It’s known as thunderstorm asthma. And it’s really weird. The theory goes that residents in an area will suffer a wave of asthma attacks and other respiratory ailments in the days before a thunderstorm arrives.

The phenomenon was first reported in Great Britain in 1983. In Melbourne, Australia in 2016, thunderstorm asthma was blamed for eight deaths and 8,000 visits to emergency rooms.

Now, a group of researchers have published a study that lends a little credence to what had until now been anecdotal evidence.

Scientists analyzed insurance and Medicare data of ER visits tied to acute respiratory diagnoses and matched it to government weather data. The weather information covered all U.S. counties between 1999 and 2012.

That covered a whole lot of weather — 822,000 days with major thunderstorms — and an ocean of ER visits, more than 22 million.

Investigators found emergency room visits appeared to spike in the days leading up to thunderstorms for people reporting respiratory problems such as COPD or asthma. They estimated 52,000 additional ER visits tied to approaching thunderstorms for the 13 years studied. It’s a modest but not insignificant number.

Of course, since scientists examined Medicare data, these additional visits involved people over the age of 65. But the study’s authors said the phenomenon would be expected to involve younger people, too.

The researchers suggest thunderstorm asthma might be related to rises in temperature and particulate matter in the air before storms arrive. But they aren’t certain.

It’s always wise to keep an eye to the weather forecast. It can tell us when to keep an umbrella handy — or an inhaler.