People with asthma should be aware that thunderstorms could trigger their asthma symptoms

National Asthma Campaign spokeswoman

Dr Guy Marks, from the Institute of Respiratory Medicine at the University of Sydney, Australia, found the thunderstorm outflows were created by down draughts of cold air which sweep up and collect a concentrate of atmospheric particles, which are then released through the rain.

Asthmatics, particularly those with an allergy to grass pollen start to suffer as they breathe in the concentrated pollens.

People allergic to grass pollen are more prone to attacks

Dr Marks found that these outflows preceded a third of all asthma epidemics and that it was worse in late spring and summer, when the pollen counts were even higher - then the outflow preceded almost half of all epidemics.

The scientists found the hourly pollen counts trapped during one epidemic showed the grass pollens trapped in a thunderstorm were between four to 12 times higher.

Thunderstorms without the outflows had no impact at all on asthmatics.

Pollen triggers

Dr Marks said: "These findings support our hypothesis that thunderstorms trigger epidemics of exacerbation of asthma during the pollen season by sweeping up allergenic particles up and concentrating them in a narrow band of air close to ground level.

"Subjects allergic to pollen who are in the path of the thunderstorm are likely to inhale air which is heavily loaded with pollen allergen and, consequently to experience an airway asthmatic response."

Now experts are calling on asthmatics to take special precautions during thundery weather.

Inhaler use might need to be increased during thunderstorms

Dr John Harvey, of the British Thoracic Society (BTS) said: "Spring and summer can be a particularly difficult time for asthmatics because of the high pollen and air pollution episodes; thunderstorms must now also be taken into account.

"In light of the recent weather, the BTS urge all asthma sufferers to increase their preventer inhaler, visit their GP if necessary and seek immediate medical attention if they experience an extreme attack. More research is needed into this interesting area."

A spokeswoman for the National Asthma Campaign agreed: "People with asthma should be aware that thunderstorms could trigger their asthma symptoms.

"They should keep a close eye on their condition either by taking regular peak flow readings or recording their symptoms.

"It may be necessary to increase their dose of preventive treatment, in consultation with their doctor or nurse.

"If they are at all worried about their asthma they should contact their doctor for further advice."