used. The atmosphere is an important part of the hydrologic cycle that can transport
pesticides from their point of application and deposit them in unintended areas. Aver-
age annual concentrations of pesticides in air and rain are generally very low, although
elevated concentrations may occur during periods of high use, usually in the spring
and summer months. The environmental effects of long-term occurrences of low levels
of pesticides in the atmosphere are not yet well understood.
Air has the ability to move particles over long distances. Most of the time this abil-
ity aids mankind. It causes rain, for example. Unfortunately for the pesticide applica-
tor, wind also causes drift. Pesticides in the air are not controllable and may settle
into waterways, homes, lawns, wooded areas, and so on. Drift must be avoided.^9Pesticide NamesMost pesticides that have been studied in the atmosphere have been detected, and
many pesticides from several different chemical groups have been found at more than
half the locations of samples nationwide. Results for different groups and individual
pesticides reflect a range of influencing factors. Because of their widespread use dur-
ing the 1960s and 1970s, and their resistance to environmental degradation, organo-
chlorine insecticides have been detected in the atmosphere in every state where
measurements were made. The most heavily used organochlorine insecticides during
this time were toxaphene, DDT, and aldrin. Because of their reduced effectiveness
and regulatory restrictions, their total use in agriculture declined steadily from 63
percent of insecticide use in 1966 to less than 5 percent in 1988. The most fre-
quently detected organochlorine insecticides have been DDT, alpha-HCH, gamma-
HCH (lindane), heptachlor, and dieldrin. Despite their widespread use, toxaphene
and aldrin were detected less frequently than other organochlorine compounds, par-
tially because of their chemical properties. Toxaphene is a complex mixture of more
than 200 different compounds and is difficult to sample and analyze. Since toxaphene
use was banned in 1982, the analytical ‘‘fingerprint’’ of environmental samples often
differs considerably from analytical standards due to changes over time in its chemical
nature. Also, the analytical limit of detection is much higher than for other organo-
chlorine insecticides. On the other hand, aldrin in the environment degrades rapidly
into dieldrin, which is more chemically stable. This is why dieldrin was detected
more frequently than aldrin even though it was used in much lower quantities.
Organophosphorus insecticides also have been used heavily for decades and
account for 65 percent of insecticide use today. Generally, they are not as long-lived
in the environment as organochlorine insecticides, but nevertheless have been
detected in the air and rain in many states, even though they are not often included
as specific targets. The organophosphorus insecticides detected most often in air, rain,
and fog were diazinon, methyl parathion, parathion, malathion, chlorpyrifos, and
methidathion. Diazinon, methyl parathion, parathion, and malathion have been
among the most widely used insecticides in each of the last three decades, although
parathion, malathion, and diazinon use is declining.^10192 | Pesticides