Permethrin is a synthetic insecticide engineered to mimic natural insecticidal compounds. It is widely used as a topical treatment for parasitic infestations like head lice and scabies. Permethrin is also incorporated into clothing as an insect repellent and utilized in various pest control products. The product is designed with specific dwell times, after which it must be removed to prevent unintended consequences from prolonged contact.
Localized Effects of Prolonged Skin Contact
The most immediate and common reactions to leaving permethrin on the skin longer than recommended are localized to the application site. Extended contact can lead to contact dermatitis, characterized by visible symptoms like redness, itching, and a mild rash. This dermal reaction is often compounded by a mild, transient burning or stinging sensation.
A distinct effect of prolonged pyrethroid contact is paresthesia, a temporary sensory phenomenon. This is often described as a tingling, prickling, or “pins and needles” feeling at the site of exposure. This sensation occurs because permethrin acts directly on sensory nerve endings, disrupting the normal function of sodium channels.
This localized sensory effect is a direct result of the chemical’s mode of action, causing repetitive firing of the nerve fibers. These reactions, including the paresthesia, typically resolve once the substance has been thoroughly removed from the skin.
Risks of Systemic Absorption
While localized effects are common, the more serious concern with prolonged exposure is the potential for the chemical to be absorbed through the skin and enter the systemic circulation. Permethrin exhibits low toxicity in mammals because the body possesses efficient mechanisms to rapidly break it down. Human liver enzymes, such as carboxylesterases and cytochrome P450 enzymes, quickly metabolize permethrin into inactive compounds that are easily excreted.
Leaving high concentrations of the product on the skin for extreme durations, or accidental ingestion, can overwhelm the body’s metabolic capacity. Studies show that less than two percent of the applied dose is typically absorbed through the skin in humans. The rate of absorption is affected by the product’s concentration and the vehicle it is dissolved in, with agricultural formulations posing a greater risk than medicinal creams.
If a sufficiently large amount is absorbed systemically, symptoms of toxicity can develop as the compound affects the central nervous system. These effects include headache, dizziness, fatigue, and gastrointestinal distress such as nausea or vomiting. True systemic toxicity is rare following standard topical use but requires immediate medical attention if severe symptoms, like muscle weakness or difficulty breathing, manifest.
Decontamination Protocol and Emergency Action
The primary action following the discovery of unwashed or excessively prolonged permethrin exposure is immediate decontamination to limit further absorption. Contaminated clothing must be promptly removed to prevent the chemical from remaining in contact with the skin. The exposed area should be washed thoroughly with large amounts of soap and water. Since permethrin is often formulated in oily carriers, repeated washing may be necessary to ensure all residue has been completely removed from the skin.
If permethrin has splashed into the eyes, they must be flushed continuously with copious amounts of water for at least fifteen minutes, and contact lenses should be removed during the initial minutes of rinsing. In cases of accidental ingestion, which carries the highest risk of systemic toxicity, vomiting should not be induced unless specifically instructed by a medical professional. If an individual experiences severe or worsening systemic symptoms, such as difficulty breathing or neurological changes, they should seek emergency medical care immediately. For advice on managing exposure, contact a regional Poison Control Center.