While Beijing residents continue to struggle through a period of unusually high air pollution, similarly high smog levels are a fact of life for many in different parts of the world where Beijing-style warning systems aren't in force.

In the past week, during which Beijing's air pollution has been at its most severe, air quality has been one-and-a-half times worse in Delhi, India's capital, The Hindustan Times reports.

These figures are based on the particulate matter, or PM 2.5 levels, in both cities. The '2.5' refers to the particles' diameter of 2.5 micrometres or less. These tiny particles are considered to be the most deadly, as they penetrate much further into the lungs than larger pollution particles - seriously increasing the prevalence of respiratory diseases and the risk of lung cancer.

In the last week, the air in Delhi has had an average PM 2.5 concentration of 230.9 micrograms per cubic metre. By contrast, the average in Beijing over the same period was 139.8 micrograms.

Both cities have seen unusually high pollution levels recently, resulting in schools and businesses being shut down in Beijing as local authorities try to protect residents.

The average annual PM 2.5 level in Beijing is around 56. That's still far higher than the World Health Organisation's 'healthy' rating of 25, but much lower than Delhi's annual average of 153.

Of the top 10 most polluted cities in the world, six are in India, with three in Pakistan and one in Iran.

People told to stay indoors as air pollution in Beijing reaches hazardous levels Show all 5 1 /5 People told to stay indoors as air pollution in Beijing reaches hazardous levels People told to stay indoors as air pollution in Beijing reaches hazardous levels beijing-1.jpg A woman wearing a mask crosses a road during severe pollution in Beijing GETTY People told to stay indoors as air pollution in Beijing reaches hazardous levels beijing-2.jpg A woman wearing a mask walks on a street during severe pollution in Beijing GETTY People told to stay indoors as air pollution in Beijing reaches hazardous levels beijing-3.jpg Parking attendants stand on a roadside during severe pollution in Beijing GETTY People told to stay indoors as air pollution in Beijing reaches hazardous levels beijing-4.jpg Traffic makes its way through Beijing GETTY People told to stay indoors as air pollution in Beijing reaches hazardous levels beijing-5.jpg Severe pollution clouds the Beijing skyline GETTY

Significantly, Delhi and these other cities do not have a similar alert systems as Beijing - leaving schoolchildren and workers to bear the brunt of the pollution without much protection.

However, it is not the average pollution levels that have been worrying residents of Beijing, but the frightening extremes - PM 2.5 concentrations in Beijing have soared over 400 micrograms per cubic metre in the last few days for brief periods, in which even healthy young people can become ill simply from being outside.

The effects are even more severe for the elderly and sick, who have been encouraged to stay indoors at all times.

Some restrictions have been put on China's energy and transport industries, and the smog levels are continuing to improve, but Beijing's geographical location makes its already high pollution levels even worse.

As Forbes reports, cool air is trapped above a blanket of warm air during Beijing's winter, stopping air circulation and trapping the smog. Low average wind speeds during winter also means the pollution tends to linger and accumulate.