Health risk of inactivity is 'comparable to smoking'
18 July 2012 Last updated at 08:11 BST
The co-author of a report which suggests a lack of exercise is now causing as many deaths as smoking believes being inactive increases people's risk of developing chronic diseases.
Dr I-Min Lee said the study showed being physically inactive is a common factor in medical conditions such as heart disease and type two diabetes.
Therefore "on balance if you are phsyically inactive your risk of premature mortality is probably comparable to that of smoking".
Dr Lee added that doing exercise such as walking briskly every day can significantly improve a person's health.