Ultrasonic Humidifiers in Beijing – An Unnecessary Risk?

(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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7 thoughts on “Ultrasonic Humidifiers in Beijing – An Unnecessary Risk?”

  1. Chris this is an excellent post. I have been saying the same thing for years, ever since I learned what I did researching buying a humidifier for our home. Overall, the "basic" type back in the States, the hot steam type, seemed best to me, given the water quality and convenience factors. It spits out H2O and that is all, with all the residue left behind (yes, there is a LOT), and also kills pathogens in the boiling process.

    Amazingly this type is not sold commercially in China, or if it is, I have not found it. (There was one kind, Air-O Swiss, which I heard did fit the bill, but it came years after my initial research, was pricey, and, I think, unavailable currently if the recent BOCA posts have been correct…) Anyway, I decided to order three used vapourizers off of Ebay Australia and have them shipped here. Two out of the three units made it here. But after lo these many years, and a couple of careless essential oil accidents, they are dead and I find myself in need of new units. Any pointers would be helpful!!

  2. excellent post. i've been having incredible problems with allergies during the winter, which is very odd. i will be getting rid of my ultrasonic humidifier today, hopefully this will help.

  3. At home I've been using 4 German Venta brand humidifiers, bought them about 5 years ago and I have forgotten where, but I think they are still available in Beijing. If I find out where I will post this info. This kind works by passive evaporation of air passing over a drum immersed in water. The steam kind (like boiling a kettle) that Liora mentions are also good, but as she says not easy to get here. Tap water is fine in both these kinds since the salts stay in the reservoir.

    I don't know about medical grade De-ionized water availability, but that sounds expensive! Water delivered in plastic drums is ok I think if it is distilled since this gets rid of the salts, but watch out for mineralized waters that have had the salts put back in. With the Nestle brand water this is the default option these days, but you can still get the pure distilled variety if you ask for it. There may be good reasons for drinking the mineralized versions of course, but they are not good for ultrasonic humidifiers. The Watsons water I have at home still seems to be the pure distilled variety.

    Another option is to install a Reverse Osmosis (ROI) water purifier under your sink since this kind gets rid of water hardness. There are several brands available in the home appliance stores. Ion-exchange softeners don't help with the salt content because they just swap one kind of salt (calcium water hardness) for another (common salt). Regular water filters, water treatment jugs and so on also do not remove hardness.

  4. Great post!
    The way I have solved this issues, at least in the winter, is by putting wet towels on the heating radiators. They stay wet only for a few hours, which means that all this water evaporates very efficiently in the given time. Much better than putting vessels of water on the radiators, that take days for the same volume of water to evaporate (since the surface where the evaporation takes place is very limited).

    For the big radiators, from floor to ceiling, I use folded linen sheets, that cover them entirely. I wonder why garments like that do not get sold commercially (in the right form factor) already.


  5. The PM2.5 readings soar when I turn on my ultrasonic humidifer. I used RO purified water. So is it still safe? Without ultrasonic humidifier PM2.5 is under 10, with u/sonic humidifier on PM2.5 is 25-30.

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