Thermacell is considered safe to breathe. All insecticides carry some risk, which must be balanced with the disease prevention they provide. Allethrin, a synthetic version of a mosquito-repelling compound that naturally occurs in chrysanthemum flowers, is the active ingredient in Thermacell products. Allethrin has been evaluated by the Environmental Protection Agency and is marked as toxicity class III—AKA slightly toxic or irritating—which means users should take care to follow the manufacturer’s guidelines when using the product.