In rapidly growing economies, the amount of pollution in the air is undeniably rising, but it is a different story in most rich countries. Take, for instance, PM2.5 particulates 鈥� believed to account for most of the health burden of air pollution (see 鈥�What鈥檚 in the air鈥�). Worldwide, average concentrations rose 11 per cent between 1990 and 2015, according to a report by the Health Effects Institute and the Institute for Health Metrics and Evaluation, both in the US. The trend reflects large increases in India, Bangladesh and China: concentrations in the US, the European Union, Canada and Australia fell over the same period (see graph).
Read more: Cutting through the smog on air pollution
The bad news on bad air seems to get worse by the day, but all is not as it seems over Western cities. We take a look at the numbers
Media reports on air pollution in the West frequently don鈥檛 mention the major improvements made since the 1950s. But the rate of progress has slowed and Europe, including the UK, is showing no signs of meeting WHO guidelines for clean air any time soon. 鈥淭he data from monitoring sites across western Europe shows PM2.5 levels are going down,鈥� says Gavin Shaddick of the University of Bath, UK, who develops air pollution models for the WHO. 鈥淏ut they are not falling quickly enough.鈥�
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What鈥檚 in the air
Particulate matter (PM)
Comes from: power plants, factories, gas cookers, industry, volcanoes, dust storms, forest fires, car exhaust.
Health effects: Any dust or droplet less than 10 micrometres (碌m) across (called PM10) can penetrate deep into your lungs. Those smaller than 2.5 碌m (PM2.5) are the most damaging air pollutants. They include ultra-fine particles, which are smaller than 0.1 碌m. Long-term exposure to PM2.5 can impair lung and heart function and increase mortality, especially among those at higher risk of heart disease and stroke.
Nitrogen oxides (NOx)
Comes from: road transport 鈥� especially diesel engines 鈥� as well as indoor heating and power stations.
Health effects: NO2 is assumed to be the second most harmful pollutant after PM2.5. Exposure can trigger respiratory problems. In lab studies, volunteers given extra NO2 exhibit inflammatory responses. However, long-term effects are unclear.
Ground-level ozone (O3)
Comes from: reactions between other chemicals including NOx and volatile organic compounds, especially on warm, sunny days.
Health effects: ozone can cause wheezing, shortness of breath, inflamed and damaged airways, a range of lung diseases, exacerbated asthma. Children, elderly people and those who are most active are at greatest risk. O3 is a powerful oxidant, so damages cells and tissues.
Sulphur dioxide (SO2)
Comes from: burning fossil fuels, especially coal. As a result, levels have dropped significantly in the West where natural gas has largely replaced coal.
Health effects: irritated airways and eyes, breathing problems, heart and circulation problems. Those with asthma and other respiratory conditions are at greatest risk. SO2 can form sulphates that become PM2.5, and it causes acid rain.
Ammonia (NH3)
Comes from: decomposing organic matter livestock and fertilisers.
Health effects: At the concentrations that it is present in the air, NH3 is unlikely to harm human health. Its main effect is to acidify the soil.
This article appeared in print under the headline 鈥淐utting through the smog鈥�
