Paraquat, or paraquat dichloride, is a synthetic (human-made) herbicide.¹ It is a member of the bipyridylnium or dipyridylnium chemical family.^2,3 Paraquat kills all vegetation that it touches. This is called a nonselective contact herbicide. It is also used as a desiccant, which dries out the crop before harvest. Paraquat is sometimes used as a plant growth regulator.³ For example, it can control unwanted growth in trees and vines. It may induce or delay flowering. It has no residual activity in soil. This means paraquat is not expected to keep killing plants after it is first applied.²

Paraquat is widely used throughout the United States.³ It was first registered in the United States in 1964. The U.S. Environmental Protection Agency (U.S. EPA) classified all paraquat products as restricted-use pesticides in 1978 because of paraquat's high acute toxicity.² Many paraquat products contain a dye and/or an emetic (which causes vomiting) to reduce the risk of people accidentally drinking it.⁴ The U.S. EPA has added additional training requirements for people who use paraquat, and requires special packaging to help prevent exposure.

In the United States, there are currently 24 registered pesticide products that contain paraquat.⁵ Paraquat is only formulated as a liquid. All paraquat products are restricted-use pesticides. Restricted-use pesticides are not available for the public to buy or use. Only people who have gone through special training and certification can buy or use paraquat. There are no products for home or residential uses.³

Paraquat is used on a wide variety of crops, including grains, vegetables, peanuts, and animal feed crops. It is used as a plant growth regulator for sucker control on orchard crops, hops, and grapes. Paraquat is used as a desiccant on crops such as potatoes and cotton before they are harvested.³ It is also used in pastures and non-crop areas including fencerows, airports, and electric substations.^2,3

Paraquat is not used in organic agricultural production.⁶

Paraquat is a nonselective contact herbicide and affects all plants.³ Paraquat is quickly taken up by leaves and stems.⁷ It destroys cell membranes in the plant tissue it touches. This stops photosynthesis.²

It is absorbed quickly by foliage but kills plant tissue so quickly that it is not likely to move throughout the entire plant. It is most effective against healthy, actively growing plant tissue.⁷ The plant can still grow from undamaged parts if it is a perennial.²

You may be exposed to a pesticide such as paraquat if you get it on your skin, breathe it in, or if you eat or drink it. For a pesticide exposure to harm you, you must be exposed to enough of it. If a pesticide is very toxic, it may cause harm from only a small exposure. You may be exposed to very low levels of paraquat residues on or in food because it is used on crops and pastures. These residues are expected to be well below the level that could cause harm. See the text box on pesticide tolerances.⁴

If you are very close to an application site and there is spray you could get paraquat on your skin or breathe it in. People have died from being exposed to paraquat by accidentally drinking paraquat that had been stored in beverage containers.³

The U.S. EPA requires pesticide handlers to wear chemical-resistant gloves and respirators when handling paraquat. Handlers must also use closed loading or transfer systems when putting paraquat into application equipment. People who apply paraquat must be certified applicators and must take specific training before they can use it.³ These requirements are meant to protect workers from exposure.

The U.S. EPA classified paraquat into Category E, "evidence of non-carcinogenicity for humans."¹⁰ This means that there is evidence that paraquat does not cause cancer in people.

The U.S. EPA concluded that "there is no concern for mutagenicity" for paraquat.¹⁰

Some animal experiments that involved feeding paraquat to rats including pregnant rats did not find developmental effects. Other experiments found effects to both the mothers and their young.^2,7 The U.S. EPA reviewed a variety of studies and concluded that the studies did not show evidence of developmental toxicity, unless the mother was also visibly harmed by the exposure.¹⁰

• In one study, scientists force-fed pregnant rats 0, 1, 3, or 8 mg/kg per day of paraquat on days 7-16 of their pregnancies. Neither the mothers nor their pups were affected.² See the text box on mg/kg.
• In another study, scientists fed paraquat to two generations of rats of both sexes before mating, during pregnancy, and after birth. They then repeated those exposures in the next generation. Neither the mother rats nor their babies showed any ill effects.⁷
• Scientists force-fed pregnant rats 0,1, 5, or 10 mg/kg of paraquat during days 6-15 of their pregnancies. Mothers in the two groups with the highest doses gained less weight, had trouble breathing, and had damage to their lungs and kidneys. Their fetuses had less bone formation in their toes than fetuses from mother rats not fed paraquat.²
• In another study, scientists gave pregnant female rats low doses of paraquat by stomach tube on days 6-15 of their pregnancy. The mother rats did not gain weight during pregnancy. They had a hunched appearance, their fur stood up, and they made croaking noises. The unborn pups of these rats weighed less and had less bone development. Mothers given the higher dose showed these signs more quickly. Some of the mother rats died before the end of the dosing period at the higher dose.⁷

No studies on the developmental and reproductive effects of paraquat on humans were found.

