More adults in Wales suffer from the symptoms of asthma than anywhere else in the world, according to new figures.
More than 10 million people in the UK have asthma
A report from the Global Initiative for Asthma, or Gina, suggests that around 30% of Welsh adults display symptoms.
More than a third of Welsh teenagers also suffer from the condition, which can be fatal in severe cases.
Experts have said that more than 10 million people in the UK have asthma, and the problem is placing a multimillion pound burden on health services.
Judy Oskan, from the National Asthma Campaign, said there were a number of reasons why the figures were so high.
"Housing could be a factor - and environmental pollution, both indoors and outdoors," she said.
"Passive smoking is likely to play a part, but more likely than not it's a combination of all these things.
"As yet we don't actually know what the cause is."
In the Gina report, which is being revealed at the World Asthma meeting in Bangkok, Wales tops the chart showing the number of adults with asthma symptoms.
Around 33% of children in Wales also display asthma symptoms, which is more than three times as much as those in Spain, Poland and Denmark.
Asthma symptoms in adults
Wales - 30%
Australia - 29%
Scotland - 28%
Republic of Ireland - 28%
England - 25%
Spain - 22%
Portugal - 18%
Italy - 10%
And the proportion of people with clinical or severe asthma in Wales is above the UK average of 16.1%.
In Wales, 16.8% of people have symptoms of clinical asthma, ahead of England with 15.3%, although behind Scotland's 18.4%.
Dr Bryn John is a GP in Neath, south Wales.
"In my practice I've seen an increase, particularly in children, who have asthma," he said.
But he said a significant proportion of adults who wheezed or displayed what looked like asthma related symptoms may be suffering from smoke related or mining related diseases - especially in south Wales.
He added that asthma was easily treatable.
"There are two types of treatment, relievers and preventers. And many people who suffer badly from asthma don't use a preventer.
"Because they're steroids people don't want to use them but they're absolutely safe."
Gina's report found that the rate of asthma increases as communities adopt western lifestyles and become urbanised.
And the number of people with asthma is expected to continue increasing in the next two decades.
Gina is calling for better understanding of asthma and health measures to combat it.