(The following is a second post from a new contributor, Chris Buckley of Torana Clean Air)

Many of us rely on the Air Quality Index (AQI) supplied by the US Embassy as a guide to pollution levels in the capital. In this, the first in an occasional series of posts on this topic, I will discuss how to interpret this number in a Beijing context.

There are air quality standards and indices for several different urban pollutants, including dust particles of different sizes and gases such as NO2 and SO2. From epidemiological studies carried out in the last decade ultrafine dust (less than 2.5 microns in size) has emerged as the most important health risk, while pollutants such as nitrous oxide and sulfur dioxide that were once thought to be a problem for city dwellers have shown less correlation with health effects. The Embassy monitoring station measures these ultrafine particles, usually called PM2.5 that come from engine exhausts and industrial smoke. As far as current knowledge is concerned it represents the most useful guide to our exposure to pollution.

The simplest way to use the daily pollution reading (available on this site and on http://iphone.bjair.info/) is to focus on the brief risk assessment that accompanies each number, ranging from “Good”, through “Moderate”, “Unhealthy for sensitive groups” and so on. But if you understand a little about the index and the numerical score you can get a clearer perspective on what it is telling you.

The AQI is a scale devised by the US-based Environmental Protection Agency (EPA). It is based on a standard for air quality set by the EPA, which considers an average amount of PM2.5 particles of 35 microgrammes per cubic meter to be the maximum acceptable exposure over a 24 hour period. This figure corresponds to an AQI on their scale of 100. The first point to notice is that this refers to an average, so a brief exposure to levels above 100 is not necessarily a cause for alarm if our overall exposure averages lower than this. Nevertheless 100 is a useful number to keep in mind: anything above this score is worth paying closer attention to.

The AQI scale was devised with US cities in mind, which mainly have their air pollution well under control. On a typical day most busy cities in the US score 50 or below, and the AQI scale is designed to be most sensitive to differences in pollution levels in the 0-100 range. Here are the ranges, their health descriptions and corresponding PM2.5 dust levels:

The sharp-eyed will notice that the relationship between the AQI and the amount of pollution is not a straightforward one. For readings above 100 the index increases more slowly with increasing PM2.5 dust level. To take an example, we might intuitively expect an AQI of 200 to reflect double the level of pollution of an AQI of 100, but in fact the total amount of dust has already doubled by the time we reach 150. At an AQI of 200 the pollution level is FOUR times the level at AQI 100. This point is worth bearing in mind in Beijing since we often experience AQI scores in the range of 100 to 200 (or more).

To sum up, days with AQI scores below 100 correspond to relatively low exposures to dust particles, but above 100 the exposure is increasing significantly. I will address the question of what the numbers mean in terms of our health in a later post, but as a rule of thumb I will consider a bicycle ride or jog outdoors on days with an index below 100, but when the index is above 150 I will be postponing that exercise for another day.

For more information on this scale and how it is calculated consult the AirNow website:

http://www.airnow.gov/index.cfm?action=aqibasics.aqi