As North America enters its peak summer growing season, gardeners are planting and weeding, and groundskeepers are mowing parks and playing fields. Many are using the popular weed killer Roundup, which is widely available at stores like Home Depot and Target.
In the 1990s Monsanto began packaging glyphosate with crops that were genetically modified to be resistant to it, including corn, soybeans, cotton and canola. Farmers who used these “Roundup Ready” seeds could apply a single herbicide to manage weeds during the growing season, saving time and simplifying production decisions. Roundup became the highest-selling and most profitable herbicide ever to appear on the global market.
China still dominates the pesticide industry—it exported 46 percent of all herbicides worldwide in 2018—but now other countries are getting into the business, including Malaysia and India. Pesticides used to flow from Europe and North America to developing nations, but now developing countries export many pesticides to wealthy nations. More pesticide factories in more places leads to oversupply and even lower prices, with critical implications for human health and the environment.
The question of whether glyphosate causes cancer in humans has been hotly debated. In 2015 the International Agency for Research on Cancer, an agency of the World Health Organization, classified it as a probable human carcinogen based on “limited” evidence of cancer in humans from actual real-world exposures and “sufficient” evidence of cancer in experimental animals.
There also are questions about possible linkages between glyphosate and other human health problems. A 2019 study found that children whose mothers experienced prenatal exposure to glyphosate had a significantly higher risk of autism spectrum disorder than a control population.
However, the US Environmental Protection Agency and the European Food Safety Authority maintain that glyphosate is unlikely to cause cancer in humans and does not threaten human health when used according to the manufacturer’s directions.
A challenge for regulators
In the 1990s and early 2000s, the world community adopted several groundbreaking agreements to restrict or monitor sales and use of hazardous pesticides. These agreements—the Stockholm and Rotterdam conventions—target compounds that are either acutely toxic or persist in the environment and accumulate in animals, including humans. Glyphosate does not appear to meet these criteria, but humans may be more exposed to it because of its ubiquity in soil and water and on food.
Today a handful of countries, including Luxembourg and Mexico, have banned or restricted the use of glyphosate, citing health concerns. In most countries, however, it remains legal with few restrictions.
Scientists are unlikely to reach consensus soon about glyphosate’s health and environmental impacts. But that has also been true of other pesticides.
For example, DDT—which is still used in developing countries to control mosquitoes that spread malaria and other diseases—was banned in the US in 1972 for its effects on wildlife and potential harm to humans. But it was not thought to cause cancer in humans until 2015, when scientists analyzed data from women whose mothers were exposed to DDT while pregnant in the 1960s, and found that these women were more than four times as likely to develop breast cancer than others who were not exposed. This study was published 65 years after the first congressional testimony on DDT’s human health impacts.
Science can take a long time to reach conclusive results. Given how widely glyphosate is used now, we expect that if it is definitively found to harm human health, its effects will be widespread, difficult to isolate and extremely challenging to regulate.
In our view, growing concerns about glyphosate’s effectiveness and possible health impacts should accelerate research into alternative solutions to chemical weed control. Without more public support for these efforts, farmers will turn to more toxic herbicides. Glyphosate looks cheap now, but its true costs could turn out to be much higher.
This article has been updated to remove a reference to glyphosate detection in breast milk, which was based on a study that was not peer-reviewed.
Marion Werner is an associate professor of geography at the University at Buffalo; Annie Shattuck is an assistant professor of geography at Indiana University and Ryan Galt is a professor of geography at University of California, Davis.
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