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:
http://www.airnow.gov/index.cfm?action=smog.page1#1

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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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

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(The following is a guest piece from Chris Buckley, owner of

Recently, I’ve been doing measurements inside and outside of homes in Beijing, looking at fine particle air pollution, of the same size range (less than 2.5 microns) that is monitored by the US Embassy. Most homes have indoor air pollution levels that are similar to outside levels, as Richard has mentioned in a previous post on Beijing air pollution, but in a few cases I have seen levels of fine dust that are much, much higher than outside. In one case recently the dust in the finest range that I am able to measure 0.3 micron to 0.5 microns) was 3 times higher than the outside air, and dust in the 0.5micron to 2.5 micron range was an astonishing 28 times higher than the outdoor level.

So what is going on in these homes?

In each case the common factor was the presence of ultrasonic humidifiers filled with Beijing tap water. These humidifiers work by making a fine mist with an ultrasonic vibrating plate in a water tank. They are popular and cheap to buy in Beijing, and at this time of year department stores are full of these machines, including whimsical designs in the shape of cartoon characters. The side effect that all ultrasonic machines share is that as this mist evaporates it leaves a tiny nucleus of hard water salts suspended in the air. It’s common in ultrasonic humidifers sold overseas for them to include a cartridge that removes the hard water salts, but models sold in Beijing usually lack this feature.

Most people who own these ultrasonic machines realize that if they put tap water into these machines they will get a deposit of white dust around their humidifier. What is less widely appreciated is that the machines not only make large dust that settles out rapidly but also produce dust at very small sizes, even in the 0.3micron to 0.5micron range. These particles stay suspended in the air a relatively long time and some of them will end up in our lungs.

To put this in perspective, mineral salts from humidifiers are not as much of a health hazard as the carbon soot from diesel engines and coal burning that are a well-researched hazard of city life. Opinions differ on the level of risk that humidifer dust poses: the problem for researchers is a lack of data, and the fact that the physical properties of materials change when they are converted to very fine dusts. The US-based Consumer Product Safety Commission sums it up on their website: “The health effects from inhaling this humidifier dust are not clear” I think it is safe however to say that humidifier dust places an avoidable extra burden on the lungs, especially in Beijing where we have so much air pollution to cope with anyway.

In talking with some ultrasonic humidifier users I have found that some believe that adding vinegar to the water takes care of the problem. This is based on the logic that vinegar can be used to clean limescale from a kettle. Unfortunately, in the case of ultrasonic humidifiers it does not help, since what goes into the humidifer must come out, so this just adds to the burden of salts in the dust, as well as evaporating vinegar into the air.

So what to do? For ultrasonic humidifier owners there are two safe options: use distilled or de-ionized water in the machine, or get rid of it and get a humidifier that works by evaporation rather than ultrasonic misting. De-ionized water contains no salts and does not make dust particles as the mist evaporates. This water is expensive however, so the best long-term solution for most people will be to use an evaporative humidifier, since these types can take tap water without creating dust.

In either case, you should also clean your humidifier each time you refill the tank, to prevent the build up of molds and bacteria, and periodically de-scale the machine if there is limescale build-up.

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