The world’s most widely-used herbicide has been getting a lot of attention lately.

Last month, an international agency declared glyphosate, the primary ingredient in the popular product Roundup, a “probable human carcinogen.” The weed killer also has made recent headlines for its widespread use on genetically modified seeds and research that links it to antibiotics resistance and hormone disruption. Several national governments are planning to restrict its use, and some school districts are talking about banning it.

So what do we know about glyphosate? Five key questions and answers:

How Is Glyphosate Used?

Introduced commercially by Monsanto in 1974, glyphosate kills weeds by blocking proteins essential to plant growth. It is now used in more than 160 countries, with more than 1.4 billion pounds applied per year.

Glyphosate, often sold under the brand name Roundup, is probably in your garage or shed because it’s ranked as the second most widely used U.S. lawn and garden weed killer. These products have been promoted as easy-to-use and effective on poison ivy, kudzu, dandelions, and other weeds.

But the primary use is by agriculture. Nearly all the corn, soy, and cotton now grown in the United States is treated with glyphosate.

Its use skyrocketed after seeds were genetically engineered to tolerate the chemical. Because these seeds produce plants that are not killed by glyphosate, farmers can apply the weed killer to entire fields without worrying about destroying crops. Between 1987 and 2012, annual U.S. farm use grew from less than 11 million pounds to nearly 300 million pounds.

“By far the vast use is on [genetically engineered] crops – corn, soy and cotton – that took off in the early to mid-nineties,” says Robert Gilliom, chief of surface water assessment for the US Geological Survey’s National Water Quality Assessment Program.

In addition, some five million acres in California were treated with glyphosate in 2012 to grow almonds, peaches, onions, cantaloupe, cherries, sweet corn, citrus, grapes, and other edible crops.

View Images Glyphosate, marketed by Monsanto as Roundup, is the second most popular weed killer for residential yards and gardens. Photograph by J. Blue, Bloomberg/Getty

What Happens to Glyphosate in the Environment?

Despite its widespread use, USGS hydrologist Paul Capel said there is “a dearth of information” on what happens to it once it is used.

Glyphosate is not included in the U.S. government’s testing of food for pesticide residues or the monitoring of chemicals in human blood and tissues. As a result, there is no information on how much people are exposed to from using it in their yards, living near farms or eating foods from treated fields.

A recent USGS study sampled waterways in 38 states and found glyphosate in the majority of rivers, streams, ditches, and wastewater treatment plant outfalls tested. Not much was found in groundwater because it binds tightly to soil.

Glyphosate also was found in about 70 percent of rainfall samples. It “attaches pretty firmly to soil particles” that are swept off farm fields then stay in “the atmosphere for a relatively long time until they dissolve off into water,” Capel says.

What About Exposure Through Food?

Before genetically engineered crops, glyphosate residues in food were considered unlikely, says Charles Benbrook, research professor at Washington State University’s Center for Sustaining Agriculture and Natural Resources. But since about 2005, pre-harvest use of glyphosate “results in very high residues,” he says. Traces were found in 90 percent of 300 soybean samples.

So what is the likelihood of exposure? The people most likely to be exposed are working on or living near farms where glyphosate is used, says University of California, Irvine professor Bruce Blumberg.

What Is known About Effects on Human Health?

The U.S. Environmental Protection Agency had determined that the science “does not provide evidence to show that glyphosate causes cancer.” But now the EPA says it will analyze new findings by the UN’s International Agency for Research on Cancer, which declared in March that glyphosate probably raises the risk of cancer in people exposed.

The UN agency based its decision on human, animal, and cell studies, says National Cancer Institute scientist emeritus, Aaron Blair who chaired the IARC review committee. The studies found glyphosate in farmworkers’ blood and urine, chromosomal damage in cells, increased risks of non-Hodgkin lymphoma in some people exposed, and tumor formation in some animal studies.

Monsanto called the IARC conclusion “inconsistent with decades of ongoing comprehensive safety assessments.” The American Soybean Association and National Corn Growers Association also denounced the finding. CropLife America, a trade association representing pesticide manufacturers, says, “It’s important to remember that glyphosate acts on an enzyme that exists only in plants and not mammals, contributing to the low risk to human health.”

One study suggests that glyphosate may affect pathogens such as Salmonella in ways that can contribute to antibiotic resistance. Other recent research suggests it can interfere with hormones.

Yet the really big unanswered question is the potential health effect of low levels over extended periods of time.

So Where Does This Leave Us?

The EPA is reviewing its approved uses of glyphosate and expects to release a preliminary assessment of the human health risk later this year. This is expected to include new restrictions.