The pyrethrins are very unstable in light and air,
limiting the areas where they are effective but also
providing a safety factor against the accumulation of
hazardous residues. Uses of pyrethrum and its
synergists are regulated by restrictions under the
Environmental Protection Agency and by tolerances
for levels in food and feed under the Food and Drug
Administration. The tolerance values for post harvest
applications to various plant products are commonly
1-3 ppm for pyrethrins and 8-20 ppm for
piperonyl butoxide. In several cases, these compounds
are exempted from the requirement for tolerances
because their safety relative to the levels likely
to be present under normal conditions is acknowledged.