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Most herbicides used in wildland situations have low acute toxicity to humans, and although some formulations can cause eye and skin irritation, most herbicides do not cause immediate noticeable systemic effects in people occasionally exposed to small amounts. However, several commonly used herbicides in weed management are associated with other effects such as cancer, birth defects, or endocrine disruption (see the table below).

Chronic and Reproductive Hazards of Common Herbicides for Humans

Herbicide Active Ingredient Cancer Rankinga Developmental Toxicityb Reproductive Toxicityb Neurological Toxicityc Suspected Endocrine Disruptorc
Aminopyralid Not Likely
No
No
Chlorsulfuron Not Likely
No
No
Clopyralid Not Likely
No
No
Glyphosated Probable
No
No
Imazapyr d Not Likely
No
No
Polyoxyethylene-amine (POEA) (surfactant)e
Yes
Triclopyrd Unclassifiable
g
No
No
2,4-D Possible
g
No
Yes

Hazard summary for humans is based on animal data. An entry of “—” indicates that the chemical is not listed as having that toxicity. This may mean it is not toxic or simply that it hasn’t been tested.
a From US EPA registration documents, the International Agency for Research on Cancer (IARC), the National Institutes of Health 13th Report on Carcinogens, or the California Proposition 65 list.
b From the California Proposition 65 list.
c From the European Union Pesticide Properties Database.
d Includes salts and esters of the parent compound.
e POEA is the surfactant used in some glyphosate formulations that has been found to be an endocrine disruptor. It is also more toxic than glyphosate, especially to aquatic life.
f “Unclassifiable” means that cancer studies in laboratory animals have produced equivocal results, and insufficient data are available to resolve the uncertainty.
g These herbicides are not on any official list for developmental and reproductive toxicity, but US EPA registration documents describe developmental or reproductive toxicity in animal studies, with Lowest Adverse Effect Levels (LOAEL) less than or equal to 100 mg/kg. With the inclusion of the usual 100-fold uncertainty factor for humans, the Reference Dose (RfD) for humans is less than or equal to 1 mg/kg, an exposure that is in the range of possibility for applicators working with herbicides. Women of childbearing age should avoid use of these herbicides.

Workers applying herbicides are the most likely to be exposed, through spills, spray to exposed skin, contact with treated vegetation, or inhalation of spray mist. This type of general exposure is difficult to avoid entirely, as a number of biomonitoring studies have shown (see the USFS risk assessments for the details of these studies for specific pesticides). The plots below show the relative risks of the different herbicides for both general exposure and for exposure from direct spills.

General Applicators
General Applicators

Reading the chart: For each bar, the labeled central value is the most likely estimate. The right end of the bar assumes worst-case conditions for all underlying variables; the left end of the bar assumes best-case conditions. Mitigation is advised if risk enters the red zone.

Methodology and sources

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