Toxicity
Fipronil is classed as a WHO Class II moderately hazardous pesticide, and has a rat acute oral LD50 of 97 mg/kg.
It has moderate acute toxicity by the oral and inhalation routes in rats. Dermal absorption in rats is less than 1% after 24 h and toxicity is considered to be low. It has been found to be very toxic to rabbits.
The photodegradate MB46513 appears to have a higher acute toxicity to mammals than fipronil itself by a factor of about 10.
Toxicity on humans has been tested in few studies, more commonly involving human cells which were used in carcinogenicity studies, but with no adverse effects. Yet, fipronil has been classified as a Group C (possible human) carcinogen based on an increase in thyroid follicular cell tumors in both sexes of the rat. Furthermore, fipronil is considered slightly irritating to the skin, while moderately irritating to the eyes in both humans and animals, and there is no evidence suggesting it may cause birth defects.
Two Top Spot products were determined by the New York State Department of Environmental Conservation to pose no significant exposure risks to workers applying the product. However, concerns were raised about human exposure to Frontline spray treatment in 1996, leading to a denial of registration for the spray product. Commercial pet groomers and veterinarians were considered to be at risk from chronic exposure via inhalation and dermal absorption during the application of the spray, assuming they may have to treat up to 20 large dogs per day. Fipronil is not volatile, so there is little likelihood of humans being exposed to this compound in the air.
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