European Heart Journal published a study claiming that air pollution causes 800,000 deaths in Europe and 9 million worldwide each year, which is double previous estimates.

However, the figures do not mean that 9 million people dropped dead solely because of air pollution. Rather, they are a way of representing the harm dome by pollution.

This is not to say that air pollution isn’t dangerous. In fact, this study suggests it is a bigger killer than smoking, which using the same method is estimated to cause 7 million extra deaths worldwide each year.

Lead study author Jos Lelieveld says that air pollution has now joined the ranks of major risk factors such as high blood pressure, diabetes and obesity.

It is important to understand where these numbers come from. Working out the damage done by air pollution is much harder than something like car accidents, for which we have firm figures, because it typically aggravates the effects of common disorders such as respiratory diseases.

Many teams around the world have been doing long-running studies that compare, for example, people living in areas with different levels of particulate pollution to work out how it affects the risk of developing respiratory and cardiovascular diseases.

Recent results suggest air pollution is a far greater contributor to cardiovascular disease than previously thought. But unfortunately telling people their “hazard ratios” for air pollution – the standard scientific measure – would be a form of words to them.