Living in an unhealthy environment kills many times more people than die in road accidents, violent conflicts and natural disasters put together, but these risks rarely make headlines. Now an analysis by the World Health Organization (WHO) reveals that about a quarter of the annual health and death toll of living in such environments could be avoided. See graphic.

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The report is the result of an eight-year meta-analysis of relevant scientific literature and available health and population statistics, which went through the fraction of mortality and illness attributable to environmental causes factor by factor. More than 100 public-health experts and epidemiologists advised on the regional health impacts.

Most countries haven't yet realized that severe health problems often have quite trivial causes.

Polluted water, poor sanitation and smoke inhalation resulting from indoor use of wood-burning stoves are the main risks in most low-income countries, and poverty is a major obstacle when it comes to reducing risk exposure. But environmental factors pose an avoidable health risk in rich countries as well, says Annette Prüss, a WHO public-health scientist who oversees the project. Noise, work stress and outdoor pollution all add to the burden of ill health attributable to the environment.

The WHO's research centres on 'disability adjusted life years' (DALY) that are preventable through healthier environments. The DALY is a commonly used unit that includes years lost when someone dies prematurely and also takes account of years blighted by chronic disease or disability.

The data, says Prüss, are to help national health authorities prioritize disease prevention efforts. “Most countries, rich and poor, haven't yet realized that severe health problems often have quite trivial causes,” she says.

“The WHO exercise is helpful in that it draws attention to the fact that many health problems need a non-medical response,” says Andrew Scott, policy director with Practical Action, a non-governmental organization that helps modernize cooking stoves in Africa. “Small amounts can make a large difference.”