Air quality index may grade risks to health

The government is studying a Canadian air pollution alert system as an alternative to the existing outdated system, which does not indicate the direct health impact of various pollutants.

This emerged as the choking sandstorm that had blanketed the city since Sunday night - pushing the air pollution index off the top of a 500-point scale and forcing some pilots to use autopilot to land at the airport because of poor visibility - slowly dispersed in the face of a southeasterly wind.

A person familiar with the situation said a group of health scientists commissioned by the Environmental Protection Department to study improvements to the alert system submitted a framework for a new system last year. But the proposal has been put on hold pending the results of a review of air quality objectives enacted 23 years ago.

The recommended system is modelled on the Canadian Air Quality Health Index, which has a scale of 1 to 10 and shows four categories of health risks from low to very high. It is still not clear, however, whether this is the only proposal being considered or whether the government will incorporate contingency plans to discourage polluting activities on days of high health risks.

The city's 15-year-old air pollution index (API) is calculated based on the readings of the maximum level of any one of five selected air pollutants - sulphur dioxide, nitrogen dioxide, ozone, respirable suspended particles and carbon monoxide.

Whenever that maximum reading exceeds the level of the air quality objectives, the API will rise to a level of over 100, categorised as 'very high'. Different levels also carry different health advisories.

'What is flawed in this system is that the index only informs people about the excessive levels of a particular pollutant. It also implies there is a safety level and does not tell people the actual health impacts,' a person familiar with the study said.

The main feature of the proposed system is that the health impact for each of the chosen air pollutants would be quantified based on past studies, such as the extra number of hospital admissions when the a particular pollutant is above a certain level. The aggregated impact of all pollutants would then be converted into indexes, which could be grouped into different health risk categories.

Dr Wong Chit-ming, a biostatistician at the University of Hong Kong, said under the Canadian system any index value above zero was a clear indication that people were suffering from the impact of air pollution.

Wong said such a system was better because it took into account the total impact of multiple pollutants rather than just one and could avoid overexaggerating the index when a single pollutant skyrocketed in unusual circumstances.

'Had this been adopted, the pollution reading of the sandstorm might have not been so high.'

At the peak of the sandstorm, 10 of the 14 monitoring stations went off the top of the 500-point scale. Several stations were still above 400 at 8am and above 300 at 2pm, but all but one, Eastern District, had dropped below 200 by 8pm.