Excerpts from Jim Conrad's
from the July 10, 2011 Newsletter issued from Mayan Beach Garden Inn 20 kms north of Mahahual, Quintana Roo, México
HOW PYRETHRUM WORKS
Pyrethrum is an extract from the flower heads of certain Chrysanthemum species. It affects the nervous system.
Animal functions such as movement and feeling are possible only as long as weak electrical impulses move through the body. They pass from nerve to nerve by way of "sodium channels," which function by conducting sodium ions (Na+) through cell plasma membranes. Wikipedia's illustrated page explaining the process is at http://en.wikipedia.org/wiki/Sodium_channel.
So, nerves do their jobs only as long as sodium ions can travel from nerve to nerve.
Pyrethrum -- or more correctly the six chemical compounds known as pyrethrins that are the active ingredients of pyrethrum -- bind to the sodium channels, "clogging them up." The result for the insect is a shut-down nervous system. The insect can't move, can't breathe, and can't do a lot of other things, either, so it dies.
Human nerves also have sodium channels, so is pyrethrum safe for humans?
The general wisdom is that in concentrations humans experience when using pyrethrum-based insecticides, or being where such insecticides have been used, no danger is involved.
However, pyrethrum is extremely toxic to fish such as Bluegills and lake trout, and moderately toxic to birds such as Mallards and Bobwhites, and of course it's lethal to many beneficial insects such as honeybees and flower-pollinating wasps.
Mammals are less affected because pyrethrins are easily metabolized and don't accumulate in the body. They break down in sunlight and are unlikely to concentrate in the food chain.
Still, it's good to remember that we living things are all related, and that we share many, many chemical processes that keep us going. If pyrethrum kills insects by clogging up sodium channels, a good guess is that it at least impacts in some small way the sodium channels of the human nervous system. In fact, among humans sometimes pyrethrum does produce skin irritation, itching, pricking and local burning sensations. At least one report describes more serious chronic effects, including circulatory and hormonal effects.