The air quality index from the US Embassy (AQI) that many of us are familiar with is based on particles suspended in the air (PM2.5, meaning particles smaller than 2.5 microns). Now the Embassy has added a second AQI for ozone to the particle measurement.
The new hourly measurement from the Twitter feed now looks like this (for example):
12-25-2010; 12:00; PM2.5; 50.1; 125; Unhealthy for Sensitive Groups // Ozone; 150.1; 132; Unhealthy for Sensitive Groups
These numbers are:
Date; Time; PM2.5 measurement, PM2.5 AQI, Health Rating // Ozone concentration; Ozone AQI; Health Rating
So why the new number for ozone?
Ozone is a form of oxygen, with three oxygen atoms (O3) instead of the usual two (O2). It is a very reactive gas and an irritant. I remember its smell very well from my days as a grad student operating high voltage equipment: in those days (mid ‘80s) we knew less about its hazards however.
Ozone occurs naturally high in the atmosphere in the “Ozone layer”, and it has the useful effect of shielding us from ultraviolet light. When it is at ground level however it is less welcome. It can irritate the lungs, especially for those with asthma. There’s a description of the known health issues with ozone on the EPA’s website:
Where does ozone come from?
Ozone at ground level comes mainly from vehicle exhaust fumes reacting with sunlight. This means that it is likely to be highest on sunny, hazy days with not much breeze. These conditions are also the ones that give us high particulate pollution, so high PM2.5 levels and high ozone levels are likely to occur together.
Who needs to pay attention to this new index?
Ozone has its own index and system of health warnings that are very similar to the warnings for particulate pollution, but are slanted towards those who are more at risk, especially asthma sufferers. For most Beijing residents I think that a glance at the PM2.5 AQI will continue to tell you what you need to know about the pollution level, but asthma sufferers will probably want to look at the ozone AQI too. For schools supervising pupils with asthma it will also be worthwhile to pay attention to the ozone level and index since it may help to anticipate and avoid problems. The advice is similar to that for particulate pollutants: avoid strenuous exercise when the AQI is very high.
This is a new feature and it will be interesting to see how the index varies from day to day and through the year.
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