If your health is good, then the levels of air pollution we usually experience in the UK are unlikely to have serious short-term effects. However occassionally air pollution levels are greater than normal. it is particularly during these episodes that potential risks increase for those people with asthma, allergies, sensitivity to pollutants, lung diseases, heart conditions or the seriously ill or elderly.
For more information read DEFRA's Air pollution: what it means for your health leaflet. This also includes a section on how you can help to reduce air pollution.
To find out what the air quality is like near you go to UK National Air Quality Information Archive where data and lots more air quality information is available for the whole of the UK.