12
Pesticides, Organic Contaminants, and Pathogens in Air
FIGURE 2.5
Carbamates include carbofuran and carbaryl.
hard to know since the exposure took place 50 years or more before paralytic
effects were noticed (see the discussion of paraquat toxicity in Chapter 4).
2.3.3 Carbamates
Carbamates are a newer class of cholinesterase inhibitors. They act by blocking nerve impulses of insects, causing hyperactivity, tetanic paralysis, and
then death. Most are not persistent and do not bioaccumulate in animals
or have signifcant environmental impacts. Carbamate insecticides include,
among others, aminocarb, fenobucarb, aldicarb, carbofuran (Figure  2.5),
fenoxycarb, and carbaryl (Figure 2.5). They are generally less toxic to mammals than OPs but are toxic to bees.
Carbaryl was introduced by Union Carbide in about 1950 and is still used
today in homes and on crops. It is of low toxicity and moderate persistence.
Methomyl and related carbamates like Temik are effective low residual carbamates but have potential to leach in sandy soils and contaminate groundwater. The unorthodox use of N-methyl carbamates to hasten the growth of
cucumbers and melons has led to sporadic reversible symptoms of AChE
inhibition in consumers.
2.3.4 Synthetic Herbicides
Popular synthetic herbicides began with the introduction of phenoxy herbicides
2-methyl-4-chlorophenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic
acid (2,4-D), and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) (Figure 2.6). The
latter two are components of “Agent Orange” used by the U.S. military as
a defoliant during the Vietnam War. Agent Orange is on the “dirty dozen”
list of the World Health Organization as off-limits for nations worldwide
because it is contaminated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD;
“dioxin”)—one of the most toxic synthetic chemicals known to humans