Calls to emergency departments spiked in the wake of a thunderstorm that swept over Melbourne, Australia, in 2016. It was a rare outbreak of "thunderstorm asthma," the most severe ever recorded.

Now, a new model, published April 14 in the journal PLOS One, hints that a combination of lightning strikes, wind gusts, low humidity and popping pollen grains may be to blame for the surge of asthma attacks following the storm, which contributed to the deaths of 10 people. 

As the name suggests, thunderstorm asthma outbreaks occur when a passing storm disperses allergen particles in the air, triggering asthma attacks in susceptible people, according to the American Lung Association.

Those most at risk include: people with diagnosed asthma, particularly if their condition is poorly controlled; people with undiagnosed asthma; and those with seasonal hay fever or a rye grass allergy, according to a 2017 report from the Victoria State Government's Chief Health Officer. 

Although thunderstorms rumble through the sky fairly frequently, thunderstorm asthma events are quite rare. Since the first recorded thunderstorm asthma event in 1983, 22 accounts of the phenomenon have appeared in the medical literature, first author Kathryn Emmerson, a senior research scientist at Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO), told Live Science in an email.  

Of these 22 outbreaks, 10 occurred in Australia, so it seems the country is a "hotspot" for such events, she added.

The most severe outbreak to date occurred in the Melbourne area on 21 November 2016, at roughly 5:30 pm local time. Leading up to the storm, the weather had been hot, above 86 degrees Fahrenheit (30 degrees Celsius) and very dry, Emmerson said. The air held more than 133.4 pollen grains per cubic yard (102 grains per cubic meter), indicating that grass pollen season had reached its peak in Australia. 

"The event occurred at the peak of the hay fever season, and most patients were suffering with an allergic response in their airways," Emmerson said.

Normally, rye grass pollen grains - the main culprit behind the outbreak - are too large to reach the deep lungs and instead get caught in the nose and throat; but somehow, during the 2016 storm, the weather conditions broke down these grains into smaller particles, triggering asthma symptoms in a large number of people.

The storm pushed a wall of gusty wind through the region, but dropped very little rain, only about 0.03 to 0.15 inches (1 to 4 millimeters), according to a 2017 state government report. A wave of high humidity followed the storm, as well. But because of the sparse rain, many people remained outside as the storm passed by, which increased the number of people exposed to the pollen, Emmerson noted. 

That evening and the following day, local health care providers were suddenly inundated with a flood of patients seeking care for respiratory-related conditions. 

The public hospitals in Melbourne and nearby Geelong saw a 672% increase in patients arriving at emergency departments with respiratory problems, as compared with the average number for that time of year; that amounted to 3,365 more cases than expected based on the three-year average.

Ambulance transport services, local primary care physicians and pharmacies were also bombarded with calls regarding emergency medical care. And in the end, storm-related asthma symptoms contributed to the deaths of 10 people, according to the state coroner.

Of course, the big question is, why did this disaster occur? In the past, scientists theorized that downdrafts of cold air from the storm clouds stirred up grass pollen grains below, pushing them skyward; once captured inside the clouds, the pollen grains became saturated with water and began to burst, so the theory goes.

A 2016 study, published in the Journal of Applied Meteorology and Climatology, supported this idea, noting that wind in the clouds also contributes to the pollen-grain-popping, as well as lightning, to a lesser extent.

Following the Melbourne outbreak, the state health department wanted to create some sort of forecasting system to help predict when another outbreak might strike. Emmerson and her colleagues went to work crafting this forecast system but found that high humidity, supposedly the main driver of pollen grain ruptures, wasn't a helpful predictor for thunderstorm asthma events. 

We "found that conditions of high humidity, a measure of how much water is in the atmosphere, occurred almost every evening - not what you want from a warning system predicting a relatively rare event," Emmerson said.

So if high humidity served as the basis of their warning system, it might set off too many false alarms. To craft a better forecast model, Emmerson and her team looked for other atmospheric conditions that might set the stage for outbreaks of thunderstorm asthma. 

Using data from the 2016 event as a guide, the team crafted computer models to test how airborne pollen grains rupture under different weather conditions; they backed up these models with lab experiments, in which they subjected pollen grains to wind gusts and electrical pulses.

Based on their experiments and models, they found that several phenomena work in tandem to smash the grains to bits, namely, strong winds, lightning strikes and the build-up and discharge of static electricity brought on by low humidity, as seen just before the 2016 storm.

But notably, "the lightning method was the only mechanism to generate a pattern in [sub-pollen particles] following the path of the storm," the authors wrote.

Assuming the 2016 storm had a similar pollen-laden tail, this may somewhat explain the timing and distribution of emergency calls for ambulances that occurred during the event, hinting that lightning strikes may be a key trigger for thunderstorm asthma. 

However, during the fateful storm, not much lightning struck within Melbourne itself, where most of the asthma attacks occurred, but instead fell to the east and south of the city, the Australian news outlet 9News reported. So while there seemed to be some correlation between the lightning strikes and asthma attacks, it wasn't a perfect explanation. 

In fact, "none of the tested processes completely satisfied our requirements for a warning system," meaning none stood up as a wholly reliable signal for forecasting thunderstorm asthma events, Emmerson told Live Science. "We haven't fully cracked the code on the triggers of thunderstorm asthma yet." 

For now, the best approach for predicting such events is to monitor for thunderstorms associated with severe wind gusts, while also tracking the levels of unburst grass pollen in the air. Emmerson and her team plan to improve upon their current model, in part by better estimating the amount of whole and burst pollen grains higher in the atmosphere, close to the clouds.