Heavy smog obscures the view of downtown Harbin, Heilongjiang province, on Monday. [Photo by Kim Kyung-hoon / Reuters]

The health effects of outdoor air pollution can only be assessed after long-term monitoring and quantitative data analysis, a spokesman for the country's top health authority said.

Mao Qun'an, spokesman for the National Health and Family Planning Commission, said in a news conference on Tuesday that the authority's monitoring of the health effects of outdoor air pollution could yield results only after a long period. The monitoring began in late 2013.

In October, the authority announced a plan for establishing a monitoring network to study outdoor air pollution's effects on health in the next three to five years.

In December, Chen Zhu, former minister of health, wrote in an opinion piece in the British general medical journal The Lancet that a 2010 study finding that at least 350,000 people in China die prematurely each year as a result of outdoor air pollution "might overestimate the health effects of outdoor air pollution".

The study, conducted by the World Bank, World Health Organization and the Chinese Academy for Environmental Planning, concluded that between 350,000 and 500,000 people die prematurely annually as a result of outdoor air pollution.

Chen, who was elected vice-chairman of the 12th National People's Congress Standing Committee in March, said those findings are "a maximum international estimation".