Red-alert smog warning across parts of China

Children wearing facemasks on a school trip in Beijing, China.

Parts of China have been covered under a thick blanket of smog for the last four days.

The air quality in China can sometimes be so bad, that the government have to give people warnings about the level of pollution in the air.

When the levels are high they can close schools, stop planes taking off, close factories and limit the number of cars on the road.

In north and central China, cities like Beijing, are currently under a red alert air pollution warning - the highest level.

Millions of people, especially children and elderly people have been warned to stay indoors when the levels are high.

If they do go outside lots of people wear facemasks, to try to filter out the pollution.

What causes the smog?

Coal is used to power some Chinese factories and as it's winter now, more people are using it to heat their homes.

This means that more coal is burnt and this can increase the pollution levels in the air.

The weather can also have an effect on pollution.

As there is very little wind at the moment, the pollution is gathering in one area, rather than being blown away.

How do they measure pollution?

Pollution can be measured in different ways, but the World Health Organisation (WHO) use a scientific measurement called PM 2.5.

This measures the number of small particles in the air which can be breathed in.

They recommend that 10 micrograms is a safe amount in a year.

Recent recordings in China's capital city Beijing were higher than 400 micrograms per metre cubed, and around 600 in the city of Shijiazhuang.

When will it clear up?

Lots of factories in China use the fossil fuel coal for power.

According to weather forecasts a wind from the north should blow most of the pollution away on Thursday.

However for big cities like Beijing the air pollution levels can be high for most of the year.

In 2014 China's government said they were working hard to reduce the amount of pollution in the air.

Since then they've been trying to close down some coal-burning factories, and limit the amount of traffic on roads.