Countries urgently need to make all workplaces and indoor public places smoke-free, the head of the World Health Organization said Tuesday.

The recommendation aims to protect people from exposure to second-hand smoke, which can lead to premature deaths from heart disease and serious respiratory diseases in adults and worsen conditions such as asthma in children.

"The evidence is clear, there is no safe level of exposure to second-hand tobacco smoke," Dr. Margaret Chan, WHO's director-general, said in a statement released ahead of No Tobacco Day on Thursday.

"Many countries have already taken action. I urge all countries that have not yet done so to take this immediate and important step to protect the health of all by passing laws requiring all indoor workplaces and public places to be 100 per cent smoke-free."

The UN agency estimates almost 700 million children, or almost half of the world's children, breathe air polluted by tobacco smoke, mainly at home. An estimated 200,000 people die from exposure to smoke at work each year.

Canada, the United States, France, Spain, Ireland, Portugal, parts of Australia, New Zealand, Bermuda and Uruguay have banned smoking in public places, according to the International Union against Cancer, a non-governmental group that supports the global campaign to make workplaces and public spaces smoke-free.

"By July 1, 240 million people worldwide will be protected by smoke-free legislation," spokesperson Wayne Kao told a news conference in Geneva.

"Unfortunately, that is less than four per cent of the world population."

The WHO's policy recommendation is based on evidence from three major reports on second-hand smoke written by the International Agency for Research on Cancer, the U.S. surgeon general and the California Environmental Protection Agency.

Tobacco use is the leading preventable cause of death globally, causing more than five million deaths a year, according to WHO.