WRAL WeatherCenter Blog

"Thunderstorm Asthma" a rare but high-impact event

Posted November 28

Assorted pollen grains. Rye grass pollen broken into small pieces was apparently the principal allergen involved in the recent Melbourne thunderstorm asthma outbreak.

Some of you may have seen the news stories last week on a "Thunderstorm Asthma" event that struck the Melbourne area in Victoria State, Australia last week. Most of us in the WeatherCenter hadn't heard of such a thing before, but found that it is a phenomenon that's been recognized in the past and studied by a few specialists around the world. Fortunately, the combination of high pollen count, sudden temperature falls, wind speeds and the right kind of pollen to create the conditions seen last week are very rare.

In the Melbourne area, the thunderstorm with strong winds and rain managed to loft intense clouds of pollen into the air, and moisture led to individual pollen grains, which are typically too large to infiltrate deeply into the respiratory system breaking into hundreds of much smaller allergenic particles, with sizes anywhere from about a half-micron to 2.5 microns. These correspond to a particle size range that was recognized some time ago in the air pollution and medical communities as being able to bypass protective "screening" of the outer respiratory system, and to infiltrate deep into the lungs. In Melbourne last week, this led to some people experiencing their first asthma attack, and many of those were quite severe. In the space of five hours or so, some 8500 patients sought treatment, with the Victoria EMS system fielding around 1900 calls, overwhelming the ability to respond in a timely manner in some cases, and other patients flooding emergency rooms in the area. As of one week later, six patients had died and another 15-20 remain hospitalized.

While it seems at a glance that the basic ingredients that lead to these rare outbreaks are well known - warm temperatures, high pollen loads, the right kind of pollen and intense thunderstorms, it turns out that those who have studied the phenomena have found that a high-impact event only occurs a very small percentage of the time when most or all of those seem to be in place. Pending further research that would turn up better information on the exact conditions required, this makes it tough to issue confident warnings and to plan for such events in the medical and disaster response communities, since false alarms would be so prevalent.

For any of you who'd like to read more about it, I included a link to a fact sheet from the Australasian Society of Clinical Immunology and Allergy. A web search for thunderstorm asthma will also turn up many more details about this event and some historical information.

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