China's capital, Beijing, has announced measures to combat worsening air pollution, state-run media report.
They include taking half of the city's four million private cars off the roads on days when there are serious levels of pollution.
The system will be based on a vehicle's licence plate - odd numbers will be allowed on the roads one day, even numbers the next.
Motor vehicles are considered a major source of pollution.
The new system will give out four different degrees of air pollution warning - blue, yellow, amber and red, Xinhua news agency reports.
On days when an amber warning is given, factories will stop production and work will be halted on construction and building sites.
Restaurants which offer an open-air barbecue will be ordered to close temporarily and fireworks will be banned throughout the city.
Anger at exemptions
When a red warning is issued, the new car restriction measure will be implemented. Schools and kindergartens will also be closed.
The measure to restrict the number of private cars from using the road is proving to be controversial.
Critics have aired their concern that those who can afford to buy two or more cars will able to drive any day when the restriction is in force.
Users of Weibo, China's version of Twitter, have also criticised the restriction for targeting ordinary people as cars used by government officials and civil servants are exempted.
One user said that whenever there is a problem in Beijing, "ordinary people are the first to be forced to pay the price for it".
Beijing has almost 21 million permanent residents, according to official estimates.
There is also a large migrant population in the city, but no exact official figures are available.
Most people relied on bicycles and public transport to get around in the city before private car ownership became popular.