it's a chemical which is produced naturally by some plants, e.g. marigolds - that's why one might see marigolds planted around and in the midst of some home vegetable gardens.
it is toxic to many creatures, including insects and humans. so it is more of a poison than a repellent.
it is a nerve toxin which prevents the formation of and interferes with the action of certain neurotransmitters which form at neural synapses (junctions) and so, the little electrical impulses can't "jump the gap".
common sign of permethrin poisoning in humans is something that is similar to many nerve agents, viz. inability to swallow and foaming at the mouth. There are other more internal systemic effects also.
as far as humans go, there MAY be some who are hypersensitive to it, but supposedly this is rare. for most people, they report no problems. Look,...
in the litigious USofA (a world leader in the area of litigation, IMHO), a company like Ex Officio is comfortable marketing permethrin treated clothing under the tradename "BUZZ OFF". If there was a serious problem with humans contacting permethrin, they would have long ago been sued out of business. Makes sense, right? Well, it does to me at least.
So, what are the potential problems for humans? well, incidental skin contact is probably not an issue. Supposedly, oils and/or sweat on the skin renders permethrin harmless.
The problem (as i learned it over 30yrs ago in an Advanced Entomology course in College, dealing with insect pests) is typically with professional and non-pro (like a home owner) exterminators, and L/UL backpackers, SPRAYING permethrin (to treat homes or clothes) in either a closed environment or where the wind might blow the spray back into the face of the person spraying and a bunch of the chemical is inhaled and some finds its way into the aveoli deep in the lungs.
bottom line: be careful treating your clothes. do so, outside, on a non-windy day (or stay upwind of the spray).
after being educated on the safety of permethrin a couple of years ago by more knowledgeable folks on this website (i had learned it's a 'no-no' in that course i took), i've investigated permethrin further and now even own some permethrin impregnated clothing. i'm comfortable using it in the deer-tick infested forests and meadows of New England. I can report no problems with the clothing or with ticks and other six or eight legged critters. My personal conclusion, for me, at least, it is safe to use. I haven't had to re-treat the clothing yet - it is supposedly good for 20+ washes.
Please be sure to read Richard's and Doug's Posts which follow this one. They both have some very excellent remarks which i, unfortunately, failed to include in this Post.
Thanks guys for coverin' my six!