Storms began to spread across parts of France on Thursday and are set to be a key feature on the forecasts through to Sunday, with experts warning that they could trigger ‘storm-induced’ asthma attacks.

The combination of electrical storms and high pollen concentrations in the southern half of the country is set to increase the risk of respiratory problems from Friday, France’s Réseau National de Surveillance Aérobiologique (RNSA) warned. 

This phenomenon causes pollen grains to break up into fine particles, which spread rapidly and penetrate more deeply into the respiratory system. People allergic to pollen and asthma sufferers are among those most at risk.

On Friday, some 23 départements were under yellow alert for thunderstorms, with a southern belt stretching from the Landes, in the south-west to the Alps in the south-east. This figure is set to fall to 11 on Saturday, mainly in Auvergne-Rhône-Alpes and southern Burgundy-Franche-Comté.

“These weather conditions, combined with the high concentrations of grass pollen currently observed and forecast for this week … considerably increase the risk of storm-induced asthma,” the RNSA said in a press release. 

Pollen counts are extremely high over a large band of France, from the Atlantic coast to Alsace and the entire south-western quarter, it said.

During the first half-hour of a thunderstorm, patients suffering from pollen allergies may inhale a high concentration of the allergenic material that is dispersed into the atmosphere, which in turn can induce asthmatic reactions, often severe

Experts don’t fully understand why these events trigger breathing problems. But air flow within a thunderstorm system is thought to be important. 

Thunderstorms form when warm wetter air on the earth’s surface is beneath much cooler air higher in the atmosphere. The warm air rises quickly (known as up-draft) which causes the cool air to flow down towards the ground (known as down-draft). As the warm air rises, it cools and the moisture condenses into clouds and water molecules.

As a result pollen concentration “increases rapidly close to the ground with the strong downward winds that carry grass pollen from the upper air layers to the air layers close to the ground,” the RNSA said. 

Pollen then becomes waterlogged and bursts open under the combined effect of high air humidity, gusts of wind and variations in the electric field in the atmosphere. “The result is fine allergenic particles that can penetrate even deeper into the lungs”.

Thunderstorm asthma manifests itself as breathing difficulties comparable to an asthma attack, and occurs mainly – but not exclusively – in people allergic to grass pollen and asthmatics in general, but also in children and young adults.