According to the report of WHO, some leading cities (e.g. New York, Miami and Boston) in America have cleanest urban air in the world, where air quality have improved dramatically since the implementation of environmental regulation. For example, air condition in New York is twice as clean as in London and Berlin. But, US scientists and health experts are warning that problems could arise from the deregulation momentum. 

They attribute this to the decreasing returns from existing regulations and the Trump administration’s bid to lessen environmental regulations. Enforcement of the Clean Air Act, which aims to reduce harmful emissions from heavy industry and transportation, has seen six pollutants (that can cause health issues) drop by 50% since 1970. However, a new analysis found that this progress has slowed between 2011 and 2015. Experts said that policies that repeal emission standards and encourage more fossil fuel use could even make air quality worse. Concerns over the regression in air quality stems from deregulation initiated by the Environmental Protection Agency (EPA) under the administration of Scott Pruitt. Pruitt has also overseen attempts to lessen and or delay vehicle emissions rules and the implementation of smog standards. Given resistance from environmental groups and some states via the courts, it is unclear whether the EPA will be successful in repealing its targeted regulations.