June 24, 1994 began hot and humid in London. But around midday, the heat that dominated the morning began to dissipate. The humidity remained, however, and as the afternoon marched on, the sky turned ominous. A mighty storm was brewing.
The inclement weather that eventually rolled in as the sunlight started to wane was a Mesoscale Convective System. Similar in many ways to a tropical cyclone, systems like these are formed when smaller thunderstorms jumble together and loosely organize. The resulting weather mass is like a multi-headed monster, thrashing wildly in multiple directions with whipping winds, tumultuous rain, and frenetic lightning.
As sewers and streets brimmed with water, area hospitals soon grew inundated as well, with patients presenting strange symptoms that seemed to have nothing to do with the storm whatsoever. Over the next thirty hours, at least 640 patients visited emergency departments complaining of extreme wheezing and troubled breathing — they were suffering from asthma attacks. Several thousand more may have been affected without seeking medical attention.
The incident remains the largest outbreak of asthma ever recorded, affecting even those with no history of the condition, and catalyzed scientific study into the strange link between thunderstorms and asthma.
Nearly twenty-two years later, that link is now coming to be well understood. A recent review published to the journal Clinical and Experimental Allergy asserts that there is now sufficient evidence to suggest a causal relationship between thunderstorms and asthma outbreaks.
The recipe for an epidemic seems to rest on a few key factors: an extremely elevated concentration of grass pollen, typical during late spring and early summer, a decrease in temperatures (colder, denser air keeps pollen concentrated near the ground), and a sufficiently turbulent storm.
“Grass pollen is usually too large to enter the small airways of the lungs and is filtered out by the nose,” writes Megan Howden, a respiratory physician in Melbourne, Australia. “But stormy winds and moisture can cause the pollen to rupture into tiny particles, which are small enough to be inhaled.”
“The outflow winds of a thunderstorm then concentrate these tiny particles at ground level, where they can easily enter the small airways of the lungs and cause an acute asthma attack in those who are allergic to grass pollens.”
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