by only 25% after 20 washes and remained above minimum effective levels after 50 washes.
Permethrin Sprays
– Cotton and polyester fabric [still repelled](https://academic.oup.com/jme/article/39/6/869/846189?login=false) mosquitoes after 5 washes when treated with Sawyer Permethrin spray.
– In [one study](https://pubmed.ncbi.nlm.nih.gov/11758397/), army combat uniforms retained repellency after 6 washes. After 20 washes, effectiveness dropped by about half.
– On synthetics like nylon, Sawyer Permethrin [lasted for](https://www.ncbi.nlm.nih.gov/pubmed/15212496) 3-4 washes in a lab test.
When to Reapply Permethrin
To maintain protection, permethrin should be reapplied once the original application loses effectiveness:
– **Factory treatments:** Reapply after 70+ washes or 1-2 years of use
– **Self-applied sprays:** Reapply every 5-20 washes depending on fabric type
Check clothing periodically for decreased repellency against ticks or mosquitoes. Re-treat gear that is heavily used or washed often. For long backpacking trips or travel to high-risk areas, pretreating clothing just before the trip ensures maximum protection.
Washing Methods to Prolong Permethrin
While permethrin binds tenaciously to fabrics, certain washing methods help extend its lifespan:
– **Hand wash in cool water** instead of machine washing if possible
– **Air dry** treated clothing instead of machine drying
– Wash in **gentle soap or detergent**, avoid bleach
– **Limit wash frequency** by spot cleaning when possible
– Use a **delicate cycle** on the washing machine
– **Re-treat** gear after a set number of washes according to directions
These measures allow you to safely wash permethrin-treated items while getting the most washes possible before reapplication is needed.
Effectiveness Over Time
As permethrin breaks down with repeated washing, it becomes less effective against insects. However, clothing doesn’t go from fully protective to useless overnight. The insecticidal activity gradually declines over time.
Even when permethrin levels drop below the original concentration, treated clothing still provides some degree of repellency and protection from bites. The overall reduction in insect pressure still offers beneficial protection against diseases like Lyme disease.
While number of washes provides a general guideline, assessing real-world effectiveness over time provides a fuller picture:
– **0-20 washes** – Permethrin near full strength. Provides very high repellency and kill rates.
– **20-50 washes** – Moderate repellency and kill rates, but still substantially protective against insect bites/diseases.
– **50+ washes** – Gradual decline in effects. May still repel some insects but kill effects decreased.
Rotating permethrin-treated clothing helps maintain high effectiveness for protection against tick and mosquito-borne diseases.
The Takeaway
How long permethrin lasts depends mainly on the initial application method and how frequently clothing is washed. While factory treatments retain insecticidal activity for 70+ washes, self-applied sprays may start declining after 5-20 washes.
No matter the application method, permethrin binds tenaciously to fabric and remains protective against ticks and mosquitoes for longer than any other repellent or insecticide. Taking steps to wash clothing gently and re-treat after a set number of washes helps maximize its lifespan. But even as permethrin fades, it continues offering some degree of protection from biting insects and the diseases they can transmit.
Conclusion
Permethrin-treated clothing and gear is extremely effective for preventing tick and mosquito bites. While the number of washes it lasts for depends on many factors, proper application and washing methods can extend the lifespan of permethrin. Factory-treated fabrics retain repellent activity for up to 70+ washes. Self-applied sprays bind tightly too, but may start declining after 5-20 washes. No other insect repellent or treatment remains effective as long on clothing. So permethrin continues to protect against insect borne illnesses better than any other option, even as the original application degrades. Taking care to re-apply permethrin at the right intervals, wash gently, and rotate treated clothing are the best ways to maximize the protective benefits against tick and mosquito bites.