So, what's the big deal?
While pyrethroids may be amongst the least toxic of insecticides, they are an excitatory nerve poison, acting upon the sodium ion channels in nerve cell membranes:
by sending a train of impulses rather than a single one, they overload the pathways, blocking the passage of sodium ions across cell membranes; similar in action to organophosphates (which include the now banned DDT); inhibits ATPase, which affects the release of acetylcholine, monoamine oxidase-A and acetylcholine;
inhibits GABAa receptors, resulting in convulsions and excitability (and more 'minor' problems such as sleep disorders);
known to be carcinogenic;
liver damage
thyroid function
cause chromosomal abnormalities in mice and hamsters;
are highly toxic to insects, fish, and birds;
mimic estrogen, leading to estrogen dominant health problems in females and feminizing effects in males, including lowered sperm counts and abnormal breast development;
sublethal doses have produced a wide array of abnormal behaviors, including aggression, and disruption in learning and learned behaviors
What are the pyrethroids?
Pyrethrin is one of two liquid esters derived from Pyrethrum (feverfew) (C21H28O3 or C22H28O5) that are used as insecticides. Pyrethrum, a nonvolatile hydrocarbon related to kerosene, is a similar insecticide derived from and chrysanthemum flowers.
The fact that they are derived from plants causes some people to think - a point played up by companies using this toxin in their products - that they are safe. Lots of plants are toxic, some in small quantities, others in large. Just because it comes from something lush and ornamental does not mean it can't be lethal. Oleander, azalea, mistletoe, and foxglove are just a few of the pretty - and highly toxic - plants with which we live.
Permethrin is a synthetic pyrethrin (C21H20Cl2O3). In other words, it is a man-made poison that is a copy of two poisons found in plants.