More than 95% of the world’s population is breathing unhealthy air and the poorest countries are the hardest hit, a new report has found.

According to the annual State of Global Air Report, published by the Health Effects Institute (HEI), long-term exposure to air pollution contributed to an estimated 6.1 million deaths across the globe in 2016.

It means air pollution has become the 4th highest cause of death among all health risks, exceeded only by high blood pressure, diet, and smoking.

“Air pollution takes a huge personal toll worldwide, making it difficult to breathe for those with respiratory disease, sending the young and old to hospital, missing school and work, and contributing to early death,” Bob O’Keefe, vice president of HEI, said in a statement.

“The trends we report show real progress in some parts of the world — but serious challenges remain to eliminate this avoidable affliction,” he added.

Air pollution is a complex mixture of particles and gases which differs from region to region and even within cities, but fine particulate matter (PM2.5) is the measure used as the chief indicator of pollution levels.

It report says an estimated seven billion people, 95 per cent of the earth’s population, live in areas where (PM2.5) exceeds the World Health Organisation’s air quality guidelines, and that 60 per cent live in places where fine particulate matter exceeds even the least stringent WHO air quality targets.

The problem is most acute in Asia, with India and China accounting for over half (51 per cent) of all global deaths from ambient air pollution.