The research, published in the New England Journal of Medicine, demonstrates how stricter air quality standards led to a reduced incidence of breathing problems.

Researchers at the University of Southern California studied air pollution levels in five regional communities over a 17-year period.

They measured the breathing capacity of 2120 schoolchildren aged between 11 and 15 in 1994 to 1998, 1997 to 2001 and 2007 to 2011.

Emission standards for cars, diesel trucks, refineries, ships and trains were tightened throughout the study.

Fine particles had fallen by 50per cent and nitrogen dioxide levels had fallen by 35per cent by the time the study concluded in 2011.

The researchers found that as emission standards tightened the lung development of children born in the study’s later years was significantly stronger.

In addition, the percentage of children with significantly impaired lung function declined during the study period from 8per cent to 3.5per cent. 

National Pollution Inventory data shows a direct corelation between Upper Hunter air quality and mining activity over the past decade.

Singleton GP Tuan Au, who studied the incidence of respiratory illness in residents affected by industrial air pollution, said the US study had implications for the Upper Hunter.

Dr Au found one in six children who participated in a 2009 study of 680 Upper Hunter children had  lower than normal lung function.

‘‘I have always believed that poor air quality decreases lung function, because the cross-section of airways is smaller. I still see quite a few people who complain about shortness of breath and asthma,’’ he said.

‘‘They go on holidays and their breathing improves, when they come home they need a puffer.’’

Although air pollution remained a problem, Dr Au said he welcomed increased monitoring and awareness of pollution levels in the region.

‘‘I think there have been some improvements, but certainly more can be done,’’ he said.  ‘‘