They also have to pay a service provider to undertake remedial action to achieve standards set by the government. They are awarded a certificate which is reviewed annually.

Department figures show 157 indoor premises have been awarded indoor air quality certificates this year, 43 more than last year. They include government departments, the headquarters of the WWF environmental group and the academic buildings of the Polytechnic University.

Speaking at a presentation ceremony yesterday, Ms Wong said the review would update the emission standards of indoor pollution sources, explore the possibility of covering more indoor premises, and look at the possibility of drafting new laws to control indoor air pollution.

Principal environmental officer Pang Sik-wing said the study - expected to be completed by the end of the year - would consider making the current certification scheme compulsory, and premises like shopping malls and hotels might be covered.

He said the study also would explore the possibility of setting up a green-labelling system for indoor pollution sources such as carpets, paints and furniture. This would be aimed at encouraging consumers to choose products that emit a lower level of pollutants. The government had set aside HK$90 million last year to improve the air quality in its departments. Two-thirds of the departments are still awaiting the check.

Percy Hui Shu-wing, senior manager of an environmental service company, said most companies joining the voluntary certification scheme were large firms, adding that a compulsory check each year would mean higher operating costs for small and medium-sized firms.