(I know little science so please help me if I'm in the wrong place or if my question needs editing. I could use help with tagging, for example.)

I use two devices that measure PM2.5:

  • Kaiterra LaserEgg 2+, an air monitor which uses laser-based light scattering technology
  • Blueair Classic 280i, an air purifier which measures PM2.5 but doesn't say how

Generally, their PM2.5 measurements are within 20% of each other and the readings go up and down together.

Both devices are in a room measuring about 40sqm.

It's winter so the windows are closed and I keep the Blueair Classic on automatic mode, which varies the fan speed depending on the level of PM2.5 it measures. It is rare for the fan speed to increase above the lowest level.

The level of PM2.5 is usually under 10μg/m3 (safe, according to the WHO), as measured by the LaserEgg and the Blueair Classic.

A few days ago we felt that the air was dry (cracked skin, etc.) and our humidity sensors reports humidity around 25%, so we turned on an ultrasonic humidifier (https://www.amazon.co.uk/Humidifier-Improves-Health-Sleep-Focus/dp/B01C5YHFWO). We use tap water in the humidifier.

Within a few minutes the LaserEgg and the Blueair Classic reported a massive increase in PM2.5, to about 400μg/m3 (very unsafe according to the WHO). The Blueair Classic changed its fan speed to maximum but it seemed to have little effect. Perhaps the output of the humidifier is greater than the cleaning capacity of the purifier.

We have reproduced this situation several times since - every time the humidifier is turned on.

What could be causing the PM2.5 increase?

Could it be harmful?

P.S. This seems relevant: https://www.researchgate.net/publication/51859262_Indoor_particle_concentrations_associated_with_use_of_tap_water_in_portable_humidifiers

  • 1
    $\begingroup$ Your ultrasonic humidifier does not put out water vapor, it puts out small water droplets (which then evaporate into vapor). The Blueair (which is not a calibrated particle size distribution counter) sees the water droplets as particles. $\endgroup$
    – Jon Custer
    Dec 3, 2018 at 13:55
  • $\begingroup$ @JonCuster Thank you. Please can you give me a source for "which is not a calibrated particle size distribution counter"? $\endgroup$
    – cja
    Dec 3, 2018 at 13:56
  • 1
    $\begingroup$ @JonCuster Out of interest, does that mean the water droplets are less than 2.5 micrometers in diameter? $\endgroup$
    – cja
    Dec 3, 2018 at 13:58
  • 1
    $\begingroup$ The Blueair is not a scientific instrument to measure particle size distributions. It has some little light scattering system to get some gross average measure of particles. Real particle size distribution measurements are done with instruments costing a large multiple of your air cleaner. $\endgroup$
    – Jon Custer
    Dec 3, 2018 at 14:00
  • $\begingroup$ I tried boiling some distilled water and my air quality monitor readings shot through the roof. Very likely it is picking up particles of water vapor. $\endgroup$ Feb 21, 2023 at 0:32

3 Answers 3


All tap water contains some dissolved solids, and "hard water" contains more, particularly calcium salts. There are also microorganisms found even in treated tap water, and some organisms can build up a biofilm inside the humidifier. Ultrasonic humidifiers are effective at putting these particulates into the air. However, as others stated, you may just be detecting fine water droplets from the humidifier before they evaporate.

To find out if it's water droplets or particulates from dissolved material, do a comparison test, using distilled, filtered, low-particulate water such as High Performance Liquid Chromatography (HPLC) grade water.

  • Clean out the humidifier thoroughly, rinsing repeatedly with a dilute acid safe for that machine (perhaps vinegar, but check the humidifier manual).
  • Rinse repeatedly with distilled water and finally with HPLC water.
  • Refill with pure HPLC water.

If this does not greatly increase the particulates above background, but tap water does, then you've found the source of the issue. If not, then you're probably detecting mostly water mist.

Caveat: Buying HPLC water is not inexpensive. If you can't easily make your own on-site, perhaps just leave the dehumidifier off.

  • $\begingroup$ But is inhaling the particulates from hard water dangerous? $\endgroup$
    – jerlich
    Jan 30, 2020 at 2:27
  • $\begingroup$ Possibly, but this site does not address personal medical questions as such. That said, there is information available, e.g. epa.gov/pm-pollution/… $\endgroup$ Jan 30, 2020 at 4:34
  • $\begingroup$ Would boiling distilled water also produce this mist you mention (that would be detected by a typical air quality monitor off the shelf?). It appears to be the case when I tried this, so now I am confused as to what is considered particulate matter by my device or simply "humidity". $\endgroup$ Feb 21, 2023 at 0:36

Folks at Awair conducted an experiment with different water in their ultrasonic humidifier https://blog.getawair.com/awair-investigates-how-your-humidifiers-water-affects-your-health

According to the blogpost, it is not the water that introduces most of the particles to the air but minerals and other things in the water.

  • $\begingroup$ Interesting that according to their test, distilled water still adds particles to the air. I suppose it still can contain impurities. $\endgroup$ Feb 20, 2023 at 19:13
  • $\begingroup$ Of course, pure water will not add particles to air from the humidifier, unless the temperature is well be low freezing. It's IOTETMCO that particles are from dissolved solids or particles suspended in the tap water. $\endgroup$ Feb 21, 2023 at 1:12

It's mostly due to microscopic water droplets as when my dehumidifier is made on it shoots up the PM2.5 reading from 6-7 to 20-21


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