Thousands of people suffered freak 'thunderstorm asthma' attacks in Australia - here's why

On Monday, thousands of people in Victoria, Australia were rushed to hospital with breathing difficulties. Four people have reportedly died. Three more are still in a critical condition.

In just a four-hour period that day, more than 1,900 calls were made to Ambulance Victoria and an extra 60 ambulances were deployed, along with numerous police and firefighters.

The seemingly freak occurrence, it turns out, has a name. It's called "thunderstorm asthma," and it happens when the level of rye grass pollen in the atmosphere is unusually high and a storm hits.

How 'thunderstorm asthma' happens

Asthma is a common symptom of hayfever, the allergy to pollen. Around a quarter of people with hayfever also have asthma. And, strange as it sounds, heavy rain can make hayfever symptoms - including asthma - worse, according to the Australasian Society of Clinical Immunology and Allergy. When it rains, pollen grains can absorb moisture and burst, releasing hundreds of small allergenic particles that can irritate the lungs when inhaled.

In particular, the particles can irritate and inflame bronchial tubes, which can cause them to fill with mucus and make it hard to breathe. This is one of the reasons asthmatics sound wheezy.

Excessive pollen is a problem

In cases of thunderstorm asthma, many people affected have never experienced asthmatic symptoms before, which is why it is such a seemingly freak occurrence. It is thought that sudden asthma attacks are a direct result of the tiny particles being released into the air, making people who are not usually so susceptible suddenly flare up.

"When you have a perfect storm coming together [of] a very high pollen day, high humidity, and a thunderstorm, the grains of rye grass absorb water with the humidity and they break up into thousands of pieces," Robin Ould, chief executive of the Asthma Foundation of Australia, told CNN. "Normally with rye grass the pollen would be trapped by nose hairs," but "when it breaks up it goes straight to the lungs."

Grass pollen is the most common cause for thunderstorm asthma, but attacks can also be triggered by excessively high levels of fungal spores and other pollens.