(WASHINGTON) — The Trump administration walked back an Obama-era environmental decision that has been the legal basis for establishing federal regulation of greenhouse gas emissions.

The Environmental Protection Agency (EPA) said on Thursday it was rescinding the 2009 endangerment finding, which determined that six key greenhouse gases threaten human health and welfare.

The regulations that resulted cover everything from vehicle tailpipe emissions to the release of greenhouse gases from power plants and other significant emission sources.

President Donald Trump called the move “the single largest deregulatory action in American history” and said the repealed finding had “no basis in fact” and “no basis in law.”

The endangerment finding stemmed from the 2007 Supreme Court decision Massachusetts v. EPA, which held that the EPA could regulate greenhouse gases from motor vehicles under the 1970 Clean Air Act because those gases are air pollutants.

Some environmental scientists disagree, telling ABC News that the rescission is concerning and could have major implications for health. They add that decades of research has shown the impact climate change has on human health.

“The evidence is so overwhelming,” Dr. Ana Navas-Acien, chair of the department of environmental health sciences at Mailman School of Public Health at Columbia University, told ABC News.

“The scientific evidence showing the connection between greenhouse emissions, climate change and then the related health effects — it’s massive, it’s substantial, it has been reviewed by independent organizations,” she said. “So, the fact that this body of evidence has become so well established, it just speaks to the level of rigorous science that has been done.”

How greenhouse gases, climate change impact health

Research has shown that greenhouse gases — such as carbon dioxide and nitrous oxide — drive climate change, and, in turn, can harm human health by exposing people to events including wildfire smoke, extreme heat, flooding and waterborne diseases.

For example, rising temperatures have direct harms on health by increasing risk of dehydration and causing more stress on the heart.

During heat waves, the risk of death from heart attack increases by 64%, according to a 2024 study from researchers at Yale School of the Environment.

Additionally, a 2025 study from Yale School of Public Health found that between 2000 and 2020, there were more than 3,400 preventable deaths in the contiguous U.S. due to high temperatures.

The World Health Organization has warned there will be an estimated 250,000 additional deaths around the world annually from 2030 to 2050 due to climate change-related heat exposure.

“We have recent studies showing heat exposure can lead to heart attack events … and it goes beyond just the cardiovascular system,” Kai Chen, an associate professor of epidemiology at Yale School of Public Health and faculty director of the Yale Center on Climate Change and Health, told ABC News. “It goes beyond these kinds of physical conditions or physical health, [also] impacting our mental health.”

High temperatures can also negatively impact the quality and quantity of sleep, which can raise levels of cortisol in the body.

Studies have also shown that extreme heat can exacerbate conditions such as depression, anxiety and suicide ideation. For the latter, a 2023 meta-analysis found that even a slight bump in the average monthly temperature can lead to increases in suicide and suicidal behavior.

“So, really, heat can lead to a broad spectrum of diseases,” Chen added.

In recent decades, wildfires in the western U.S. have become larger, more intense, and more destructive due to a combination of factors, including human-amplified climate change and rapid urbanization, according to the federal government’s Fifth National Climate Assessment.

The same report found that these fires are worsening air quality in many part of the country due to wildfire smoke, which is a mixture of gases, water vapor, air pollutants and fine particulate matter. The latter, known as PM2.5, is at least 30 times smaller in diameter than a human hair.

Experts say these particles are of concern because they are too small to be seen with the naked eye and can easily enter the nose and throat. They can travel to the lungs and even circulate in the bloodstream.

PM2.5 can cause both short-term health effects, even for healthy people, including irritation of the eyes, nose and throat; coughing, sneezing; and shortness of breath and long-term effects such as worsening of conditions such as asthma, heart disease or chronic obstructive pulmonary disease flare-ups.

“Recent evidence has shown that the fine particulate matter in the air can also impact dementia and cognitive function,” Chen said. “It can also affect birth outcomes, leading to pre-term birth and low birth weight. So, air pollution is a major risk factor.”

Regulating fossil fuel combustion reduced PM2.5-related deaths by approximately 54% from 1990 to 2010, according to 2018 joint study from researchers in the U.S. and China.

Short- and long-term implications

Navas-Acien believes that, in the short-term, the rescinding of the endangerment finding will lead to higher pollution levels.

“Higher pollution levels, if we allow that to happen, that means more pollutants in the air that we breathe, in the water that we drink, in the food that we eat,” she said. “And that’s going to result in a higher burden of chronic diseases and even not just chronic disease, but also like heart attacks.”

Research has shown that in the days following an increased level of air pollutants, there are more visits to emergency rooms and hospital admissions for cardiorespiratory events, according to Navas-Acien.

“So that tells you that the impacts of pollution, in the short term, are very rapid,” she added.

In the absence of federal protections, Navas-Acien and Chen said state and local leaders can take actions to reduce greenhouse gas emissions and improve air quality.

On the individual level, to protect yourself, the experts recommend frequently checking the air quality in your area, wearing a mask if air pollution levels are high and supporting the creation of urban green spaces to combat climate change.

Chen said the EPA rescinding the endangerment finding is “very concerning” because “the scientific evidence has not become weaker, but actually became much stronger, showing the health harms from the climate change stirring from the greenhouse gas emissions. This rollback of policy will actually be threatening millions of Americans’ lives.”

“Climate change is impacting our health right now,” Chen continued. “It’s not a political debate. It is science and the science is clear. We need to take action.”

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