At least four people have died and hundreds were sent to the hospital this week in Melbourne, Australia after an outbreak of “thunderstorm asthma” struck late Monday.

The rare condition is thought to occur when high levels of pollen become saturated by rain and disintegrate into teeny fragments that are easier to inhale. Those minuscule particles can invade the deep crevices of the lungs and get lodged there. In people who are allergic to pollen and/or already suffer from asthma, the scenario can easily become life-threatening.

On Monday, the city’s rye grass pollen count was reported to be double the level normally considered “high.”