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According to one estimate, air pollution kills 9 million people worldwide each year – more than smoking. Rather than killing otherwise healthy people, air pollution tends to aggravate the effects of heart and lung diseases, so working out the death toll is not straightforward. People exposed to air pollution in early life can still have a higher death rate decades later.
Population studies show that living in polluted areas is associated with a higher risk of asthma, reduced fertility in women and lower scores on maths and language tests.
There are two main components of air pollution: particulates and gases. Particulates are split into categories based on their size: PM2.5 means particles smaller than 2.5 micrometres; PM10 means particles smaller than 10 micrometres. The smaller ones are more damaging as they can penetrate deep into your lung tissues. Power plants, factories, gas cookers, volcanoes, car exhausts and forest fires are all sources of particulate pollution.
Among the gases, nitrogen dioxide is considered the most harmful. Its main sources are diesel engines, indoor heating and power stations. Ozone and sulphur dioxide can also be damaging to health.
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Worldwide, the average PM2.5 concentration rose 11 per cent between 1990 and 2015. This reflects large increases in air pollution in rapidly growing economies like China and India. In developed countries, concentrations of particulates and nitrogen dioxide have fallen thanks to tighter controls on vehicle emissions, but the rate of the fall seems to be slowing down.
Air pollution is still a major problem in the US: 45 cities have PM2.5 concentrations above those recommended for healthy air.
And if you think you are safe from air pollution inside your home, think again. Household items like deodorant and shampoo release a mix of carbon-based chemicals into the air, although it’s unclear how much effect they have on health. Wood burning stoves are a major source of particulate pollution both indoors and outdoors.