A new report out of the American Lung Association (ALA) notes that air pollution in certain U.S. cities is so bad that it’s “too often dangerous to breathe” for civilians. The report states there has been “encouraging progress,” yet there continues to be “evidence of troubling challenges” as well, noting that air pollution affects some 138.5 million people (44 percent of the population) across the nation.

The State of the Air 2015 examined air quality between 2011 and 2013, finding mixed results: “Many cities experienced strong improvement and many others suffered worse episodes of unhealthy air,” the report states. “While most of the nation has much cleaner air quality than even a decade ago, a few cities even reported their worst episodes since the report began.”

Here’s the good news: Overall, since 1970, ozone and particle pollution levels have dropped significantly due to the Clean Air Act and more stringent standards (see the graph below). This means that even as the economy, population, energy use, and number of miles driven increases steadily, pollution drops.

Even as energy consumption, population, miles driven, and carbon dioxide emissions increase, air pollution has been dropping thanks to stronger regulations. Source: State of the Air / EPA, Air Quality Trends, 2015

However, the report notes that despite the nation cleaning up its pollution act in the past few decades, a lot of cities had a record number of days with very high short-term particle pollution — which, the authors claim, is likely due to climate change in the West. Some of the most polluted cities included Los Angeles, Fresno, and Bakersfield, all of which are located in California. The cleanest cities, meanwhile, included Bismark, N.D.; Fort Myers, Fla.; and Elmira, N.Y.

Most Ozone-Polluted Cities:

  1. Los Angeles-Long Beach, CA
  2. Visalia-Porteville-Hanford, CA
  3. Bakersfield, CA
  4. Fresno-Madera, CA
  5. Sacramento-Roseville, CA

Cities Most Polluted Year-Round With Short-Term Particle Pollution:

  1. Fresno-Madera, CA
  2. Bakersfield, CA
  3. Visalia-Porterville-Hanford, CA
  4. Modesto-Merced, CA
  5. Los Angeles-Long Beach, CA

Research shows that air pollution impacts health negatively, from damaging the aging brain and increasing stroke risk, to exacerbating mental illness and anxiety. Another study found that air pollution could impair prenatal brain development, not to mention the adverse effects of dirty air on asthma patients and its contribution to lung cancer.

The authors of the State of the Air argue that we must protect the Clean Air Act, and prevent Congress from weakening some of the act’s amendments that have made it possible to reduce air pollution in the nation. They also urge President Obama and the EPA to take more steps in keeping carbon pollution in check, by creating new stronger standards for power plants, which are the largest source of greenhouse gases in the U.S.

Putting new, stronger standards in place could help prevent 7,900 premature deaths, 1.8 million child asthma attacks, and 1.9 school days missed by 2025, the authors argue. The ALA “urges our nation’s leaders to stand up for public health and take these important steps to improve the air we all breathe.”