Permethrin concentrate is a liquid insecticide you dilute with water before spraying around your home, yard, or on clothing and gear. It comes in several strengths, most commonly 10% and 36.8% active ingredient, and is one of the most widely used insecticides for mosquitoes, ticks, fleas, ants, and dozens of other pests. The concentrate format is far more economical than ready-to-spray bottles, but it requires careful mixing to get the right strength for each job.
How Permethrin Kills Insects
Permethrin belongs to a class of insecticides modeled after natural compounds found in chrysanthemum flowers. It works by forcing open sodium channels in insect nerve cells, which causes uncontrolled firing of nerve signals. Insects exposed to even small amounts experience rapid paralysis and death. The compound is dramatically more toxic to insects than to mammals because insects have more sensitive sodium channels and much smaller body mass relative to the dose they encounter.
Common Concentrate Strengths
The two most common retail strengths are 10% and 36.8% (sometimes labeled 38%). A 10% concentrate is typically the cheapest entry point: an 8-ounce bottle runs under $8 at most farm supply stores. A 36.8% concentrate costs more per bottle but yields significantly more finished spray per ounce, making it the better value for large-area treatments like yards or barns.
There are also 40% concentrates sold in smaller 4-ounce bottles, usually priced under $25. These are popular for clothing treatment, where you only need a small batch at a time.
Mixing and Dilution
Most outdoor pest control applications call for a final solution of about 0.5% permethrin, while clothing treatment and lighter-duty applications often use 0.5% or lower. The label on your specific product is the authority here, since formulations vary, but here’s the general math for mixing a common batch.
To make 32 ounces of a 0.5% spray from a 10% concentrate, you need roughly 1.6 ounces of concentrate topped off with water. For a 0.3% solution from the same 10% concentrate, you’d use about 1 ounce of concentrate (roughly 1.8 tablespoons) per 32 ounces of water. With a 36.8% concentrate, you need far less product per batch, so measuring precision matters more.
The simplest formula: multiply the total spray volume you want (in ounces) by the desired final percentage (as a decimal), then divide by the concentrate percentage (also as a decimal). That gives you the ounces of concentrate to add. Fill the rest of the container with water and shake thoroughly.
Treating Clothing and Gear
One of the most popular uses for permethrin concentrate is treating outdoor clothing, boots, and gear for tick and mosquito protection. This is especially common among hikers, hunters, and people working in tick-heavy areas. When applied correctly, permethrin binds tightly to fabric fibers and persists through multiple washes.
A few important rules apply. Not every permethrin product is labeled for clothing treatment. Look specifically for products with instructions for treating fabrics. Apply the solution outdoors in a well-ventilated area, away from wind. Hang treated items outside and let them dry completely before wearing. Never apply permethrin to clothing while you’re wearing it, and never apply it directly to skin. It is poorly absorbed through skin on its own, but sunscreens and other topical products can increase absorption. Wash permethrin-treated clothing separately from the rest of your laundry.
Safety While Mixing and Spraying
Permethrin concentrate is far more potent than the diluted spray you’ll end up with, so the mixing step is when your exposure risk is highest. Wear chemical-resistant gloves and eye protection at minimum. Long sleeves and closed-toe shoes are standard practice. If you’re spraying in an enclosed or semi-enclosed space, a respirator rated for organic vapors adds an important layer of protection.
After spraying an area, no one (including pets) should enter for at least four hours. Many product labels specify a re-entry interval of 12 to 24 hours for general use areas. If the label doesn’t state a specific time, a 24-hour waiting period is a reasonable default for areas classified as slightly toxic. For moderately toxic formulations, 48 hours is the standard restricted period. Let all sprayed surfaces dry completely before allowing contact.
Serious Danger to Cats
Permethrin is extremely toxic to cats. This is not a mild sensitivity; it can be fatal. Cats lack a key liver enzyme that other mammals use to break down permethrin, and they also appear to process it more slowly through other metabolic pathways. The result is that doses a dog would barely notice can send a cat into neurological crisis.
Symptoms of permethrin poisoning in cats include extreme sensitivity to touch, full-body tremors, muscle twitching (often visible in the ears and whiskers first), overheating, and seizures. These signs can appear within hours of exposure or take up to 72 hours to develop, and they generally last two to three days even with veterinary treatment. Many poisoning cases happen when cat owners unknowingly apply a dog flea product containing permethrin, or when a cat grooms or cuddles a recently treated dog.
If you have cats, keep them away from freshly treated clothing, sprayed areas, and any dog that has recently been treated with a permethrin product. Store concentrate bottles where cats cannot contact them.
Toxicity to Fish and Aquatic Life
Permethrin is highly toxic to fish and aquatic invertebrates at very low concentrations. It binds tightly to soil particles, which limits how far it travels through groundwater, but surface runoff into ponds, streams, or storm drains can deliver lethal doses to aquatic ecosystems. In soil, permethrin breaks down through microbial activity and sunlight, with a half-life that varies from a few days in sunny, biologically active soil to several weeks in shaded or compacted ground.
If you’re spraying near any body of water, including ornamental fish ponds, maintain a generous buffer zone. Avoid spraying before rain, and don’t rinse spraying equipment where runoff could reach waterways.
EPA Classification and Long-Term Exposure
The EPA classifies permethrin as “likely to be carcinogenic to humans” by ingestion, based on benign lung and liver tumors found in mice at high doses and supporting chemical-structure analysis. This classification applies to oral exposure, not to the skin contact or inhalation levels typical of home use. The EPA’s established safe daily exposure level is 0.25 mg per kilogram of body weight per day, derived from animal studies with a 100-fold safety margin built in.
For a typical adult, normal use of diluted permethrin spray, with gloves during mixing and proper drying time before re-entry, results in exposure levels well below that threshold. The concentrate itself, however, is orders of magnitude more potent than the finished spray, which is why careful handling during the mixing step matters so much.