Seven million people died prematurely in 2012 from air pollution caused by fossil fuel combustion, according to a 2014 report by the World Health Organization. So President Trump’s decision to halt U.S. compliance with the 2015 Paris climate agreement is a blow not just to climate science and international diplomacy — it’s also a looming disaster for public health.

“It’s a huge mistake for the United States to pull out of the Paris agreement for lots of reasons,” says Jonathan Patz, director of the Global Health Institute at the University of Wisconsin-Madison. For 15 years, Patz served as a lead author on the Intergovernmental Panel on Climate Change, and has been leading research on the links between health and climate change for more than two decades.

Under the Paris accord, ratified thus far by 147 parties or nations, the U.S. pledged to cut its greenhouse gas emissions to 26 to 28 percent below 2005 levels by 2025. Clean energy policies decrease not only carbon pollution linked to climate change but also other types of pollution that harm human health, such as sulfur oxides, nitrogen oxides, and fine particles that can damage the airways of humans and other organisms.

So even without concerns about the warming effect of carbon pollution in Earth’s atmosphere, the Paris agreement goes a long way toward reducing harmful air pollution worldwide.

One could narrow the focus to the Upper Midwest of the U.S. alone, and the public health benefits would still be huge, Patz says. A switch from driving to walking or bicycling for round trips under eight kilometers (about five miles) in just the 11 largest cities in that region could save nearly 1,300 lives and $8 billion annually in costs associated with sick days, hospitalizations, and deaths, according to a 2011 study led by Maggie Grabow, a researcher also working at the Global Health Institute.

But as Patz puts it, “Americans instinctually jump into their cars even for the shortest of trips, missing out on the large public health dividend that comes from daily exercise of active low-carbon transportation, let alone the 7 million lives that could be saved globally every year from low-carbon energy.”

Climate change clearly harms human health in many ways. A review of dozens of studies published between 2009 and 2014 link climate change to increases in a wide range of health problems, including asthma and other respiratory disorders, heart disease induced by heat stress, infectious diseases, waterborne diseases that can cause dangerous bouts of diarrhea in children, and mental health issues such as depression and PTSD following climate-related natural disasters such as hurricanes.

The authors of that roundup, including Patz, called for health professionals to play a stronger role in explaining the links between climate change and human health and in advocating for policy changes. And there has been quite a bit of progress on that front, he says, with the formation of groups such as the Climate and Health Alliance, the Healthier Hospitals Initiative, and the Medical Society Consortium on Climate & Health, launched just two months ago.

In his climate announcement this afternoon, Trump’s main target was the Obama administration’s environmental agenda. But the health of millions of people worldwide — including many Americans — will be collateral damage.

A version of this article originally appeared at Undark, a digital science magazine published by the Knight Science Journalism Fellowship Program at MIT.