By Alex Kirby BBC News Online environment correspondent

Just 30 minutes of moderate physical activity daily can cut the risk of heart disease by half, the World Health Organization (WHO) says.

In a message to mark World Health Day, it says exercise should be acknowledged as "a pillar of a healthy lifestyle".

In developed countries, physical inactivity is the second most serious threat to health.

WHO says policymakers can help people to exercise with little effort.

Dr Mark Danzon, WHO's regional director for Europe, said a simple way to get enough exercise was to walk and cycle more.

Exercise doesn't have to be like this...

The recommended daily half-hour need not all happen at once. It could be achieved by walking or cycling to work and back, by using stairs instead of lifts, doing stretching exercises while seated, or simply by house-cleaning.

Exercise as modest as this, WHO says, stimulates both physical and mental wellbeing. It cuts the risk of inactivity-related disorders (including heart disease, adult diabetes and obesity) by about 50%.

Immobile generation

It also substantially reduces hypertension, blood pressure, stress and depression.

Globally, WHO says:

• a sedentary life is one of the 10 leading causes of death and disability
• the proportion of adults who are sedentary "or nearly so" ranges from 60-85%
• lack of activity leads to more than 2 million deaths annually
• a combination of improper diet, lack of physical activity, and tobacco use is estimated to cause up to 80% of cases of premature coronary heart disease
• under one-third of young people are active enough to benefit their present and future health and wellbeing.

It can be as idyllic as this

One encouraging example, it says, is the growth of "walking buses" in the UK - groups of young children who walk with their parents to school along recognised routes, with stops to collect other children.

In the Netherlands, the national cycling organisation runs older cyclists' courses to encourage them not to abandon their bikes.

The courses include practical skills training, and checks on eyesight, hearing and reaction times.

WHO says two things are needed to get more of us on to our feet and into the saddle: "A greater understanding of the strict link between health and physical activity, and an appreciation that walking and cycling are 'real' means of transport."

Reducing "the real and perceived dangers posed by traffic", it says, needs a mixture of approaches: enforcing a 30 kilometres per hour (18.5 miles per hour) speed limit, introducing slow-speed streets, improving road design, and educating road users.