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type of formulation, and warning properties (odour and taste) are other
important factors.
Key toxicity parameters and other features of representatives of four groups
of pesticides that have been developed since the 1940s are compared to
illustrate the progress made in improving the safety of pesticides (Table 3).
Dieldrin represents organochlorines, a highly effective group of insecticides
that includes dichlorodiphenyltrichloroethane (DDT) and has been in use
since the early 1940s. Dichlorvos belongs to the large group of organophosphates
that became available in the 1950s (it is not possible to find a true
representative of this group because of the large differences in toxicity among
the related substances). Cypermethrin is a pyrethroid, a group that was
developed in the 1960s and 1970s. The rodenticide, flocoumafen, represents
the new generation of anticoagulants, the so-called "super-warfarins," developed
in the past decade.
Dieldrin has oral and dermal LD50 values (dose lethal to 50% of animals tested)
that place it in the highly hazardous class lB in the World Health
Organization's (WHO) classification system. Low NOAELs ("No adverse
effect level") in the 90-day and chronic feeding studies also point to a high
systemic toxicity. In spite of its toxicity and rapid skin penetration, lethal
intoxication from occupational exposure has been rare over 40 years of use;
only one case has been reported (IPCS 1989; van Raalte, unpublished paper
presented at the Conference on occupational health, Caracas, Venezuela,
1965). Clinical recovery from intoxication, occurring during manufacture and
formulation, was always complete (lager 1970).
The persistence of dieldrin in the environment was initially viewed as an
advantage because it was associated with long-lasting pesticidal activity.
However, in light of ecotoxicological consequences, it is now considered a
major disadvantage. Although dieldrin and the other organochlorines have
good records of safety in regard to human health, their use has been restricted
because of ecotoxicological considerations and, in North America, because of
a perceived cancer hazard.
Dichlorvos should also be classified as highly hazardous, because of its acute
toxicity in laboratory animals and its skin penetrating properties. The 90-day
and chronic feeding studies show NOAELs 10 and 20 times higher, respectively,
than dieldrin, although observed carcinogenic effects in laboratory
animals are not relevant to man. No reports of lethal work-related intoxications
with dichiorvos are known, but other organophosphates, in particular
parathion and malathion, have caused many deaths despite the availability of
specific antidotes. Certain organophosphates, now withdrawn, cause irreversible
neurological defects in patients recovering from intoxications (fl'CS 1986).
Not all organophosphates represented a step toward safer pesticides. However,
their rapid degradation in the environment represented an improvement
from an ecotoxicological point of view.
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