Toxicology information used for the
calculation of drinking water standards and the acceptance of food tolerance
values is largely provided by herbicide manufacturers, Using
test protocols provided by the U.S. EPA. As part of herbicide
registration requirements, batteries of animal toxicity tests are conducted,
including studies of both acute and chronic effects.
Acute effects appear shortly after
ingestion of the herbicide, and are in response to relatively high doses
of the pesticide. From such studies, a lethal dose is identified
which results in 50% mortality within a specified time period (LD50).
The oral LD50 of 2,4-D in the rat ranges from 375-666mg/kg; 370mg/kg in
the mouse; and less than 320 to 1,000 mg/kg in the guinea pig. The
rat and rabbit have dermal LD50 values of 1,5000 mg/kg and 1,4000mg/kg,
respectively. In humans, prolonged breathing of 2,4 - D causes coughing,
burning, dizziness, and temporary loss of muscle coordination. Symptoms
of poisoning can be fatigue and weakness with perhaps nausea. On
rare occasions there can be inflammation of the nerve endings with muscular
effects following high levels of exposure.
For chronic studies, groups of test
animals are exposed to various doses of the pesticide, so that the relationship
between pesticide dose and animal response can be determined. Animals
are examined for a variety of responses including developmental, reproductive,
and mutagenic effects, and for carcinogenicity. Rats given moderate
amounts (50mg/kg) of 2,4 - D in the diet for two years had no adverse effects.
Some dogs fed lower amounts of the compound in their food for two years
died, probably because dogs do not excrete organic acids efficiently.
A human given a total of 16.3 grams in 32 days as "desperation therapy"
lapsed into a stupor, showed signs of uncoordination, weak reflexes, and
urinary incontinence.
Reproduction effects after administration
of drinking water dosed with moderate levels of 2,4- D (about 50 mg/kg)
to pregnant rats did not result in any adverse effects on birth weights,
or litter size. Rats fed higher levels (188 mg/kg) had fetuses with
abdominal cavity bleeding and increased mortality. DNA synthesis
in the testes was significantly inhibited when mice were fed large amounts
(200 mg/kg) of 2,4 - D. Rats fed 150mg/kg on days 6-15 of pregnancy
had an increase in skeletal abnormalities such as delayed bone development
and wavy ribs which are a function of general toxicity. Traces of 2,4 -
D have been found in the milk of lactating animals for six days following
exposure. This indicates that humans may be at risk with 2,4
- D exposure though no direct evidence of reproductive problems associated
with 2,4 - D exposure exists.
In the case of carcinogenicity testing,
high doses are used so that if a compound is carcinogenic, statistically
significant increases in cancer can be detected in relatively small groups
of animals. The EPA then uses a model to estimate increases in cancer
risks that would occur for humans at doses such as those associated with
drinking water concentrations. The studies of 2, 4
- D carcinogenicity show a low incidence of brain tumors at moderate exposure
levels (45mg/kg/day) over a lifetime.
Ecologically 2,4 - D has a half -
life of ten days when applied to turf and seven days in the soil where
soil microbes are primarily responsible for its disappearance in soil.
In fish and oysters the half - life is less than 2 days when exposure is
discontinued. Despite its short half - life in soil and in aquatic
environments, 2,4 - D has been detected in groundwater supplies in at least
five States and in Canada. It has also been detected in surface waters
throughout the United States at very low concentrations. The maximum contaminant
level, 70 MCL(ul/L), is the maximum concentration allowed in drinking
water according to EPA drinking - water standards. The lifetime health
- advisory, 70 HA(ul/L), is the maximum concentration in drinking
water that would not cause adverse human - health effects, excluding cancer,
based on a 150 - pound adult consuming about 2 quarts of water per day
for a period of 70 years.
Return to Main Page
EXTONET ,1995
Pesticide information Notebook: Cornell
University, http://www.uoguelph.ca/GTI/urbanpst/24d.htm..
Hippe, D.J., 1997
National Water - Quality Assessment
Program:Water - Resources Investigations, U. S.
Geological Services, 1997, Report
94 -4183.