The summer heat often brings with it swarms of biting insects. There is debate on the use of insect repellents and FR clothing. Below is an article I wrote helping clarify the matter – Mark

There has been frequent discussion about the use of insect repellants, particularly ones with DEET, and their affect on FR garments. Many FR wearers can be exposed to biting insects like mosquitoes and ticks and therefore have a need to protect themselves with an effective repellant, but one that does not compromise the effectiveness of the FR. The US Forest Service performed flammability testing on FR fabrics with both DEET based products and Permethrin based products applied to FR fabric.

What is DEET? 

DEET is the chemical abbreviation for N,N-diethyl-meta-toluamide, a common active ingredient used in many popular insect repellants like Off. DEET was developed by the US Army back in World War II. It is highly flammable which is obviously not good for FR clothing. DEET is typically applied to either the skin or to clothing.

What is Permethrin?

Permethrin is the synthetic version of pyrethrum which is derived from crushed and dried flowers of the daisy. Unlike DEET, Permethrin is designed to be applied to fabric not the skin.

US Forest Service Testing

The testing performed by the US Forest Service comprised of taking several weights of Nomex IIIA fabric and applying 100% & 40% concentration of DEET, and .5% concentration of Permethrin to the fabric samples. The samples were also allowed to dry in varying drying times and varying numbers of application. After the samples were prepared they were submitted to a vertical flame test (Federal Test Method 191, Method 5903.1). The measurement of the flame-resistance using this method is to measure the inches of char length and the seconds of after flame which was done.

Test Results

The testing resulted in several interesting findings. The first was all but one of the samples using DEET experienced a phenomenon they dubbed flash flame. This phenomenon is one where the entire length of the sample showed signs of burning during the test which is considered a failure. The second finding was most of the samples failed because the after flame was too long. The third finding was the one DEET sample that passed was the heavier weight (7 oz) fabric with a 1 hour wait time between the DEET application and testing. Lastly, none of the Permethrin samples failed the test and showed no signs of the flash flame phenomenon.

There was also a field trial in performed in Alaska where a combination of DEET applied to the skin and Permethrin applied to the clothing was shown to create an excellent barrier to mosquitoes. In this trial they found this combination had a 99.9% protection rate. The DEET used was a 35% concentration and the Permethrin was applied (outdoors) to both sides of the fabric enough to moisten and allowed to dry for 2 to 4 hours before wearing.


This testing would indicate that the use of DEET on FR fabrics/garments would pose a potential hazard to the wearer. If used, DEET should be applied to the skin as directed. On the other hand Permethrin products can be applied to the FR fabric/garment without diminishing the protective performance of the clothing. Permethrin must also be applied following the manufacturer’s instructions. It should also be noted that although Permethrin has low toxicity to mammals and birds, it is know to be toxic to both fish and bees. If you have a need, either of these products can be effective if used properly.