On a stopover Wednesday in Ireland after his state visit to London, President Trump said that “we have the cleanest air in the world in the United States, and it’s gotten better since I’m president.”

Trump, who was addressing a question about his stance on climate change, is wrong on both counts. The United States ranks 10th in air quality, according to the Environmental Performance Index, a collaboration of Yale and Columbia universities and the World Economic Forum, and the latest data suggests that it’s worsening.

It’s true that the United States has far cleaner air than it did before the passage of the landmark Clean Air Act nearly 50 years ago. But in some ways, that law’s remarkable success has given Americans a false sense of complacency — not necessarily regarding climate change overall but certainly when it comes to their health. Problem solved, they can be forgiven for thinking. But research shows that the air is not nearly as clean as it needs to be and that its impact on health goes well beyond our lungs. Air pollution kills an estimated 100,000 Americans every year. As the Trump administration rolls back regulations on polluters, that figure could rise.

Rigorous science links dirty air to illness and death, but the naked eye can’t make those connections easily, and it’s usually impossible to say in any individual case that pollution is what made someone sick. This invisibility creates a political challenge for efforts to build on the Clean Air Act or push back against the administration’s efforts to ease even the current restrictions on polluters.

“You see one person run over in the street, and you’ll never forget it,” said Joseph Lyou, president of California’s Coalition for Clean Air. But thousands dying from the effects of dirty air “will never even faze you.”

The benefits of cleaning up are similarly hard to see: We never know when we don’t get sick, or don’t lose a loved one, because the air is cleaner than it would have been without effective regulation.

Dirty air is strongly linked to increased rates of heart attacks, strokes, cancer and premature birth. But there’s much more. Seventeen years ago, a neuropathologist examining the brains of puppies who had breathed Mexico City’s awful air found the same markers doctors use to diagnose Alzheimer’s disease in humans: plaques, twisted proteins and degenerating neurons. A more recent study, from 2012, showed that women in their 70s who lived with moderate pollution levels did as badly on cognitive and memory tests as they would have if they been two years older and breathing cleaner air.

But as the research on pollution’s harmful effects on health moves forward, the Trump administration is taking the United States backward. The consequences of its aggressive regulatory rollbacks, its push to cast doubt on solid science and an exodus of expertise from the Environmental Protection Agency are predictable: The Trump administration will make the country’s air dirtier. The science tells us that means more illness and more deaths.

The Clean Air Act passed the Senate unanimously in 1970 and drew just one no vote in the House. Since then, it has underpinned decades of progress. With the authority that the powerful law gave it, the EPA demanded that car companies, power plants and factories spew less pollution into our air. The slow, steady march of that regulation, grounded in science, brought dramatic improvements in air quality across the country. It is one of the world’s great environmental — and health — success stories. Cleaner air has saved millions of U.S. lives and trillions of dollars; official studies found that the law’s benefits have been dozens of times larger than its costs.

But the job is not finished. Because, as our air has gotten cleaner, scientists have learned that pollution wreaks harm on the body even at levels once thought to be safe.

The latest air-quality survey from the American Lung Association found that more than 43 percent of Americans — about 141 million people — breathe unhealthful air. Globally, air pollution cuts short about 7 million lives every year.

A vast body of evidence shows that cleaner air means better health, and longer lives. A new study in May found air-quality improvements in Southern California between 1993 and 2006 were associated with a 20 percent drop in the number of new asthma diagnoses in children.

But under the leadership of a former coal lobbyist, Andrew Wheeler, the EPA is easing rules that have forced polluters to clean up. At the same time, the agency’s enforcement muscle is withering, as its scientists and health experts leave and are not replaced. The most radical element of the administration’s anti-regulatory push is its effort to cast doubt on solid scientific research. Science has been the foundation of both the Clean Air Act and the EPA, providing evidence that has justified powerful health-saving regulations.

That is clearly why the Trump administration has sought to undermine it. When it comes to air pollution, the administration doesn’t like the answers science is providing. So it is ditching widely accepted methodologies used to calculate cleaner air’s benefits and adopting new ones that produce numbers that lowball pollution’s harm, and make it easier to justify administration plans to relieve industries of the burden of complying with regulations.

History shows that to maximize profits, companies will pollute as much as we allow them to. So loosening rules and easing enforcement will inevitably mean more pollution.

Europe offers a cautionary tale. Despite its reputation for environmental progressivism, its air is significantly dirtier than United States.’ The European Union lacks a strong, well-resourced enforcement agency with authority and expertise like our EPA has historically had. The Volkswagen diesel cheating scandal is a powerful case in point: The company programmed millions of cars to detect when they were being tested and then switch on pollution controls that were idle the rest of the time. While there were far more emission-evading vehicles in Europe, it took U.S. regulators to bring VW’s egregious violations to light.

While the United States forced Volkswagen to pay billions of dollars in compensation and retrofit rule-breaking vehicles, Europe has been slower to respond, allowing the company to get away with cheap, ineffective software tweaks. So, across Europe, millions of diesel cars continue to pollute at levels far over the legal limit. Europeans are paying with their health and their lives.

It’s a lesson Americans would do well to heed as we unravel the rules and weaken the enforcers that have delivered so much progress.