It was known as the Great Pea Soup. In 1952, a thick, greenish-yellow fog smothered London, halting traffic and daily life. At the time, when households burned cheap coal for heat, factories spewed unregulated smoke, and buses burned diesel fuel, Londoners were used to a certain degree of greasy haze. But the Great Smog or Big Smoke, as this 1952 pea-souper was also known, was unprecedented. Bitterly cold air “soaked up the pollution and held it like a blanket over the city” for four days straight, according to the Daily Mail. Twelve thousand people died.

Sixty-five years later, our scientific understanding of air pollution has advanced immeasurably. We now know—because of events like the Great Pea Soup, but also a groundbreaking 1993 Harvard University study of smog-ridden U.S. cities and countless research papers since then—that short-term and long-term exposure to air pollution can kill people, particularly those with pre-existing conditions. “The evidence is so large,” said C. Arden Pope, a professor at Brigham Young University world-renowned researcher of air pollution’s impacts on human health. “There are very few people conducting this research and publishing it in the peer-reviewed literature who don’t think fine particles pollution can lead to death.”

There are, indeed, very few people who believe air pollution—specifically “fine particulate” pollution, or PM2.5—doesn’t cause death. Those who do, however, are getting louder and gaining influence in conservative political circles and inside President Donald Trump’s administration. These air-pollution deniers have just one hope: the repeal of clean-air regulations that have long protected Americans’ health.

At last month’s Conservative Political Action Conference (CPAC), during a little-noticed panel on climate change and environmental regulation, air pollution denial was rampant and went unchallenged. Steve Milloy, formerly a paid flack for the tobacco and fossil fuel industries and member of Trump’s Environmental Protection Agency transition team, argued that excessive air pollution is not linked to premature death. “My particular interest is air pollution,” Milloy said, alleging that EPA’s scientists are inherently biased. “These people validate and rubber-stamp the EPA’s conclusion that air pollution kills people.” Milloy also said, baselessly, that EPA scientists are “paying for the science it wants,” and that Trump must change the research process at the agency.

It is extensively proven, and widely accepted, that air pollution can harm humans, which is why the government regulates it. PM2.5 refers to tiny particles that are 2.5 micrometers or less in diameter—small enough to penetrate deep into the circulatory system and potentially infiltrate the central nervous system. The particles range in composition, originating anywhere from cement dust to tobacco smoke to pollen. They are currently regulated under the Clean Air Act, a widely popular law passed in 1963 that has seen major amendments receiving unanimous or overwhelming support in the Senate. The CAA currently requires Congress to set what’s known as National Ambient Air Quality Standards for particulate matter.