That’s because, as the World Health Organization (WHO) notes, the effects of climate change cut across multiple kinds of health care needs. A freak flood precipitated by changing climate patterns could simultaneously kill and injure people, spread disease-causing pathogens, and devastate local economies. The ensuing disruptions to physical and economic security can take a serious toll on mental health. Even in more developed countries like the U.S., environmental factors like poor air quality (which is associated with fossil fuel burning) have been linked with diminished medical outcomes that are exacerbated by income inequality and a lack of access to health care.

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The climate change-related Paris accords reached in late 2015 by more than 190 countries put public health front and center. (For a more complete explainer on exactly what the agreement is and what it does, read my colleague Laura Entis’ rundown here.) While the energy industry would be most affected by the deal, the WHO and other public health agencies lauded its potential to curb death and illness in advanced nations and emerging markets alike.

“The world now has a global climate agreement that will have a major public health policy impact as countries take action. As stated in the agreement, ‘the right to health’, will be central to the actions taken,” wrote the global health organization last year.

But the “as countries take action” part is critical to achieving the WHO’s ultimate stated goal. And a U.S. departure from the climate agreement could lead to a downstream effect where other countries feel less compelled to implement the policies necessary to meet the Paris deal’s thresholds, according to its supporters.

The United States would join just two other nations—Syria and Nicaragua—in pulling out of the climate agreement.