Air quality was classed as "unhealthy for sensitive groups" in Beijing on Sunday, as the population braced for several days of choking smog.

An "orange" alert, the second highest in China's four-tier warning system, was put in place on Thursday.

The warning was issued by the Ministry of Environmental Protection, and included Tianjin, Hebei and surrounding areas, valid from Friday until Wednesday.

As a result of the alert, heavy industries were forced to reduce output by 30-50 percent. More than 700 companies are affected by the directive in Beijing alone.

The warning refers to the concentration of small particles referred to as PM2.5, which are particulate matter with a diameter of less than 2.5 micrometres. This is less than the width of a human hair.

These particles, which are small enough to be ingested directly into the lungs, have a direct correlation with premature deaths from heart and lung disease. They have also been shown to have a negative effect on chronic conditions such as bronchitis and asthma.

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PM2.5 particles have a variety of sources. Many are ejected directly into the air by power stations, motor vehicle emissions, wood burning fires and agricultural crop burning.

Weather conditions may also enhance their formation, such as when water vapour reacts with sulphur dioxide emissions by power plants.

Beijing's smog is expected to range between "unhealthy" and "hazardous" until Wednesday afternoon. There will then be some respite, with a return to "moderate" air quality through much of Thursday, before it rises to decidedly unhealthy levels once again.