Rainfall is known to clear the air of pollen and make breathing easier for those with asthma. But sometimes heavy precipitation can do the contrary: growing evidence shows that rain and thunderstorms can exacerbate asthma attacks. A thunderstorm in 2016, for instance, sent more than 8,500 individuals to the hospital for asthma-related distress in what is known as the Melbourne epidemic thunderstorm asthma event.
Scientists have been trying to piece together how storms lead to worsening asthma symptoms, and recent research offers new insight. For a study published in September in GeoHealth, researchers analyzed data on asthma-induced emergency room visits, alongside precipitation, in New York State during nonwinter months between 2005 and 2014. The results suggest that storms and heavy rain could be related to an increased risk of severe asthma attacks.
There are many different triggers for an asthma attack, says Camellia Hernandez, an allergy and immunology specialist at Brigham and Women’s Hospital, who was not involved in the GeoHealth study. For some people, such attacks can be brought on by pollen, temperature changes or humidity—all of which play a role in heavy rain or thunderstorms.
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The meteorology of thunderstorms sets the stage for respiratory irritants. When cool air high in the atmosphere floats over warm, wet air near Earth’s surface, the warm air quickly rises and pushes the cool air toward the ground. As the warm air ascends and begins to cool, it condenses into clouds and water droplets. The sinking cold air increases pressure; because air travels from high to low pressure, this difference creates a strong wind along the ground. That wind picks up respiratory irritants such as pollen grains, fungal spores and air pollutants, including particulate matter.
The increased humidity can also affect breathing in those with asthma. When you exhale, your lungs contract and decrease in size. But people with asthma have hyperresponsive airways that are chronically inflamed. Inflammation makes your respiratory system “a little less pliable,” Hernandez says. “That inflammation basically prevents air from getting out.” That can be exacerbated by humidity; people with preexisting inflammation have a harder time breathing that denser humid air, triggering an asthma attack.
Rainfall may not always occur alongside heavy wind, and instead it can pick up and clear the air of particulate matter that causes asthma attacks or allergy symptoms. “However, rain or humidity may hydrate pollen grains,” says Temilayo Adeyeye, corresponding author of the GeoHealth study, an assistant professor at the University at Albany and a research scientist at the New York State Department of Health. Adeyeye further explains that rain can also rupture the pollen grains, breaking them into smaller particles or releasing their contents into the air. This results in a flurry of breathable, allergenic aerosols that could induce asthma attacks, she says.
Other studies, Adeyeye adds, demonstrate that humidity can increase mold growth, which also impacts respiratory health. Raffi Tachdjian, an immunologist at the University of California, Los Angeles, and Providence Saint John’s Health Center in Santa Monica, Calif., who was not a part of the study, says that mold releases volatile organic compounds that “can exacerbate respiratory symptoms for even non-asthmatics.”
Hernandez offers another point that some asthma attacks are a delayed secondary reaction. The immune system is upregulated, or made to work harder, even minutes or hours after an exposure to an allergen or asthma trigger. “If you get a particularly heavy rainfall for, like, three hours, is it that that’s really causing the asthma attacks, or is it really a later inflammatory response” triggered by something else? Hernandez says.
Limited thunderstorm data kept Adeyeye’s team from comparing asthma-related incidents with those from rainstorms.
Hernandez suspects that many of the same factors are at play in both kinds of storms. She also explains that asthma is likely influenced by where someone lives—exposure to pollutants as well as the types of pollens or allergens can differ from region to region. A separate study published in August found that socioeconomic factors, such as poverty and racial-ethnic background, are associated with asthma rates.
Ultimately, it’s difficult to isolate specific causes of increased asthma attacks from these storms. Both rainfall and thunderstorms bring a slew of environmental changes. Tachdjian says that he often warns his patients with asthma about the increased risk of severe symptoms and attacks when a storm rolls in. More controlled studies that safely induce an asthma response to a small amount of allergens or irritants from a storm could improve our understanding of the relationship between rainfall and asthma around the world, he says.
For anyone with chronic lung inflammation or a respiratory condition, “any type of irritant is going to upset the status quo and can very likely lead to a significant asthma exacerbation,” Hernandez says.
Editor’s Note (11/3/23): This article was edited after posting to clarify Temilayo Adeyeye’s affiliation with the New York State Department of Health. The text was previously amended on November 3 to correct the spelling of Raffi Tachdjian’s last name.