More than 800 cancer patients nationwide are involved in a class-action lawsuit that accuses the chemical giant Monsanto of failing to adequately warn them about a possible link between their disease and glyphosate, the key ingredient in its enormously popular Roundup herbicide.

The lawsuit was sparked by a 2015 determination by the World Health Organization's International Agency for Research on Cancer (IARC) that glyphosate was a probable carcinogen, with research tying it to non-Hodgkin's lymphoma, a type of blood cancer, in humans. The IARC also found "convincing evidence" that glyphosate can cause cancer in laboratory animals, while other studies it reviewed found the chemical damages human DNA.

Monsanto maintains that glyphosate is safe, as industry-funded studies have found. But the class-action lawsuit has unearthed documents that cast doubt on its safety — and on the handling of its potential risks by the U.S. Environmental Protection Agency. As the New York Times reported earlier this year:

The court documents included Monsanto's internal emails and email traffic between the company and federal regulators. The records suggested that Monsanto had ghostwritten research that was later attributed to academics and indicated that a senior official at the Environmental Protection Agency had worked to quash a review of Roundup's main ingredient, glyphosate, that was to have been conducted by the United States Department of Health and Human Services.

The documents also revealed that there was some disagreement within the E.P.A. over its own safety assessment.

The EPA is currently reviewing glyphosate's registration and is scheduled to publish the draft human health and ecological risk assessments for public comment some time this year. It currently classifies glyphosate as having low toxicity.

Glyphosate is the most heavily used agricultural chemical in the world, with over 1.7 million tons applied in the U.S. since 1974. Worldwide, glyphosate use has climbed almost 15-fold since Roundup Ready crops genetically engineered to tolerate glyphosate were introduced in 1996; two-thirds of the total amount of glyphosate applied in the U.S. from 1974 to 2014 has been sprayed in just the last 10 years.

As of 2016, herbicide-tolerant varieties represented 94 percent of the U.S. soybean acreage and 89 percent of the cotton and corn acreage, according to the U.S. Department of Agriculture's Economic Research Service. Other crops that have been genetically modified to withstand glyphosate applications include canola, alfalfa and sugar beets.

Not surprisingly, given the importance of agriculture to the South's economy, glyphosate is used widely across the region, as the U.S. Geological Survey map above shows. Its use is particularly concentrated in the Mississippi Delta region and in the Black Belt region running along the Southeastern coastal plain. That has important environmental justice implications, since the residents of those areas are disproportionately African-American.