Secretary for the Environment, Transport & Works Dr Sarah Liao says it is impossible to directly compare Hong Kong's air quality objectives with those of other places because they have different ways and requirements for setting their standards.

Dr Liao today told lawmakers that the best way to make such a comparison among cities is to compare the actual concentration of pollutants, which allows us to differentiate the levels directly. The public will be misled if the standards adopted are substantially different.

She said the air pollution index (API) adopted by Hong Kong is divided into five bands with values ranging from naught to 500. European cities like London and Paris have their indices presented in values between one and 10, but their methods of grading and computation differ.

Although US cities adopt an index with values ranging from naught to 500, their computation method is different to Hong Kong.

"Hong Kong's index, based on real-time computation, is released hourly while some cities, such as Singapore, compute and release their APIs once a day. Some cities like Tokyo publish only the concentration levels of specific pollutants without establishing APIs," she said.