Pesticide exposure may contribute to other chronic health effects aside from cancer or developmental effects. Epidemiology looks at what people have been exposed to and what health problems they have later to see if the two are related. Researchers have studied relationships between paraquat exposure and several health effects. They have looked at Parkinson's disease, respiratory illness, and end-stage renal disease.¹⁰

The U.S. EPA reviewed 74 published epidemiology studies that looked for possible links between paraquat and human health effects. This included 26 studies examining Parkinson's disease, 17 that looked at respiratory effects, eight cancer studies, and 25 studies of other health effects.²² They also examined studies using live animals and studies of cells in the laboratory in the case of Parkinson's disease.²³

The U.S. EPA concluded that there was some evidence that people exposed to paraquat on the job may have a higher risk of developing Parkinson's disease. However, they determined that the data are not strong enough to conclude that paraquat exposure causes Parkinson's disease. This means that although a few studies reported that paraquat exposure was associated with Parkinson's disease, more research is needed to show that paraquat exposure actually causes Parkinson's disease. The U.S. EPA concluded that the evidence did not support a relationship between non-occupational exposure to paraquat and Parkinson's disease.²³

Other scientists have also conducted reviews of studies that look at the relationship between exposure to paraquat at work or at home and Parkinson's Disease. They used different combinations of studies and different methods than used by the U.S. EPA. They also concluded that there is evidence of a relationship between Parkinson's disease and paraquat exposure.^24,25 All reviews concluded that more studies and better data are necessary to determine if paraquat can cause Parkinson's disease.

The U.S. EPA also reviewed studies looking at paraquat and other health outcomes. The agency felt that current knowledge does not support that paraquat exposure could be associated with chronic bronchitis or cancer. However, the agency concluded that there is "limited but insufficient evidence at this time to conclude that" paraquat exposure is associated with end stage renal disease.²²

The U.S. EPA concluded that there is no evidence paraquat is more toxic to children than adults.¹⁰

As with any pesticide, young children may act in ways that put them at greater risk of being exposed to pesticides. For example, they may spend more time near the floor. They may also be more likely to place their hands in their mouths after touching treated surfaces or pets.

The form of paraquat used in pesticides is paraquat dichloride. Paraquat dichloride is a salt form that breaks apart into a positively charged ion, called a cation, that is the active form of the herbicide.⁷ It is highly soluble in water.^7,26

The vapor pressure of paraquat is very low.^7,27 This means it is unlikely to turn into a vapor. Paraquat did not vaporize from soil in laboratory tests.⁷

Plants do not break paraquat down into other chemicals.¹⁹ However, sunlight will break down paraquat residues on vegetation even after the plants are dead.²⁶ Microbes on the plants also break down residues of paraquat.²⁸

UV radiation intensity affects paraquat. In strong sunlight, two-thirds of an application of paraquat broke down within 3 weeks.²⁶ In another study, sunlight broke down 50% of paraquat in surface soils in 3 weeks.²⁹ Paraquat breaks down into chemicals that are low in toxicity and that degrade easily.²⁶

Paraquat will break down in alkaline waters.² Sunlight does not help break paraquat down if it is in water.^2,10 It may enter surface waters if it is bound to soil that washes into streams or ponds.²

Paraquat binds tightly to clay and organic matter in soil.^2,10 This limits its availability to plants and animals in the soil.⁷ Microbes may break down paraquat in soil if it is not bound to soil particles or organic matter.²⁶ This process is very slow because so little paraquat is available to the microbes. The half-life of paraquat in soils ranged from 1.4-7.2 years.³⁰

Paraquat is corrosive to metals.²

Because paraquat binds so strongly to soil, sediments, and organic material, it is thought to have low potential for wildlife or non-target plant exposure if there is no spray drift. However, impacts to non-target plants and animals have occurred.⁷ Sensitivity to paraquat varies widely among different kinds of animals, even among closely related species.^7,29,31