Environmental authorities in North China's Shanxi Province said that the extremely high density of sulfur dioxide (SO2) last week was mainly caused by household coal burning, following days of public criticism over heavy air pollution.

The density of SO2 in the air in Linfen, Shanxi, reached up to 1,303 micrograms per cubic meter on January 4, which is 21 times higher than the national standard, and was comparable with London's Great Smogof 1952.

The SO2 in Linfen was 65 times higher than the recommended density by the World Health Organization, which is 20 micrograms per cubic meter within 24 hours, for human health.

"The high density of SO2 can directly lead to poisoning, because the SO2 can be transmitted to various forms of sulfate with water, and it is also a source of PM 2.5 - airborne particles smaller than 2.5 microns in diameter and a major air pollutant," Wang Gengchen, a research fellow at the Institute of Atmospheric Physics under the Chinese Academy of Sciences, told the Global Times.

Facing huge pressure from the public, Linfen's environmental protection bureau said the excessive emissions are mainly caused by household and industrial uses of coal, while the city's geographical location is also unfavorable for pollutant dispersion, news portal thepaper.cn reported on Monday.

Zhang Wenqing, an official from the bureau, said that 70 percent of the SO2 emissions in the city are caused by burning of coal by households, as the monitoring of pollutants shows the daily peak is from 8:00 pm to 11:00 pm. More than 100,000 households in and close to the city are using coal for heating, Zhang added.

Lin Boqiang, director of the China Center for Energy Economics Research at Xiamen University, told the Global Times that "Shanxi accounts for one-third of the coal production in China. The most important step to prevent large amounts of SO2 emissions is to control the use of coal by households, and to replace it with electricity or natural gas during heating season."

"China should expand the use of clean energy, as coal accounts for nearly 63 percent of overall national energy resources. It would take about 40 years to reduce the coal consumption to 30 percent at the current pace," Lin said.

During the 2016-20 period, the ratio of coal in the energy mix is expected to be brought under 58 percent. The share of non-fossil sources is targeted to account for more than 15 percent of the nation's total energy consumption, according to the National Energy Administration.