Understanding how the WHO Air Quality Guidelines apply to Hong Kong’s high daily levels of air pollution
The following is an explanation from Professors Hak kan Lai, Chit-ming Wong and Anthony Hedley from Hong Kong University about why we should be comparing our daily air pollution levels, which are extremely high, to the ANNUAL, rather than short-term guidelines for each of the 4 pollutants studied by the World Health Organization. If you look at the Hedley Environmental Index to see how the annual standards compare to the short-term standards, you will see that the annual standards permit signficantly lower levels of pollutants. Obviously, when critical periods of high pollution are averaged out over a much longer span of time, the overall average will be much lower.
In fact, the short-term WHO standards are devised to ensure compliance with the annual standards and assume 3-4 exceedances per year ONLY: “The WHO short-term levels (e.g. 24 hour for PM10, PM2.5 and SO2) are designed to limit daily pollutant levels (through compliance) in a way which leads to compliance with the annual guideline. You can demonstrate mathematically that compliance with the short term limits for particulates will lead to the annual guidelines being achieved.”
“For example, the annual level for SO2 will be around 3 to 6ug/m3 IF there is compliance with the 24hr limit of 20ug. So the notional annualised guideline for SO2 is <6ug. (NOTE: There is no formally stated WHO annual guideline for SO2, in fact. Presumably, it is because the annual level is so low.)”
“HOWEVER, nitrogen dioxide is a problem and the short-term 1-hour guideline of 200ug/m3 doesn’t work in a way which ensures that the annual guideline of 40ug will be achieved.” THAT’S BECAUSE DAILY LEVELS OF NO2 ARE FAR ABOVE THE ANNUAL GUIDELINE BUT GENERALLY FALL JUST BELOW THE 1-hr GUIDELINE LIMIT, thus failing to trigger an exceedance event. The HKU team is not certain why such a high exceedance level of 200ug for NO2 was set. But there is no doubt that it is set too high to maintain compliance with the annual guideline limit. “The reason for this high setting for the short term is not clear but it may stem from the fact that nowhere in the world are there such high sustained levels as we experience in Hong Kong . The original calculations may have estimated a lower level but added an extra margin (i.e. up to 200) to take account of uncertainty arising from some studies. However, this doesn’t work for us in Hong Kong’s intensely polluted environment. With continuously high daily levels we estimate that the short term must be much lower if daily controls are to achieve the annual limit of 40ug.”
“We must emphasize that the short term limits are designed only to be exceeded on a few days in the year (strictly about 4 days to 7 days).” Thus, in a very high pollution environment we need to use the annual guidelines to judge everyday levels, trends and exposures.
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I am a big fan of your site. I’d like to bring up a point about the bus system in Hong Kong. Everyday, most of the buses are empty and yet they continue to pollute our air. I hope you can bring this up in a video so that the hong kong people can see that actually these bus companies like KMB, Citybus are probably making money just being a billboard. We should get a boycott going.
While all important for a global standard in pollution measurement, I think we are trying to teach a drowning man not to drink too much.
The levels are above the chart on any measure, even the API. Bus boycotts do not help in my point of view. That will cause only more empty buses.
We should rather pressure the government to force btter pollution standards in cars, to make driving private cars less convenient, and combat traffic jams around happy valley, the central tunnel etc.