Fears over a potentially deadly 'thunder fever' have sparked the warning to those suffering asthma.
Charity Asthma UK says thunderstorms can trigger attacks in people with asthma - especially young children and adults.
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"Thunderstorms can trigger asthma attacks in people with asthma, especially children and young adults. It's not fully understood why this happens.
"One reason could be that when it's very humid, the windy conditions during a thunderstorm blow lots of pollen and mould spores high into the air. The moisture higher up in the air breaks them into much smaller pieces.
"As these smaller pieces of pollen and mould particles then settle back down, they can be breathed in, irritating the smaller airways of the lungs. This can trigger asthma symptoms.
"If a thunderstorm is forecast:
"Stay indoors if you can, before, during and after the storm, and keep the windows closed.
"Change your clothes and have a shower when you've been outside to wash off any pollen.
"Avoid any of your other asthma triggers, such as exercise, alcohol or stress.
"Have your reliever inhaler close by and ready to use if you need it.
"If you have hay fever, take your usual hay fever treatments such as a nasal spray and/or antihistamines. If you're not sure, speak to your pharmacist or GP about the best hay fever treatment for you.
"Don't smoke or let other people smoke around you because it can make asthma symptoms worse.
"Make sure you know the signs that your asthma is getting worse and what you need to do if it is."