Glyphosate, the world’s most widely-used herbicide, has become a major target for environmentalists. But despite growing evidence of the chemical’s potentially dangerous health effects—the pesticide was deemed a “probable human carcinogen” by the World Health Organization last year—efforts to ban the chemical in the U.S. have repeatedly failed. Supporters of glyphosate, which is often used together with genetically modified crops, respond that no study has proven that the chemical definitively causes cancer in humans, and note that the pesticide effectively protects crops that feed millions around the world.

But on the Hawaiian island of Maui, nearly 5,000 miles from the offices of federal regulators in Washington D.C., a small group of activists have figured out how to use a little-known provision of a federal law to stop some uses of glyphosate in their community. The law is known as FIFRA, short for the Federal Insecticide, Fungicide, and Rodenticide Act, and it requires pesticide sprayers to do something you might imagine they’d be doing anyway: follow the safety instructions on the product’s label down to the letter. But because those safety instructions can be difficult to meet, activists have been able to use the label to force sprayers to change their ways.

“Most people are unaware that that’s a requirement of the label. They just spray it and think it’s safe,” says Brad Edwards, a social worker turned anti-pesticide campaigner in Hawaii. “This now means that government, individuals, condo associations—they need to take measures to keep people and pets off sprayed areas until they dry.”

For products containing glyphosate, following the label means keeping people away from the area where the product has been used for four hours after applications for agriculture, or until the product dries when sprayed for non-agricultural purposes—a time period that varies. That can be difficult in places like long stretches of roads and highways where extended monitoring to keep people away from recently sprayed areas is virtually impossible. But for a long time in Maui, as in many communities across the country, pesticide sprayers simply sprayed and moved on—and no one bothered to hold them to the safety directions of their own product.

Enter Hawaii’s activists. The effort on Maui to curb glyphosate gained momentum last year as parents began to circulate photographs of government employees spraying Round Up, the primary commercial product containing glyphosate, according to Elle Cochran, a Maui County council member. Footage showed authorities spraying on highways, roads and near schools without any visible effort to keep people away.

Edwards first grew concerned early last year when he saw a truck spray along a road near his home, shortly before children were due to walk through the area for an event. He began a long process of complaints, taking the issue to various state agencies, but received little response. Finally, he and other activists reached Stephen Rodgers, who oversees pesticide application on Maui’s state highways. The uncertainty raised by activists over the labeling issue was enough to convince Rodgers to make the eventual switch to organic pesticides. His department no longer purchases Roundup and will stop using the product entirely once the existing supply has been exhausted.