Smog is implicated in causing asthma

Environmental charity Friends of the Earth reported Aston Hill, near Montgomery in mid Wales, and Cwmbran in south Wales were among 25 stations which saw smog levels exceeded for seven days in a row.

Across the UK, 76 out of 80 testing sites reported breaches of the levels at some point during the week, including Cardiff, Narberth, Port Talbot and Swansea.

Smog is the result of strong sunlight reacting with pollutants created mainly by vehicle exhausts which combine to create ground level ozone.

Friends of the Earth pollution campaigner Tony Bosworth told BBC Wales the smog problem was improving in some areas but getting worse in others.

Scientists estimate that [pollution] could be causing 12,000 premature deaths every year in the UK Tony Bosworth

"Pollution levels are improving overall but we are still seeing big problems with smog particularly in the summer, not just in towns and cities like Cardiff and the valleys but in parts of rural Wales.

"When we have weather like last week when it's hot with still air and there is high pressure, that is exactly the sort of weather that will lead to smog levels building up.

"It's the perfect climate for it, and it's why we have seen the problems we have in the last week," he said.

Pollution from cars and industry is blamed for the major cause of smog, which is implicated in breathing difficulties.

"This sort of pollution definitely affects asthma," said Mr Bosworth.

"That has been known for many years, but scientists are now starting to think it could be one of the things that causes asthma.

"Looking beyond asthmas, government scientists estimate that [pollution] could be causing 12,000 premature deaths every year in the UK so it's certainly bad news."

Smog is more commonly associated with cities

Ozone pollution can be more apparent in rural areas than in cities, making it an untypical pollutant, according to Friends of the Earth.

It can take a long time for the reactions which cause ozone to be carried out, meaning it can gather quite a long way downwind of the original chemicals which formed it.

Ozone is also destroyed by other pollutants in vehicle exhausts, which are more prevalent in cities, meaning the "mopping up" process takes longer in the country.

Scientists in California who studied children playing sports in areas high in ozone found they developed asthma at a rate three times higher than those in low-ozone communities.