North China air pollution off the scale

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SUBSCRIBERS TO CHINA Air Quality Index (AQI) apps on their smart phones or tablets will have seen the pollution indicators go off the scale above 500 μg/m3 (micrograms per cubic metre) at times over the past few days as parts of North China including Beijing are enveloped by smog. 

As the images accompanying this article show, both the US embassy and Beijing government air quality indices registered as being 'beyond index' (off the scale) on a scale from 1 to 500 today (26 February). Sometimes the figures reported by the US Embassy in Beijing and the Beijing authorities deviate quite a bit but in this case the gap between the two indices was negligible. 

The headline figure of 500 refers to the PM2.5 reading - the particulate matter (PM) of most concern - since these particles are so small (at 2.5 μg/m3 or less) that they do not get filtered out and pass straight through the lungs into the blood stream. The other cause for concern with PM2.5 is that there is no minimum level below which no damage to health is observed.

So, for example, the World Health Organisation (WHO) sets very low target levels for both PM2.5 and PM10 in its standard air quality guideline (AQG) values for particulate matter as follows:

PM2.5 - 10 μg/m3 annual average; 25 μg/m3 daily average

PM10 - 20 μg/m3 annual average; 50 μg/m3 daily average

NO2 - 40 μg/m3 annual average; 200 μg/m3 hourly average 

SO220 μg/m3 daily average; 500 μg/m3 10-minute average

Of course, while the WHO norms above are very low compared to the peak values seen in Beijing in recent days, it is worth noting that the WHO target values are based on a measured amount of matter in the air over a specified period of time. And while China PM2.5 levels occasionally reach 300 to 500, that's normally only for a few hours at any given time and would not normally persist for a whole day.

Similarly, with the NO2 (Nitrogen Dioxide) and SO2 (Sulphur Dioxide) levels shown in the AQI pictures at the top of this article and in the WHO AQG values above, human tolerance to high levels of particles in the air is much greater for periods of 10 minutes to an hour than for prolonged periods averaged out over a year.

So rather than be alarmed over peak PM levels for an hour or less during the day, what really matters is the long-term average level of particles in the air.  And, on the positive side, the fact that all these air pollution levels in China today are so closely monitored and so widely reported shows that the authorities are serious about acknowledging and tackling the problem.

But, just as Rome wasn't built in a day and a journey of a thousand li starts with a single step, this air pollution problem isn't going to be resolved in a hurry so, for the time being, keep that AQI app handy so you can plan to avoid the worst of the smog at peak hours as and when it occurs.  







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