Beijing, Jan 24 (IANS): Beijing has issued a yellow alert for heavy air pollution that took effect from Sunday, local authorities said.

A series of measures will be taken from Sunday, including the suspension of a number of outdoor construction operations, and halting or restricting production by manufacturing companies, according to the municipal air pollution emergency response office.

The air quality of the Chinese capital, which was once one of the most polluted cities on the planet, is expected to gradually improve on Tuesday, reports Xinhua news agency.

Under China's four-tier warning system for air pollution, red is the most severe, followed by orange, yellow and blue.

According to the UN, Beijing began an intensified air pollution control programme in 1998, and over the last 20 years the city has implemented a series of measures including energy infrastructure optimization, coal-fired pollution control and vehicle emission controls.

In the five years from 2013 to 2017, fine particulate pollution (PM2.5) in Beijing fell by 35 per cent and by 25 per cent in surrounding regions.

Its clean air efforts have been hailed as a model for other cities to follow by the UN Environment Programme.