America’s leading cities have some of the cleanest urban air in the world but huge advances made in reducing smog are in danger of falling backwards, experts are warning.

New Yorkers breathe air that is 800 times less polluted than Delhi’s and twice as clean as in London and Berlin, the World Health Organization reported.

Nine out of 10 people around the world are breathing in high levels of pollutants. This amounts to a sprawling health crisis that causes about 7 million deaths a year, according to the latest global data from the WHO, which warned that air pollution inequality between the world’s rich and poor is widening.

Many of the luckier one in 10 people are found in US cities such as New York, Miami and Boston. Here, air quality has improved dramatically since landmark environmental regulations in the 1970s started to disperse the choking smogs that commonly lingered there and over cities such as Los Angeles, confining people indoors on warm days, the WHO said.

But US scientists and public health officials have warned that the stunning improvements in American cities have already starting to slow and are even in danger of reversing. They point to diminishing returns from existing regulations and the Trump administration’s zeal in demolishing recent rules designed to improve air quality and combat climate change.

“The actions of this administration are extremely counterintuitive and worrying,” said Christine Todd Whitman, who was administrator of the Environmental Protection Agency (EPA) under president George W Bush. “Time will be lost, pollution will be increased and lives will be endangered. It’s not that we will turn into Beijing or Delhi tomorrow but people take clean air for granted now and we can’t slide back to the smogs we once had.”

In New York, levels of haze-causing airborne particles called PM2.5, expelled from vehicle exhausts and power plants, are about 800 times less than they are in Delhi, where is the air is considered a significant risk to human health, the WHO found. New York also compares well to Berlin and London. Those cities have struggled with a major air pollution problem and have PM2.5 levels around double that of the US metropolis – referring to microscopic atmospheric particulate matter.

This agenda is very detrimental. It will slow progress and in some instances make things worse Dr George Thurston, NYU

Enforcement of the Clean Air Act to reduce harmful emissions from heavy industry and transportation has seen levels of the six most common air toxins, including lead and particulate matter, drop by 50% in the US since 1970 despite growth in population and economic activity, according to the EPA. These six pollutants can cause or worsen a host of health issues, ranging from runny eyes to asthma and heart disease.

However, this progress has slowed in recent years. A new analysis of satellite data by the National Center for Atmospheric Research found that reductions in pollutants that cause ground-level ozone, or smog, “slowed down dramatically” between 2011 and 2015.

This slowdown, of around 76% compared to 2005 to 2009, is sharper than is shown in EPA data, which typically uses ground-level monitoring. The study’s authors suggest the lagging progress could be down to decreasing relative contributions from vehicle emissions reduction technology and a worse than expected impact from heavy duty diesel trucks.

“More reductions are still necessary – ozone is still too high in many places in the US,” said Noelle Eckley Selin, an air pollution expert at the Massachusetts Institute of Technology, who wasn’t involved in the study.

“Continued improvement is not a given even with existing regulations; repealing relevant standards could even make air quality worse. Policies that repeal emission standards and encourage more fossil fuel use move us in the wrong direction for air quality and human health.”

Concern over a regression in air quality stems from a whirlwind of deregulation instigated by the EPA under administrator Scott Pruitt, a self-described opponent of the “activist agenda” of the regulator.

The clean power plan, the centerpiece of the Obama administration climate change effort, is in the process of being dismantled, with Pruitt claiming that it improperly uses clean air laws to curb emissions from coal fired power plants. The EPA previously estimated that the plan would prevent 3,600 premature deaths, 1,700 heart attacks and 90,000 asthma attacks per year.

Boston has undergone significant improvement in its air quality since the 1970s. Photograph: DenisTangneyJr/Getty Images/iStockphoto

Pruitt has also overseen attempts to delay or scrap vehicle emissions rules, new regulations to prevent the release of methane from oil and gas drilling operations and the implementation of smog standards. The agency is also erasing requirements that companies continue to use pollution reduction methods once emissions dip below a certain level.

“This agenda is very detrimental. It will slow progress and in some instances make things worse,” said George Thurston, who specializes in environmental hazards at the New York University School of Medicine. “The problems won’t be evident to the average person, because people have heart attacks and asthma attacks for various reasons. But that harm will be there, it will happen.”

Thurston said the reversals could even hamper air quality improvements overseas. “The US is showing a failure of leadership,” he said. “If the developing world sees we are not serious about climate change and air pollution, they will be less willing to do something. It’s really irresponsible on the part of the administration.”

It remains unclear how successful Pruitt’s EPA will be in repealing its targeted regulations, given stiff resistance from environmental groups and some states via the courts. The agency didn’t respond to a request for comment on its agenda.

On Tuesday, the EPA revealed that 51 areas in 22 states do not meet ozone standards, after being ordered by a federal court not to delay the publication of its assessment. The administration is facing a further fight after 17 states, led by California and New York, sued to protect California’s waiver to enforce stricter vehicle emissions rules than the federal standard.

“The administration will lose in court over a lot of these regulations because you can’t just get rid of them on a whim,” said Whitman. “What will be lost, though, is the institutional knowledge at the EPA. A lot of people are leaving in frustration and young people looking for a career are hardly looking to the EPA because it’s deemed unimportant. The damage has been done.”