Beijing, Jan 6 (IANS): Several schools in northern China are installing huge domes where children can play and exercise during recess to protect them from the smog outside, officials reported on Friday.

The domes resemble inflatable greenhouses and are as big as a stadium (50 metres long, 20 metres wide and 10 metres high), the officials said.

The domes, manufactured by MetaSpace Air Dome Corp, a Beijing-based company, uses filtration systems to keep the air inside pollution-free, Efe news reported.

Dozens of cities in central and northern China have been witnessing levels of polluting particles of over 500 micrograms per cubic metre, five times above the acceptable level, but inside the domes this level is less than 10.

The popularity of the domes increased a few weeks after a school in the city of Linqi in Henan province hit the headlines for making its students take an exam in the playground amid heavy smog.

Images of the children, seated at their desks amid a thick cloud of smog, shook the world and led to the principal of the school, Feng Jisheng, being sacked.

Persistent smog in cities, including capital Beijing and Tianjin has sparked alarm among the people of northern China, who are concerned about the long-term effects of the pollution on health.

In Beijing, parents of students have been asking schools to install air purifiers in classrooms, a demand the capital's Education Commission accepted on Friday.