China's air quality improved across the country in 2017, the environmental protection ministry said Thursday, after the problem was so dire in previous years that some periods were dubbed an "airpocalypse".

The average level of PM2.5 particles -- which penetrate deep into the lungs -- in 338 cities stood at 43 micrograms per cubic metre last year, falling 6.5 percent year-on-year, according to a ministry statement.

The World Health Organization recommends a maximum average exposure of 25 micrograms per cubic metre in a 24-hour period.

The average level of slightly larger PM10 particles in the cities declined to 75 micrograms per cubic metre, 5.1 percent less than in 2016, the ministry said.

Pollution has plagued China for years, with the dramatic fouling of the country's air, water and soil representing the dark side of breakneck economic growth that has lifted hundreds of millions out of poverty.

Analysts said recent improvements were sparked by favourable weather conditions and more aggressive enforcement of environmental rules.

Authorities have ordered polluting factories to leave Beijing and its surroundings, and designated "no-coal zones" where more than three million homes have abruptly switched to gas or electric heating.

Questions remain, however, on whether the government's measures are sustainable. On Thursday evening, heavy smog blanketed the capital and surrounding northern areas.

As China nears the end of its five-year plan for combating smog, experts and activists have called for a new strategy for long-term success.

A Greenpeace analysis released this month found that while PM2.5 levels in Beijing, Tianjin and 26 surrounding cities declined 33.1 percent year-on-year in the last three months of 2017, for the full year it fell just 4.5 percent around the country -- the lowest rate of decline since 2013.