ORIGINAL: mcraddock

This might help:

The ThermaCELL Mosquito Repellent utilizes the active ingredient known as allethrin. It belongs to a class of materials called pyrethroids, which are synthetic analogs of pyrethrin, which is a naturally occurring insecticide found in certain types of chrysanthemum known as pyrethrum flowers. Mosquitoes really do not like this material. The allethrin is contained in the blue mats, and when they are heated it causes the allethrin to "volatilize", which is similar to evaporation. The volatilized allethrin forms a zone of protection around the unit The amount of active ingredient that is released is small - not enough to kill mosquitoes, but enough to repel them. There is 0.018 oz. of active ingredient on a mat.