Harbin air pollution: Air Quality Index explained

Air quality in the northern Chinese city of Harbin has risen above hazardous levels in the last few days, closing schools and grounding flights in the municipality of 12.6 million people.

Kindergartens, primary schools and junior high schools were closed for a second day by local authorities, while Harbin airport canceled 258 flights yesterday as visibility was reduced to just 50 meters.

Pollution has surged in the city since it turned on its public heating system on Oct. 20, according to the Xinhua News Agency, which cited the local environmental bureau.

The heating system burns coal to produce steam that's piped into homes and offices for heating. Farmers burning straw in their fields after harvests has also contributed to the smog.

A measurement above 100 is considered unhealthy, while measurement above 300 is considered dangerous meaning children and the elderly are advised to remain indoors.

Exhaust fumes and the burning of coal for heating during the cold winter has been cited as the major contributing factors to the blanket of pollution across northern China. Combustion of such fuel results in high concentrations of solid and liquid particles such as sulfate, nitrates, ammonia, sodium chloride, carbon, mineral dust and water and create the particulate matter.

These particles are identified by their aerodynamic diameters and are linked directly to health problems. The finest of such particles, at 2.5 micrometers and smaller, are extremely hazardous to health as their size means when inhaled, they can travel deep into the lungs. They are 30 times smaller than an average strand of hair. Long term effects include cardiopulmonary disease, lung cancer, acute respiratory infection and more.

This animation explains PM2.5 pollutants and their potential effects on health.

According to the World Health Organization, 1.3 million deaths each year are caused by urban outdoor air pollution.

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