America's great strides in cutting smog at risk of being eroded, experts warn

Scientists and health experts say Trump administration's bid to undo pollution rules are 'extremely counterintuitive and worrying'

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.