Reducing air pollution can dramatically improve the lives of people with asthma, a study suggests.
There is a debate over whether air pollution causes asthma
Health Protection Agency researchers examined the health of people living near an iron foundry in Birmingham.
They found that the number of patients admitted to hospital with asthma fell by 30% after the foundry cut emissions.
The researchers told the HPA's scientific conference in Warwick that the findings showed that improving air quality can also improve health.
People living in Sandwell in Birmingham have expressed concern in the past about the effects of emissions from a nearby iron foundry on their health.
In 1997, the company, which operates the foundry, introduced a programme to reduce emissions.
Patrick Saunders and colleagues at the HPA's chemical hazards and poisons division, examined the impact this had on hospital admissions from asthma in the area.
They examined figures for the two years before the improvements were made and for the two years after the improvements had been operating for one year.
They found that hospital admissions for asthma fell by almost one third after emissions were reduced.
"The population that was most likely to be exposed to emissions from the foundry showed around a 30% decrease in hospital admission rates from asthma after the introduction of emission cleaning," said Mr Saunders.
"Air pollution is well documented as a potential health hazard and has been associated with increased respiratory sickness and death, which is of growing concern for those who live close to sources of air pollution.
"This study has suggested that improvements in emission cleaning technology have had a beneficial and measurable impact on the health of the local community."
However, the researchers acknowledged that they could not rule out other factors.
They didn't examine whether many people moved in or out of the area during this period, which could affect hospital admission rates.
In addition, they didn't examine whether the high admission rates before the foundry cut its emissions could have been caused by something else.
Nevertheless, the findings will add to the growing debate over whether there is a link between asthma and air pollution.
While a number of studies have suggested a link, others have not.
For instance, asthma rates have soared in Britain since the introduction of the Clean Air Act in 1956, which significantly cut air pollution across the country.
The National Asthma Campaign welcomed these latest findings but said more research is needed.
"Many asthma sufferers say pollutions triggers their asthma," a spokeswoman told BBC News Online.
"However, there have been very few specific studies which have shown that air pollution causes asthma. It is difficult to prove.
"This is an interesting study and we would certainly welcome measures to reduce emissions and air pollution."