What is 2,4-D?
The pesticide 2,4-D (short for
2,4-dichlorophenoxyacetic acid) comes in ammonium (amine) salt or ester forms.1
Its salts are readily absorbed by the roots, while esters are readily absorbed
by the leaves, and the active ingredient is then transported throughout the
plant where it acts as a growth inhibitor by mimicking the plant hormone auxin.1
It selectively controls broad leaf weeds in cereals (after the crop has
emerged), pastures, forestry, parks and home gardens; it is also used to control
broad-leaved aquatic weeds.1
Usage
2,4-D is among the most widely used herbicides in the
world. It was first introduced in the 1940’s and is the’biggest seller’
amongst the chlorophenoxy acids or esters, a class of similar compounds used as
herbicides.2
Most of these are sold in mixture with other herbicides to increase the spectrum
of weed contol.2
In the non-crop sector 2,4-D was among the five most used pesticide products
globally.3
Formulations of herbicides based on 2,4-D
Surfactants or ‘surface-active agents’ are added
to reduce the surface tension, helping to emulsify liquid solutions and
increasing their ability to spread over surfaces.4
Some 2,4-D-based products contain kerosene as surfactant,5
which is hazardous to health: its inhalation may cause neurological damage and
skin contact may cause dermatitis if prolonged or repeated; and when ingested or
inhaled it may cause respiratory swelling and pneumonitis, and may lead to lung
oedema (excess fluid in tissue).6
Triisopropanolamine is used as surfactant in other formulations of 2,4-D,7
and this may cause irritation of the respiratory tract; its inhalation may be
fatal as a result of spasm, pneumonitis and oedema.8
Dimethylamine is also used,9
and exposure to this may lead to severe skin irritation and burns; when inhaled
it can cause injury to the upper airways and lung oedema.10
Short-term toxicity of 2,4-D
Acute toxicity refers to the immediate effects (0-7
days) of exposure to a substance. The World Health Organisation (WHO) ranks
2,4-D in toxicity class II: ‘Moderately Hazardous’.11
2,4-D can irritate the eyes, skin and breathing passages.12,13
The acid and salt forms are 'considered to be severe eye iritants' and
consequently are in toxicity category I ('Danger') for eye irritation.14
Breathing in 2,4-D can cause coughing, burning, dizziness, and temporary loss of
muscle coordination; other symptoms of poisoning can be fatigue and weakness
with possible nausea.15
It has been found that ‘agricultural exposure to commercial
2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA)
formulations may exert short term immunosuppressive effects’.16
Inhalation of the evaporated active ingredient may be more likely with 2,4-D
esters as these are more volatile than the salt forms.17
Prolonged exposure to 2,4-D and chronic effects
Long-term exposure to 2,4-D has resulted in nervous
system effects in humans and animals.18-20
Exposure to large amounts causes stiffness of the limbs21,22
and damages the liver and kidney.13 Hepatitis and a form of sclerosis
have been observed in persons exposed to lawn pesticides.23,24
2,4-D has been found to have an effect on the immune and endocrine systems in
tested animals,25-27
and there is evidence that 2,4-D causes birth defects and affects reproduction
both in animals and humans.28-30
Harmful effects on development have been observed in animals.31-33
Studies in the USA and Canada found an association
between cancer mortality and the area treated with herbicides,34,35
and the incidence of non-Hodgkin's lymphoma (NHL, a malignant tumour) and
several types of cancer were found to be higher among farmers who spray
herbicides.36-42
Chlorophenoxyacetic acid (ester) herbicides have been linked to an increased
risk for NHL, soft tissue sarcoma (tumour of connective tissue) or myeloma
(tumour of bone marrow).43-46
An increased risk of bladder cancer in dogs has been linked to phenoxy
herbicides.47
Two studies found an excess of NHL among farmers exposed to 2,4-D,48,49
and a correlation was observed between the use of 2,4-D and cancer incidence in
California among Hispanic men (who traditionally work on farms in California).50
Opinion is divided on the potential of 2,4-D to cause cancer, as an observed
increase in cancer risk among workers in the agrochemical industry is likely to
have been caused partly by contaminants.51
In 1994 three out of eight samples from 2,4-D products were found to contain
dioxins.52
In tests on plants and chick embryos 2,4-D caused chromosomal aberrations and
other genotoxic effects.53-55
White blood cells from pesticide applicators who were spraying only 2,4-D during
