I have always found myself short of breath when exercising. I had asthma when younger, but now worry I may have damaged lungs.

If you have any concerns about the possibility of lung damage or disease, I would advise you to check with your GP. Individuals who have lung disease often feel it is impossible to exercise because of muscle weakness, fatigue and shortness of breath. However, subsequent inactivity may only make the symptoms worse.

Regular moderate exercise encourages the lungs to work better because more blood capillaries grow around the lung tissue, helping oxygen to be used more efficiently. For lung-disease sufferers, walking is the most appropriate form of exercise, placing little undue stress on the musculoskeletal system.

When exercising, try to breathe deeply, breathing out twice as long as the in-breath, and at no time hold your breath. When jogging or running, avoid snatching your breath by breathing in over four strides and then out over four strides.

Air quality can exacerbate breathing problems when you exercise outdoors, so minimise potential difficulties by exercising away from busy roads - as a rule of thumb, try to stay 20-40 metres from any car whose engine is running.

While being short of breath when working out is not unduly harmful - more often than not it is a signal that your lungs are working hard - if you have known lung disease and feel tired or short of breath, you should stop and relax for a few moments before continuing at a lower intensity.

· Joanna Hall is a fitness expert (joannahall.com). Send your exercise questions to Weekend, 119 Farringdon Road, London EC1R 3ER (weekend@theguardian.com).