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Heat, humidity affect absorption of insecticide from Army uniforms in study

Army soldiers wearing permethrin-treated uniforms absorb more of the insecticide through their skin when they are in hot or humid environments than under room temperature conditions, according to a study e-published in late March by the American Journal of Tropical Medicine and Hygiene that could also have implications for civilian populations.

The multi-institutional research team included scientists from the US Army Research Institute of Environmental Medicine, the Henry M. Jackson Foundation for the Advancement of Military Medicine, the Centers for Disease Control and Prevention's National Center for Environmental Health, the VA Boston Healthcare System, and Boston University.

Since 2013, all Army personnel have been required to wear uniforms that have been factory-treated with permethrin to protect soldiers from biting insects and vector-borne diseases in a variety of training and deployment environments. The acute toxicity of permethrin is low when absorbed dermally, but there may be a risk for longer term health issues, particularly with extended exposure. Permethrin has been classified by the U.S. Environmental Protection Agency as a potential carcinogen because some evidence suggests it may cause cancer in animals.

The study used simulated environmental conditions to examine the effects of high temperature and a combination of high temperature and humidity on permethrin dose among 27 enlisted soldiers wearing permethrin-treated uniforms over 33 hours (to reflect a deployment or training scenario). The researchers used urine samples to assess biomarkers for permethrin absorption in three different testing environments: simulated high temperature (35°C and 40% relative humidity), simulated high temperature and high humidity (30°C and 70% relative humidity), and ambient conditions (13°C and 60% relative humidity)

Soldiers in the heat and heat/humidity groups showed increasing biomarker concentrations shortly after entering the simulated environmental conditions; those concentrations returned to baseline levels only after leaving the simulated environmental conditions. Biomarker concentrations were 60–90% higher in the heat and combined heat/humidity groups than the ambient group. Also, the average daily permethrin dose, calculated 12 hours after removing the treated uniforms, was significantly higher in the heat and the heat/humidity groups than the ambient group.

Although the permethrin biomarker concentrations measured in this study were well above U.S. national values, the observed significant increases in permethrin biomarker concentrations among the heat group and the heat/humidity group remained on average 3–3.5 times below the World Health Organization's (WHO) acceptable daily intake (ADI) for oral permethrin ingestion. Even the maximum average calculated daily dose in the heat/humidity group was almost two times lower than the WHO ADI.

However, the long-term effects of extended dermal permethrin exposure in soldiers—and civilian populations who work in similar environments—remain unclear.

"As the long-term use of these uniforms continues to be a vital part of protecting soldiers from disease-carrying insects, it is important to examine factors that affect permethrin exposure and absorption and, thus, the conditions that contribute to higher dose," the authors wrote.
Read the study here: https://www.ajtmh.org/content/journals/10.4269/ajtmh.19-0543

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