How Glyphosate Works

Glyphosate is a herbicide used to control weeds and grasses. It is the active ingredient in the popular weed killer Roundup and other similar products.

Although the U.S. Environmental Agency (EPA) and other international health authorities regard glyphosate weed killers as safe when used as directed, the International Agency on the Research for Cancer (IARC) has listed it as a "possible carcinogen" (cancer-causing agent).

This article explains what glyphosate is, how it is used, and where it is found. It also takes a closer look at the potential risks and why public health authorities have differing views on the widely used herbicide.

What Is Glyphosate?

Glyphosate is a non-selective herbicide. This means that it is not selective in the types of plants it kills; it can kill all plants it comes into contact with. Glyphosate works not by "poisoning" a plant but by preventing it from making certain proteins that are needed for plant growth.

Glyphosate is also a crop dessicant. This is an agent applied to a crop just before harvest to kill the leaves so that the crop dries out more quickly and evenly. This makes harvesting more efficient, particularly for grain crops like wheat, barley, or oats.

The main difference between the two applications is their concentration, which can range from 0.25% to 41%. Generally, the concentrations of glyphosate are lower for crop applications.

How Glyphosate Works

Glyphosate is also classified as a systemic herbicide. This means that it is absorbed by the plant rather than just sticking to it like some potentially toxic agents used in the past, such as DDT.

The absorption occurs mainly through the leaves of the plant rather than the roots. Because of this, it is more effective in killing leafy weeds, like crabgrass or dandelion. Thereafter, the herbicide moves systemically (through the whole plant) and blocks the synthesis of proteins it needs to survive.

When sprayed onto individual weeds, the effects of the herbicide remain relatively localized. When used as a crop desiccant, the herbicide can be scattered to the soil where it can remain for up to six months (although bacteria in the soil will quickly break it down).

Glyphosate also breaks down quickly in food crops, with a half-life of only around eight or nine days (meaning that the concentration of glyphosate is reduced by 50% during that time). The mechanical drying of grain crops speeds the process, reducing glyphosate to trace levels.

When used for crop applications, glyphosate is unlikely to get into groundwater because it binds tightly to soil.

What Products Contain Glyphosate?

Glyphosate is used worldwide, where it is sold in liquid, concentrate, and solid forms. It is the active ingredient in many weed killers sold in hardware and garden stores, including brands like:

• Roundup Weed & Grass Killer
• Roundup QuikPro Herbicide
• Ragan & Massey Grass and Weed Killer
• RM43 Total Vegetation Control
• FarmWorks Grass and Weed Killer
• Ranger Pro Herbicide
• Ortho GroundClear Vegetation Killer
• KleenUp Weed & Grass Killer

Glyphosate is also used for industrial farming, although the way it is used can vary from one country to the next. Where some countries limit the concentration to 1% or less, others have used concentrations of up to 11%.

Possible Risks and Hazards

Due to its widespread use, glyphosate is continuously being studied for possible long-term effects in humans, animals, and the environment.

For its part, the EPA conducted extensive investigations into glyphosate since its introduction in 1974. After reviewing the available data, the agency determined the following:

• When used according to the product label, glyphosate does not pose any risk to adults. 
• There is no reason to believe that children are more sensitive to the chemical than adults or that it poses any special risk to them—whether exposed during pregnancy or after.
• There is no risk to children who eat foods with glyphosate residues or who play in residential environments where glyphosate has been used.
• There is no indication that glyphosate disrupts the endocrine (hormonal) system and causes problems like abnormal periods or infertility. 
• There is no indication that glyphosate disrupts the neurological (nervous) system and causes conditions like Parkinson's disease.
• Glyphosate is not likely to cause cancer in humans.

Other organizations supporting the EPA decision include the European Chemicals Agency, the Canadian Pest Management Regulatory Agency, the European Food Safety Authority, the Joint Food and Agriculture Organization/World Health Organization (FAO/WHO), and the German Federal Institute for Occupational Safety and Health.

Controversy

In contrast to the conclusions reached by the EPA, the International Agency on the Research for Cancer (IARC) deems glyphosate to be “probably carcinogenic to humans.” In fact, the agency gave it its second-highest classification rating (Group 2a), avoiding the Group 1 classification of known carcinogens.

The IARC evidence is largely based on mice studies which showed that the high consumption of glyphosate was linked to certain types of cancer. The same was not seen in rats. (Exposure through the skin was seen to be a less likely source as only 1 %to 3% of the herbicide is absorbed through the skin.)

The IARC also cited a possible link in humans wherein long-term, high-level exposure to glyphosate may increase the risk of a blood cancer called non-Hodgkin lymphoma (NHL). Even so, the agency concluded that the evidence was "limited."

Opponents of the IARC classification argue that the evidence is weak, citing studies in which glyphosate does not accumulate in the body and dissipates to 1% or less after seven days.

Amid the controversy, the Agency for Toxic Substances and Disease Registry (ATSDR)—housed within the U.S. Department of Health and Human Service—concluded in 2019 that there is a potential cancer risk associated with the use of glyphosate.

Food Safety

Glyphosate is widely used, so trace amounts may be detected in fresh fruits, cereals, vegetables, and many other food products. According to the EPA, these residues pose no known risk.

As long as foods and food products comply with the limits set by the EPA, they’re safe for consumption. The EPA describes this as the acceptable daily intake (ADI), meaning the amount taken by mouth within a 24-hour period.

Currently, the ADI for glyphosate is 0.5 milligrams per kilogram of body weight per day (mg/kg/day).

Studies from the European Food Safety Authority (EFSA) suggest that higher levels may be acceptable. Studies from the EFSA reported that there were no observable negative effects with glyphosate levels of 100 mg/kg/day.

Legality

Glyphosate herbicides are legal in the United States and most other countries around the world. A few countries have either banned it completely or set out specific restrictions regarding its use. Among them:

• France, the Netherlands, and Belgium have banned glyphosate for household use.
• In 2021, the sale and use of glyphosate herbicides was banned in Vietnam.
• The German government announced in 2019 that glyphosate would be prohibited from sale by the end of 2023. 

Currently, glyphosate is not banned in any U.S. state. Even so, Monsanto (the manufacturer of Roundup) currently faces billions of dollars in class action suits, some of which have been or are being settled out of court.

The state of California was one of the major litigants, along with several cities and counties that have banned or restricted glyphosate use. This includes Los Angeles County which banned the use of the herbicide on county property, citing concerns about public safety and cancer.

Summary

Glyphosate is a herbicide used as a household and commercial weed killer, sold under the brand name Roundup and others. It is also used in farming as a crop desiccant to help speed the drying of crops before harvests to improve yields.

The U.S. Environmental Protection Agency (EPA) declared that, when used as indicated, glyphosate is safe and poses little risk of cancer. The International Agency on the Research for Cancer (IARC) differed, concluding that glyphosate is “probably carcinogenic to humans.”