Environmentalists on Wednesday expressed disappointment at new clean-air targets for Hong Kong, as research showed pollution-related illnesses killed more than 3,000 residents a year.

In the face of mounting public criticism and allegations that it is soft on polluting industry, the government on Tuesday announced its first revision to air quality objectives (AQOs) in 25 years.

But the tighter standards will not be implemented until 2014 and fall well short of World Health Organisation guidelines.

"We welcome these measures as a small step in the right direction, but I have to say they are disappointing," Friends of the Earth Hong Kong senior environmental affairs manager Hahn Chu said.

"These measures fall far short of WHO guidelines and do not guarantee public health safety."

The new objectives impose more stringent limits on the atmospheric concentration for seven pollutants including sulfur dioxide, nitrogen dioxide, carbon monoxide and lead.

For the first time the city will measure airborne particles smaller than 2.5 micrometres in diameter, known as PM2.5, which are more harmful than the larger particles currently measured.

Heavy polluting vehicles will be phased out, hybrid or electric vehicles will be promoted and more use will be made of natural gas.

Mike Kilburn, head of environmental strategy at independent think-tank Civic Exchange, asked why two years had passed since the government pledged to update its air quality targets.

"These are the same air quality objectives that the government put out in 2009 and they came out unchanged. Frankly we see this as the government's reaction to Beijing rather than the demand of Hong Kong people for clean air."

Beijing bowed to a vocal online campaign earlier this month and said it would provide hourly updates of PM2.5 measurements ahead of the Lunar New Year on January 23.

Kilburn said the 2014 timeframe for Hong Kong's new standards would allow infrastructure projects like a planned third airport runway to go ahead despite pollution concerns.

Secretary for the Environment Edward Yau said the WHO standards were a "distant target" for a city like Hong Kong, which is regularly blanketed in smog from factories and power stations in mainland China.

"Given the surrounding environment of Hong Kong, we cannot set a goal that is unachievable," he told reporters on Tuesday.

Meanwhile, the University of Hong Kong released updated research showing 3,200 Hong Kongers died each year from illnesses related to bad air, up from the previous estimate of 1,000.

Economic losses to the southern banking and finance centre were estimated at HK$40 billion (more than $5 billion) annually.

"Hong Kong's air quality has been deteriorating over 20 years and today presents a serious daily threat to the health of the public and for future generations," the university said in a statement.

It said official air quality standards used to communicate health hazards to the public were "primitive and seriously misleading".

"The risk categories... bear no relationship to the currently measured bad health outcomes in the population," it added.