California has the most polluted cities in the United States, a report issued on Wednesday said, as President Donald Trump's administration seeks to force the state to weaken its vehicle emissions standards.
The study published by the American Lung Association—which covers the period from 2014 to 2016, the year before Trump took office—said Los Angeles remained the city with the worst ozone pollution, and ranked fourth in terms of year-round particle contamination.
Bakersfield, California, number two for ozone pollution, also maintained its position as the city with the worst short-term particle pollution.
That city and nearby Fresno and Visalia are subject to emissions from the powerful agricultural and petroleum industries.
San Diego and Sacramento were also among the most contaminated by ozone.
Outside of California, whose roughly 40 million residents make it the country's most populous state, Phoenix, Las Vegas, Denver, New York and Houston had the worst ozone pollution.
Pittsburgh, Lancaster and Philadelphia, all in the northeastern state of Pennsylvania, along with Cleveland, Indianapolis and Detroit, had the worst year-round particle pollution.
"The United States must continue to fight climate change and support and enforce the Clean Air Act to protect the nation from unhealthy air," the report said.
Almost 134 million people were exposed to unhealthy levels of air pollution in the period studied, up from 125 million in the previous report, it added.
The "State of the Air 2018" report found that ozone pollution significantly worsened between 2014 and 2016, because of increased temperatures resulting from a changing climate.
At the same time, improvements continued in year-round particle contamination thanks to cleaner power plants and increased use of cleaner vehicles and engines, the report said.
"Continued progress to cleaner air remains crucial to reduce the risk of premature death, asthma attacks and lung cancer," it said.
Trump withdrew the United States from the global Paris Agreement on climate change, has defended coal-fired power plants, and in early April rolled back Obama-era pollution and fuel efficiency rules for cars and light trucks, saying they were too stringent.
California last week filed suit against the Environmental Protection Agency over its retreat on air quality protection.
California has operated under a waiver that allowed it to impose tougher requirements than called for under the Clean Air Act.
But EPA director Scott Pruitt has said one state cannot "dictate standards for the rest of the country."
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