SHANGHAI, Jan 20 (Reuters) - Air pollution levels fell in most cities in China last year, environmental group Greenpeace said on Wednesday, but a humid and windless winter shrouded swaths of the country in choking smog, slowing improvement in the second half.

Beijing hoisted its first ever “red alerts” on air pollution in December, when a blanket of humid, still air trapped haze over the capital. The episode followed heavy criticism of the government for inaction during a prior bout of hazardous smog.

“Despite Beijing’s choking winter of red alerts, data from 2015 clearly shows a continued positive trend in Beijing and across the country,” said Greenpeace climate and energy campaigner Dong Liansai.

“However, air quality across China is still a major health hazard.”

Annual average levels of PM2.5 - particulate matter of 2.5 micrometers in diameter that can penetrate deep into the lungs - dropped by 10.3 percent last year compared with 2014, Greenpeace said in a report published on Wednesday, citing official data from 189 cities.

In the first half of the year, PM2.5 concentrations were down 16 percent from the corresponding 2014 period, the group had reported.

Decades of growth-at-all-costs economic development in China has spawned massive problems of air, water and soil pollution that the ruling Communist Party has only in recent years begun to tackle.

The challenge is large and the topic sensitive, with thousands of protests sparked every year by concerns about environmental degradation. Uncontrollable environmental factors, such as wind, can also play a role.

Beijing’s fourth-quarter pollution woes were mirrored elsewhere in northern China, where concentrations of PM2.5 were significantly higher than in 2013 and 2014, Greenpeace said.

Even so, Beijing was among roughly 90 percent of the cities Greenpeace analysed where air pollution improved overall last year.

That was a sign the capital’s “serious efforts” to curb pollution were paying off, Dong said.

China has worked to toughen environmental protection laws in recent years. Amended legislation took effect this month giving authorities more power to punish firms and officials responsible for violations.

Some local authorities, including Beijing, have been trying to limit emissions and forcing polluting factories to close or move.

Still, 80 percent of the 366 cities whose data Greenpeace analysed fell short of China’s ambient air quality standards last year, it said.

The national average concentration of PM2.5 was 50.2 parts per cubic metre, exceeding the World Health Organisation’s guideline of an annual average of less than 10 micrograms.

China’s financial hub, Shanghai, was among the cities where air quality worsened in 2015. Its average PM2.5 concentration increased 3.14 percent over the 2014 data, Greenpeace said.

“Greenpeace recommends Shanghai to implement a solid coal consumption cap target and aggressive measures to solve the air pollution problem,” Dong said.