Seven million people globally are dying prematurely every year due to toxic levels of air pollution, according to the World Health Organization (WHO). New data compiled from 795 cities across 67 countries has revealed that levels of air pollution have increased by 8% worldwide.

"The situation has deteriorated, " said Maria Neira, WHO's Director for Environmental and Social Determinants of Public Health. "We have a terrible figure. WHO has estimated that seven million deaths occur annually. Premature deaths are occurring every year and this is obviously a major, major public health problem", she said.

Some progress has been made, Neira said, as more countries are monitoring and reporting the levels of pollution in their air. In 2014 when WHO published its last study, its database consisted of 1,600 cities – while it now holds information on 3,000 cities from 103 countries.

India and China need to make "massive efforts" Niera said, but she would not be drawn on which city was the world's worst, since many cities – likely to be among the worst – were still being left out of the study because they were not collecting data. "We need to increase the measures being taken, and the countries need to move on monitoring air quality," she said.

"We know that there are several cities in China and several in India that are very much at above the levels of pollution recommended by the air quality guidelines at WHO, Niera continued. "Therefore I think these two countries need to do massive efforts because the situation at the moment is really bad for the population."

According to the WHO: "Air pollution is a major environmental risk to health. By reducing air pollution levels, countries can reduce the burden of disease from stroke, heart disease, lung cancer, and both chronic and acute respiratory diseases, including asthma."