Some experts think that the electrical charge in thunderstorms may make mold and pollen particles more likely to stick in the lungs when inhaled. (Gene Blevins/Reuters)

Allergy sufferers often wish for rain, hoping it will wash away all the pollens and molds that stuff up their noses. While rain can indeed provide relief, a violent thunderstorm may have just the opposite effect: An unlucky few may experience a little-known threat called thunderstorm-related asthma.

Not fully understood by scientists, thunderstorm asthma can cause labored breathing for those with asthma and with allergies — including some who have never had breathing difficulties before.

“The phenomenon exists . . . it’s not entirely predictable,” said Elizabeth Matsui, an associate professor of pediatrics, epidemiology and environmental health sciences at the Johns Hopkins Children’s Center in Baltimore. Not every thunderstorm increases emergency room visits or cases of asthma, Matsui said. It’s rare, she said, and most who may be vulnerable will not experience it.

Experts are not certain about the exact mechanics of thunderstorm asthma. They hypothesize that heavy thunderstorm winds create updrafts that lift pollen and mold particles from the ground. Beating rain saturates and bursts the particles into tiny pieces. A downdraft then spreads those small particles into the environment and the air we breathe. Some think that the electrical charge of the storm may make these tiny particles more likely to stick in the lungs when inhaled.

“There’s a lot of small respirable particles floating around that folks can inhale and[that] contain those allergens that cause them to react,” said Susan Kosisky, chief microbiologist with the U.S. Army Centralized Allergen Extract Lab in Silver Spring.

Common molds like alternaria may be transformed by the rain, winds and lightning of a thunderstorm to trigger bronchial spasms in a rare phenomenon called thunderstorm asthma. (From Susan Kosisky, U.S. Army Centralized Allergen Extract Laboratory)

Some of the main allergen culprits behind thunderstorm asthma are thought to be grains of pollen, especially grasses and weeds, and mold spores, particularly Alternaria and Cladosporium, two of the predominant molds in the Washington area, according to Kosisky. Such molds are also associated with asthma in medical studies.

Thunderstorms’ ability to set off asthma attacks has been shown by increased emergency room visits and ambulance calls around the world, according to an overview published in 2012 in the British journal QJM. The authors noted that thunderstorm asthma is an uncommon event: Their review of medical research found only 35 publications. The phenomenon has been followed particularly in Australia, the United Kingdom and Italy.

As reported in the journal Thorax in 2008, researchers in Atlanta analyzed 12 years of data and found 3 percent more emergency room visits for asthmatic attacks in the 24 hours following thunderstorms than on days without such storms. The scope of thunderstorm asthma might well be broader, said study co-author Stefanie Ebelt Sarnat, an assistant professor of environmental health at Emory University, because the ER figure doesn’t include individuals who might have self-medicated or seen their personal physicians in the wake of a storm.

Since thunderstorms are local, fleeting events, it is a challenge to organize research around them. “The thing about these studies,” Matsui said, “is that they are very hard to do; that they’re all retrospective means that it’s very hard to extrapolate any information about one particular study to all thunderstorm asthma.”

Part of the puzzle of thunderstorm asthma is whom it hits. “One important tenet of allergy and asthma is that different patients have different allergic sensitivities,” Matsui said. According to the Centers for Disease Control and Prevention, 23.4 million Americans have asthma. But very few of them are affected by thunderstorms. “It’s not a common phenomenon,” Matsui said. And curiously, thunderstorm asthma sometimes strikes people who haven’t been diagnosed with asthma before.

Under normal conditions, pollen and mold particles are too big to get into the lungs and mostly trouble the noses of people with hay fever. People with asthma who also have hay fever may experience coughing and wheezing from pollen and mold.

It’s hypothesized that when those pollen and mold particles are broken up by a thunderstorm, the particles are then small enough to get into the lungs so that even someone with mild hay fever may experience respiratory distress. “This is kind of an exceptional condition where you have somebody who doesn’t have asthma on a day-to-day basis and all of a sudden in a thunderstorm, they develop severe symptoms,” said Joe Turbyville, an allergist in Louisville.

Experts think some of the upsurge in ER visits related to thunderstorm asthma can be traced to the fact that people with mild asthma or hay fever may not carry rescue inhalers and are therefore unprepared for the impact of the storm.

Turbyville first got interested in the phenomenon when he was chief of allergy, immunology and immunizations at the Fort Knox Army installation. He recalled an instance when several of his patients had more severe reactions than normal to their allergy shots on a day of huge thunderstorms. Turbyville knew that the risk factors for such reactions are similar to those for asthma, and he wondered whether there was a connection between reactions to allergy shots and thunderstorms. Now he’s working on a study on that possible link.

The warming of the climate adds another variable. “I definitely think this is something that might become worse with climate change,” said Andrew Grund­stein, lead author of the Atlanta study and a professor in the Department of Geography at the University of Georgia. He cited studies that suggest that climate warming might put more pollen in the air and cause more strong thunderstorms with heavy rains and strong winds.

And increased urbanization, Grundstein noted, creates what experts call an urban heat island effect. That increases the likelihood of thunderstorms, perhaps creating the conditions for thunderstorm asthma as well.

Levingston is a writer in Bethesda.