You’d be forgiven for thinking that a visit to the great outdoors – the vast Great Smoky Mountain National Park, straddling the border between Tennessee and North Carolina in America, for example – would be the perfect place for plenty of fresh air.

Not so, it seems. According to a survey carried out in 2004, it was considered the most polluted national park in the country with, it was estimated, no fewer than 150 ‘unhealthy air days’ in the previous year.

It’s not just the ten million annual visitors who were at risk but also the park’s natural plants and its local inhabitants, including one of the largest populations of black bears in the United States.