Infographic: The 100 most polluted cities in the world

Every year, a thick smog covers India’s capital New Delhi. Last week, it got so bad for the 20 million residents that authorities shut schools.

New Delhi’s concentration of PM2.5 particles, which damage people’s lungs, is 34 times the World Health Organization’s (WHO) acceptable levels. The toxic haze is especially bad during the winter as farmers burn stubble left in their fields.

How is air quality measured?

Air quality is determined by the levels of air pollutants PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide.

Particulate matter (PM) comprises tiny particles that negatively impact health. PMs vary in size, most damaging are PM2.5 and PM10 – with a diameter of less than 2.5 μm and 10μm respectively. A human hair’s diameter is 50-70 μm.

PM2.5 levels lower than 12 are considered good, 55-150 unhealthy and 250 or above is hazardous.

The most polluted cities in the world

In 2020, India had 46 of the world’s 100 most polluted cities, followed by China (42), Pakistan (6), Bangladesh (4), Indonesia (1), and Thailand (1), according to air quality tracker IQAir. All these cities had a PM2.5 air-quality rating of more than 50.

Nine out of the top 10 most polluted cities are in India.

Hotan, in western China’s Xinjiang, had the worst average air quality in 2020, with 110.2.

India’s deadly pollution

In 2019, 1.67 million deaths in India were caused by air pollution, according to the Lancet.

While replacing solid fuels with alternatives has lowered deaths linked to household air pollution since 1990, deaths related to ambient PMs have increased.

Fifteen of the 20 most polluted cities are in India, mostly in the north. Stubble burning spikes pollution in autumn and winter. Vehicle emissions, industry, and burning rubbish also contribute to high levels of PM2.5 and other pollutants.

What is a smog tower?

Some Indian and Chinese cities have installed smog towers to try to tackle air pollution.

New Delhi installed two after an order by India’s Supreme Court – one is in a busy shopping area.

The $2m 25-metre (82-foot) high tower’s 40 fans take in particle-laden air at 1,000 cubic metres (35,000 cubic feet) per second and pass it through filters.

The smog tower works within a one kilometre (0.6-mile) radius, supposedly cutting PM2.5 levels by 50 percent. But questions remain over how efficient they really are.

Air pollution’s health impacts

According to the WHO, some 7 million people die annually as a result of air pollution. More than 90 percent of the world’s population lives in areas where air pollution exceeds WHO limits.

Air pollution is linked to a number of illnesses including asthma, diabetes, and heart disease.