Toxicodynamics There are two types of pyrethroids: those with and without α-cyanogroup. The
neurotoxicity of bifenthrin is based on the affinity to the voltage-gated
sodium channels (in insects as well as mammals). The pyrethroids with an α-cyanogroup block the sodium-channel permanently, causing the membrane to be permanently hyperpolarized. The resting potential will not be restored, and no further action potential can be generated. The pyrethroids without an α-cyanogroup, to which bifenthrin belongs, are only able to bind to the sodium channel transiently. This will result in after potentials and eventual continuous firing of axons. The resting potential is not affected by these pyrethroids.
Toxicokinetics Numerous studies have been conducted on the half life of bifenthrin in soil, water, and air under different conditions, such as aerobic or anaerobic, and at different temperatures and pH. It is more likely to remain in the soil and not so much in water (it is hydrophobic), nor in the air (it is unlikely to volatilize because of its physical properties). Because of the water-insolubility of bifenthrin, it will not rapidly cause contamination of ground water. However, some contamination might occur by soil bound bifenthrin to surface water through runoff. For an overview of the environmental degradation of bifenthrin, see figure below. The main path of degradation results in 4'-hydroxy bifenthrin.
Biotransformation Pyrethroids are much less toxic in mammals than they are in insects and fish, because mammals have the ability to rapidly break the ester bond in bifenthrin and break the substance into its inactive acid and alcohol components.
Toxicology Toxicity in animals ;Mosquitoes Bifenthrin is an effective pesticide to use against malaria and filaria vector mosquitoes. It is still effective when resistance to other pyrethroids is found. Mosquito nets and indoor walls can be treated with bifenthrin to keep more mosquitoes away. Bifenthrin is an effectively used insecticide, but the risk is high of it working only for a short time. Mosquitoes can develop a resistance to it, as well. ;Aquatic life Bifenthrin is hardly soluble in water, so nearly all bifenthrin will stay in the sediment, but it is very harmful to aquatic life. Even in small concentrations, fish and other aquatic animals are affected by bifenthrin. In cold water, bifenthrin is even more dangerous. pH and calcium concentration are also factors that influence the toxicity. Vertebrates are less sensitive to the effects of bifenthrin as ATPase-inhibitor. ;Bees In bees, the lethal concentration (
LC50) of bifenthrin is about 17 mg/L. At sublethal concentrations, bifenthrin reduces the
fecundity of bees, decreases the rate at which bee larvae develop into adults, and increases their immature periods. However, because of its success, they are being used more often (also indoors) and high exposure of bifenthrin to humans can occur.
Carcinogenicity The U.S. EPA classified bifenthrin as a Category C, possible human
carcinogen. This rating is based on an increased rate of urinary bladder tumors in mice, adenoma and adenocarcinoma of the liver in male mice, and bronchoalveolar adenomas and adenocarcinomas of the lung in some female mice.
Potential for neurotoxicity Bifenthrin can be absorbed by humans either by skin contact or ingestion. Skin contact is not toxic, causing only a slight tingling sensation at the point of contact. Ingestion in concentrations below 10−4
M is not toxic. However, commercially available bifenthrin products formulated for household use (such as Ortho Home Defense Max, sold as a liquid pump spray), can induce toxic effects due to
other chemicals added to improve the sustainability of bifenthrin or are toxic on their own. Symptoms of excessive exposure are nausea, headaches, hypersensitivity for touch and sound, and irritation of the skin and the eyes. ==Regulation==