April Showers Bring More Than Flowers

From the WebMD Archives

May 18, 2001 -- It's a curious phenomenon: Asthma suffers often report that spring thunderstorms trigger severe attacks. And hospital emergency room data back up the claim.

Several studies have pointed to the release of grass pollens during these storms as the culprit, but it has not been clear whether the rain itself or the electrical activity associated with thunderstorms is responsible for this release.

Now a study from Australia suggests it is neither. Researchers from the Institute of Respiratory Medicine at the University of Sydney found that downdrafts of cold air within a thunderstorm sweep up grass pollens that have been ruptured by the rain, releasing allergens into a band of air called the outflow.

"These ruptured pollen grains are sucked up into the storm, and then pushed down again in the downdraft that runs in front of the storm," lung specialist Guy B. Marks, PhD, MBBS, tells WebMD. "The air from this downdraft is filled with pollen, so asthma sufferers who are allergic to grass pollen are affected."

Marks and colleagues began studying the link between thunderstorms and asthma attacks following a 1997 asthma attack epidemic in the southeastern Australia town of Wagga Wagga. During a 12-hour period following a spring thunderstorm, 215 asthmatics in the town of 55,000 were seen at the local hospital for asthma-related symptoms. Forty-one patients required admission.

"When we investigated this epidemic, we found that almost all of those affected were allergic to rye grass pollen," Marks says. "What we did not know was why the phenomenon appeared to be particularly linked to thunderstorms, as opposed to other rain."

In this study, appearing in the June issue of Thorax, the researchers identified weather conditions during asthma epidemics in six southeastern Australian towns and compared them to days with few asthma cases. Thunderstorm outflows were detected on 33% of the epidemic days, compared with 3% of the nonepidemic days. The association was strongest in late spring and early summer, when grass pollens are present.

Detailed examination of one especially severe epidemic showed that it coincided with the arrival of the thunderstorm outflow and a four- to 12-fold increase in grass pollen grain concentration.


Meteorologist John Colquhoun, who also was involved with the study, says a long dry spell preceding a spring thunderstorm also may be a factor in storm-related asthma attacks.

"With [the Wagga Wagga] episode, there was very little rainfall prior to the storm," he tells WebMD. "That allowed the pollen to build up."

Based on these findings, Marks advises asthmatics who have grass pollen allergies to take extra precautions in the spring and early summer when thunderstorms are expected.

"What we recommend to people who have hay fever and asthma -- those who wheeze and sneeze, as we say -- is that they should take regular preventive medications during the spring and early summer," Marks tells WebMD.

Asthma and Allergy Foundation of America Executive Vice President Jeffrey G. Demain, MD, says four out of five asthmatics have allergies. Demain, who is also director of the Allergy, Asthma, and Immunology Center of Alaska, recommends that these people stay indoors in the mornings and evenings during pollen season.

If possible, Demain recommends that asthmatics plan outdoor activities for the middle of the day, keep doors and windows closed during those high pollination times, and use free-standing HEPA air filtration systems, which work well.

"We also recommend that people change their shirt when they come in from outside, and that they be aware that pets and kids can bring pollen in," Demain says.

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