Officials said on Tuesday that the Chinese capital Beijing met state air quality standards for the first time last year, a milestone lauded by experts.

After a series of smog buildups in Beijing and elsewhere sparked widespread public outrage, China declared war on pollution in 2014. This prompted concerted efforts to reduce coal consumption, reduce transportation emissions, relocate heavy industry, and a crackdown on violations.

"The improvements are real...and happening across the industrial belt surrounding Beijing, as well as in much of the rest of the country," said Lauri Myllyvirta, lead analyst at the Center for Research on Energy and Clean Air. But he warned that pollution remained high enough to "constitute an ongoing health risk for residents".

The annual average of PM2.5, or small, hazardous airborne particles, was 33 micrograms per cubic meter, nearly seven times higher than the World Health Organization's recommended level of five micrograms.

However, this was down 13% from a year ago and met China's interim standard of 35 micrograms for the first time on record, officials said during a press conference on Tuesday.

Beijing goes green

The city's efforts over the last decade, as well as the speed with which it has improved, have been described as "unprecedented" by Yu Jianhua, deputy head of Beijing's environmental protection bureau.

Last year, Beijing residents had nearly four months more of clear skies than in 2013.

Beijing promised in 2015 that it would use the 2022 Winter Olympics to help drive environmental improvements, with Chinese President Xi Jinping vowing to host a "green" Games.

As a measure of progress, average PM2.5 readings in 2016 were 71 micrograms but frequently exceeded 500 micrograms during the winter months, when coal-dominated heating systems were turned on throughout the region.

Since then, Beijing and the neighboring province of Hebei have made efforts to switch to cleaner natural gas and have planted a large number of trees throughout the region.

They have also imposed strict new fuel standards on automobiles and forced steel mills and other industrial facilities to install emission-controlling equipment.

"Further improvement will require a shift from coal and oil to clean energy," Myllyvirta said.