The nation's average PM2.5 readings came in at 61 micrograms per cubic meter for January and February, according to a Ministry of Ecology and Environment survey of 337 cities, with only 83 reaching the national standard of 35 micrograms.

Levels of PM2.5 at 28 cities in the key pollution control region of Beijing-Tianjin-Hebei soared 24 percent over the two-month period from the same time a year ago to an average of 108 micrograms, more than 10 times 10 micrograms recommended as safe by the World Health Organization.

Meanwhile, PM2.5 levels in the 11 cities of the Fenwei Plain, another major smog control zone, surged by 26.6 percent over the period, hitting an average of 119 micrograms.

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China forced smog-prone northern cities to implement special emissions restrictions from October 2018 to March 2019 in order to offset rising levels of coal combustion from state heating systems during the winter.

But Reuters calculations based on official data showed that PM2.5 readings in the 39 key northern cities still rose 13 percent over the October-February period.

The 39 cities are under pressure to make year-on-year PM2.5 cuts of around 3 percent from October to March, but Reuters calculations show that only three - Changzhi and Luliang in Shanxi province, and Jining in Shandong - were on course to meet their targets at the end of last month.

The government has previously blamed unfavorable weather conditions for the poor air quality over the period, saying that "a weak El Nino effect" and a subsequent increase in temperature and humidity has made it harder to disperse emissions.

The ministry has promised to crack down on regions that fail to meet targets, regardless of weather conditions, but it remains unclear what punishments they will face.

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The provinces of Hebei and Shanxi, where eight of China's smoggiest cities were located in 2018, have established a "punishment and reward system" in which the worst-performing districts pay fines to regions that have performed the best.

On Wednesday, Hebei published a list of 14 districts that failed to meet 2018 targets, including major industrial zones in Tangshan, China's biggest steel-producing city.

Communist Party bosses from each of the 14 districts were summoned to the provincial environmental protection bureau to receive public criticism, the provincial government said in a notice.

REUTERS