People with breathing disorders, such as asthma, are being warned to take care in the current heatwave.
Asthmatics mat gave to use their inhalers more
They have been warned high levels of ozone could make their condition worse.
It is early in the year for a smog warning to be issued.
The smog is the result of strong sunlight reacting with pollutants created mainly by vehicle exhausts which combine to create ground level ozone
Asthmatics are being advised to carry their inhalers with them.
Over 80% of people with asthma find air pollution makes their asthma symptoms worse
Dr Martyn Partridge, National Asthma Campaign
And all people with breathing difficulties are being advised to avoid taking outdoor exercise in the afternoon.
The National Asthma Campaign said there were other steps people could take to control their condition.
It said they should also:
- Monitor their condition by checking their peak flow
- Talk to their GP about increasing the amount of preventative medication (brown/orange inhaler) they take, if their asthma seems to be getting worse
- Check pollution levels daily in the media or by calling the Pollution Helpline on 0800 55 66 77
Dr Martyn Partridge, chief medical advisor at the National Asthma Campaign, said: "Over 80% of people with asthma find air pollution makes their asthma symptoms worse.
"This is of particular concern during the summer months when ground level ozone increases."
He added: "One way in which people with asthma can help to minimise the effects of air pollution is to use a written personal asthma plan which helps you maintain control of your symptoms and tells you what to do when your asthma gets worse.
"If your doctor or nurse has not given you a plan ask them to develop one for you, or you can download one from www.asthma.org.uk.
"The asthma plan will show you how to control your symptoms by adjusting your asthma medication yourself to prevent pollution or other factors from worsening your asthma."
The National Asthma Campaign's Asthma Helpline also offers advice on 08457 010203.