Pittsburgh’s air pollution was once so bad that the manufacturing metropolis was labeled “hell with the lid off.” Los Angeles was nicknamed “Smell-A” in the days of its worst smog. The U.S. has made tremendous progress cleaning up our air over the past 50 years—a feat even more remarkable because we simultaneously experienced dramatic economic and population growth.

How did we do it? Smart, targeted regulation. Although some pollution hotspots remain, our effective laws and rules mean the nation enjoys better air quality than most of the developed world. This is an American success story!

Sadly, we cannot declare victory, because citizens’ health remains imperiled by what we breathe but cannot see—and the government is dropping the regulatory ball.

A pollutant of special concern is fine particulate matter 2.5 micrometers or less in diameter, known as PM2.5.These microscopic particles come in complex mixtures of different sizes, shapes and chemical composition. They are emitted by manmade sources—think of the black cloud behind an old diesel bus as it pulls away from a stop—but also form less noticeably when chemicals combine in the atmosphere.

Fine particulate matter penetrates deeply into the lungs, causing respiratory and cardiovascular disease; of the several million premature deaths attributed to air pollution globally each year, PM2.5 is responsible for the vast majority. In the U.S., roughly 100,000 people a year are thought to die early from exposure to fine-particle pollution. In addition to the health impacts, the particles also affect climate and visibility.

The Environmental Protection Agency’s current primary standard for PM2.5 is an annual average of 12 micrograms per cubic meter of air. Past regulations have dramatically reduced PM2.5 concentrations, but roughly 22 million people in the U.S. still live in areas that do not meet this standard.

Under the Clean Air Act, pollution standards must not be based on an economic cost-benefit analysis but rather on protecting human health, with an adequate margin of safety for sensitive populations such as children and the elderly. This makes sense—everyone has to breathe. The EPA is also required to review the standards every five years to ensure they accurately reflect the latest scientific knowledge.

We are in the midst of such a review right now, and the Trump administration is rushing to complete the assessment of particulate-matter standards by late 2020, before the end of the current presidential term. Unfortunately, in October 2018 the administration fired the panel of experts who were going to provide scientific input on fine particulate matter. They recently appeared to change course, however, publishing, in the Federal Register, a call for consultants to support the review.

There is emerging evidence of significant public health impacts of fine-particle pollution at levels below 12 micrograms per cubic meter, including a paper I co-authored in July and another recent study led by scientists at Imperial College London. These studies used two very large, nationally representative data sets to examine U.S. mortality related to fine particulate matter. They linked air pollution estimates with health records for millions of Americans. And they analyzed publicly accessible data so that the results can be tested and replicated by other scientists, addressing concerns raised by legislators about “secret” science justifying environmental regulations.

The studies estimated that more than 30,000 premature deaths could be avoided by tightening the standard—with greater impacts, on average, in counties that have lower incomes and higher poverty rates. The Clean Air Act therefore mandates setting a more stringent limit, not weakening regulations as the Trump administration is likely hurrying to do.

Critics often emphasize that the health effects of fine particulate matter are uncertain. But there is always uncertainty. For example, epidemiological studies of air pollution such as the recently published ones can never truly determine cause and effect. Because of the uncertainty, the EPA rightly uses a weight-of-evidence approach to establish standards. This approach has been litigated in the Supreme Court, and the EPA has won. Many air pollution experts are concerned that the agency may be turning away from weight-of-evidence analysis for the latest review.

Although the vast majority of studies link fine-particle pollution to premature mortality, there are a few that do not. That is the nature of science. Would the merchants of doubt move their families to a neighborhood downwind of large industrial facility or immediately adjacent to a highway with high pollution levels? A similar question was posed years ago to tobacco company CEOs who claimed the link between smoking and lung cancer was uncertain: Would they allow their children to smoke? We’ve been down this path before.

Another frequent argument against tightening air pollution regulations is about costs and jobs. The costs are real, but someone is getting paid to build, install and operate the equipment that controls emissions. And there are equally real costs associated with the health impacts of pollution. These costs are ubiquitous and involuntary—everyone has to breathe. Furthermore, the costs of achieving much better air quality since the 1970s have not prevented the U.S. economy from outpacing many countries with dirtier air and less regulation.

The health benefits of a stricter standard are also relevant to the climate debate. A substantial so-called co-benefit of many strategies to address climate change (such as the Clean Power Plan) is reduced traditional air pollutants. There has been a push to ignore this benefit when evaluating the cost-effectiveness of regulations, but it should be included.

We have made impressive improvements in air quality. Do we need to do more? According to the Clean Air Act, we do. It requires that standards aim to protect health, not minimize costs. We face ongoing environmental injustice and shortening of human lives in our most vulnerable communities. The right response is that we can—and will—do more.