Gary Cohen is not a doctor or a nurse. He has never worked in a hospital, and, he admits, he thinks hospitals are kind of scary, in part because both of his parents died in one. But when the Environmental Protection Agency released a draft report in the mid-1990s, citing hospital incinerators as the country’s No. 1 source of carcinogenic dioxin emissions, Cohen, a longtime environmental activist, simply couldn’t abide the irony. How could the industry that existed to heal people be doing so much harm?

In 1996, he and colleague Charlotte Brody founded the nonprofit Health Care Without Harm, and Cohen quietly made his way to hospitals around the country, explaining to aghast administrators how their operations were hurting patients, employees and communities. Then he supplied them with cost-effective steps they could take to remedy the situation, as well as the promise of a lasting partnership.

Some 15 years later, Health Care Without Harm is a coalition of nearly 500 hospitals, universities, health professional organizations and environmental groups working in 52 countries around the world. When Cohen began the work, there were 5,000 medical-waste incinerators in the U.S. By 2006, there were 83. Back then, every nurse carried a thermometer filled with highly poisonous mercury; the instruments lined the shelves of pharmacies and went home from the hospital with new mothers. Today, mercury thermometers are practically obsolete in the United States, and Cohen, partnering with the World Health Organization, is on the cusp of making them disappear around the world. Cohen helped hospitals leverage their massive buying power to get medical supply companies to stock hospitals with greener products. Even as his work with hospitals deepened in the U.S. and expanded worldwide, Cohen has a more ambitious goal. “If the health care sector can clean up its own house,” he says, “they can be powerful messengers in the broader society on how we can detox the entire economy.”