Shanghai has fine-tuned its emergency air pollution plan, vowing to release a timely warning against smoggy weather and intensify mandatory measures to help reduce pollution.

At a news conference on Wednesday, the city's environmental protection bureau released the updated emergency air pollution plan that has been approved by the municipal government.

Experts said Shanghai took the lead in the country by setting an example and putting forward a practical and detailed emergency plan.

Under the plan, Shanghai set up a four-tier warning system for polluted weather, with red being the most serious, followed by orange, yellow and blue.

A blue alert will be released when the city's air quality index reading is forecast between 201 and 300 for 24 hours, a yellow alert for AQI of 201-300 for 48 hours, an orange for AQI of 301-450 for 24 hours, and a red for AQI of 451 and above for 24 hours.

In line with the four-tier warnings, the authority also set up a four-level response system, and made clear compulsory measures to be taken on polluted days.

For example, when a red alert is raised, the government will take 50 percent of government vehicles off roads, ban activities involving large crowds, suspend classes at kindergartens, primary and middle schools, and halt work at high-polluting enterprises and at most construction sites.

The original plan, which took effect in April, generated a public outcry as residents said the warnings came too late.

The old version was also vague on whether the measures to be taken on polluted days are compulsory or advised.

Qian Hua, former director of the atmospheric environment research institute under the Shanghai Academy of Environmental Sciences, said on Wednesday that Shanghai has made progress against air pollution. "The updated plan is more detailed, makes improvements in timeliness and is more practical," he said.

"The environmental workers are not gods. They need to learn knowledge about weather and come up with a practical emergency plan. But Shanghai leads other areas in the country with such a plan and it has been quick to make adjustments when it found defects in the original one."

Some residents welcomed the improved plan.

"In the past, we were always told about a heavily polluted day — on which children should stay at home — when we had already sent my child to kindergarten," said Yan Xuwei, a father. "We need a warning about the smog well ahead of the time it is happening to make a correct decision about our life," he added.

Shanghai also vowed to work closer with its neighbors to fight and control air pollution.

Shanghai and the provinces of Jiangsu, Zhejiang and Anhui, as well as eight ministry-level agencies, launched a coordinated mechanism to fight and control air pollution in the Yangtze River Delta region last week.

Under the framework, the four regions will work together to push forward the nation's policies on air pollution control, study the challenges the region faces and report to each other on air quality and the progress they make in their own areas.