Industrial enterprises, vehicles and sources from outside Shanghai are the cause of 80 percent of the city's air pollution, the local environment monitoring center said.

Based on samples collected over the past two years, the environmental watchdog found that emissions from industrial plants in Shanghai generated 32.9 percent of the city's air-borne pollutants, and those from motor vehicles, ships and planes accounted for 25.8 percent of the pollutants, according to figures released on Monday.

Dust, cooking and the agricultural sector generated 19.8 percent of air pollutants. The remaining 21.5 percent came from outside Shanghai.

The center will announce the final research results for 2012 to 2014 at the end of this year, as required by the Ministry of Environmental Protection.

A similar research result for the third quarter of last year, unveiled by the Guangzhou environment monitoring center earlier this year, indicates that emissions of industry and transportation contribute to 56.2 percent of the air pollutants in the southern city, close to Shanghai's 58.7 percent.

Fu Qingyan, chief engineer at the Shanghai center, said the final result will be somewhat different because the "main pollution sources change throughout the year".

"We started collecting samples and data from 2009. But the research is still in its primary stage compared with developed countries," she said.

"We must do thorough research on each of the 150 kinds of pollutants."

Hu Ming, one of Fu's colleagues, said a more detailed classification of pollution sources should cover "different industries, automobiles, and even different ways of cooking".

Hu said all these efforts are necessary "to give the findings more practicable reference values for the making and implementation of pollution control measures".

Smog has become more frequent in cities in northern and eastern China over the last two years. But a lack of research on pollution sources has made it difficult for government to address the issue effectively.

Wu Xiaoqing, vice-minister of environmental protection, said at a work conference of environment monitoring earlier this year: "For the time being, we do not know where the pollutants come from, how they evolve into smog, and we don't have the necessary technology to deal with it."

Some research reports on pollution sources issued by various organizations differ greatly, making it imperative for the central government to establish a more reliable research system.

In Beijing, Tianjin and Hebei, where smog is more serious, local environmental monitoring centers will have to submit their findings on pollution sources to the Ministry of Environmental Protection before July.

The deadline for the monitoring centers in the Pearl River and Yangtze River delta areas is the end of this year.

Releasing the findings to the public before the deadline can help raise the citizens' awareness of air pollution, said Zhang Quan, director of the Shanghai Municipal Environmental Protection Bureau. "To be informed is the first step to take action," he said.

Analysts believe the central government will have to coordinate national action to clean the smog in northern China.

An integrated development plan for Beijing, Tianjin and Hebei province is the latest effort to tackle regional pollution as a whole.

Although environmental watchdogs in the three regions are yet to release their findings, iron and steel works and the burning of coal for heat and power generation are believed to be the main causes of the worst smog in the world.

Hebei produces 40 percent of China's crude steel and is responsible for 6 percent of the world's total coal consumption each year.

The measurement of PM2.5?-- particulate matter with a diameter less than 2.5 microns, which can penetrate deep into the lungs and is extremely harmful to health?-- is often 300 to 600 in the region throughout the year, sometimes hitting 1,000.

The World Health Organization's Air Quality Guidelines suggest that readings above 100 are hazardous to human health.