(The following is a guest piece from Chris Buckley, owner of Torana Clean Air Center in Beijing)
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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