a 3-month season were found to have a higher proliferation than before exposure
and compared to a control group.56,57
A study evaluating cancer risk from 2,4-D exposure concluded that 'evidence for
2,4-D causing cancer in humans is inadequate to classify it for its cancer
risk'.58
The International Agency for Research on Cancer (IARC) classifies 2,4-D as
'possibly carcinogenic to humans' (class 2B).59
In a preliminary risk assessment the US Environmental Protection agency (EPA)
has ‘identified several limited use scenarios where risks appear to be
elevated’ and states that these ‘are confined to uses that involve direct
application to water, which leads to potential concern for drinking water
derived from these water bodies’.60
The EPA is currently considering a Special Review of 2,4-D.61
Environmental fate and ecological impact of 2,4-D
In soil 2,4-D is degraded quite rapidly (the half-life
or time for reduction by 50% in different soils ranged from 3 days to 22 days).62
2,4-D was found to be 'weakly sorbed by soil' and sorption 'generally increased
with increasing soil organic carbon content'; the rate of 2,4-D degradation
depended on the composition and pH of soils.62Degradation
has been found to proceed more slowly in soil under trees.63
A study that measured residues of herbicides in water of soil samples from
Canadian prairie farms found that 2,4-D was the most frequently detected
herbicide (it has been used in large amounts on the Canadian prairies for more
than 50 years), and that 'under appropriate environmental conditions, the bottom
sediments may act as a source of herbicides to the water column'.64
The authors of this study observed that 'on occasion maximum herbicide
concentrations did exceed aquatic life and irrigation water guidelines'.64It
has been reported that in addition to agricultural usage urban usage 2,4-D and
MCPA [4-chloro-2-methylphenoxyacetic acid] 'contributed to loadings along the
Red River'.65
2,4-D has been measured in pond water at concentrations of up to 0.29 part per
million (ppm).66
2,4-D is 'Moderately hazardous' to certain bird species (e.g. to pheasants, as
classified by the WHO),67
and it is 'Moderately toxic' to some fish species (according to Kamrin 1997).68
Esters are toxic to fish,69
especially the butoxyethyl (butotyl) ester.70
In Greece the survival rates of young tortoises exposed to annual spraying with
2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were 'significantly lower
in the affected areas'.71
The US EPA has assessed the environmental risks of 2,4-D and believes that
'2,4-D exposures through spray drift and runoff present the greatest potential
risks to terrestrial plants, mammals, and birds compared to the other taxonomic
groups evaluated in this assessment, while exposures to 2,4-D through the direct
application to water for aquatic weed control present the greatest potential
risk to aquatic plants and animals'.72
And the authors state that 'it should be noted that there will still be
exceedances of LOCs [Levels of Concern] for non-target terrestrial plants, small
mammals, and birds for the lower label rates for major uses'.71
Conclusions
A safety evaluation concluded that 'the
epidemiological data do not indicate that a measurable public health threat is
presented by continued use of 2,4-D' and that 'the levels of 2,4-D to which
humans would be exposed under normal operating conditions would not be expected
to result in any immunological, neurological, reproductive, or developmental
effects'.73
However, a study has shown that exposure levels for young children indoors and
one week after 2,4-D was applied to the lawn were 'estimated to be about 10
times higher than the preapplication exposures', and it was estimated that
non-dietary exposure contributed to a major extent to the overall exposure after
application.74
2,4-D has been banned in Denmark and Norway and cancelled in Sweden,75
and in view of the available evidence of its hazards to health and the
environment 2,4-D should be substituted by safer alternatives and its use
significantly reduced, especially in developing countries where farmers'
protective equipment is often insufficient.
(Article written by Richard Isenring, based partly on a draft by Ronald Macfarlane for PAN Asia and the Pacific)
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67 op. cit. 1
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69 op. cit. 1
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75 Pesticide Action Network (PAN) North America, Pesticides database, San Francisco 2002 (http://www.pesticideinfo.org/Index.html).[This article first appeared in Pesticides News No. 65, September 2004, pages 20-21]