Question: Is there a correlation between humidity and particulates in the air?

sarasage is asking a question about air-quality
Follow this topic

by sarasage | August 10, 2021 17:28 | #27451

Wondering about indoor AQ, specifically.


Hi Sara!

Yes, in two broad ways that I've seen -- both in being able to distinguish particulates /from/ humidity, and potentially in humidity affecting how much particulate is actually in the air.

For example breathing into a Plantower/Simple Air Sensor/Purple Air will spike the detected particulates. I believe this is because the droplets of moisture are big enough to be detected and assumed to be particulates (literally they are, in a sense, just particles of water). Using a room mister (humidifier) can cause the same.

Humid days are sometimes strongly correlated with high particulate readings from this type of device (laser-based particle counters). But it's hard to say if that's because humidity is increasing the actual particulate count somehow, or if it's just setting off the sensors with tiny water droplets.

One theory could be that humidity may form around particulates (which is almost a 3rd scenario - particulate in the air causing mist/fog??) but, if that increases their weight (they get wet), maybe they'd be more likely to fall out of the air sooner.

I think there are papers about this and some devices dry out the air with a heater (or dry out the samples) before measurement.

There's some discussion of this by @Jdahlen @jefffalk @DavidMack here:

Also some including input from @crispinpierce here:

Interesting notes on humidity being less of an issue than fog, from @lincomatic and notes from @guolivar:

Very in-depth discussion of humidity here too!

Hope this helps!!

Is this a question? Click here to post it to the Questions page.

Reply to this comment...

@Sara, I believe your question specifically refers to indoor air quality, specifically particulates, and humidity and because @Warren 's comments referenced a long ago comment of mine I will comment here. Unless your "indoors" is very unusual, eg a kitchen, laundry, or bathroom, a large number of animal bodies in a small room, or in a very humid outdoor location, the humidity will not vary as much as outdoor humidity. It will seldom rain inside your house or have moisture running down the walls. On the other hand for some people it may be so dry (for them) that they use a humidifier. So, in general, indoor humidity will not vary a whole lot. Will the humidity, whatever it is, effect the quantity of particulate matter? I would ask, where is the particulate matter supposed to be coming from? and how would humidity add or subtract from this? There could be high humidity and low particulate concentration (a bathroom after a shower) or low humidity and high particulate concentration (a very warm northern home in winter with burning food on the stove). I believe @Warren 's comments refer to measuring particulate concentration and how humidity might effect a monitoring instrument. That's an important but different question. In short, I would say humidity at whatever level neither creates nor destroys particulate matter. And as to monitoring particulate matter, most indoor situations will not have humidity varying enough to make a difference of meaningful size in the measured concentrations.

Is this a question? Click here to post it to the Questions page.

Thanks for this really great clarification/distinction. Another question, for outdoors in this case, but related more to the first of the above distinct questions, is:

Does a humid day (outdoors) influence how much particulate matter is actually in the air?

That is, could moisture in the air cause particulates to fall out of the air? Or stay in the air longer? Or have any other effects on particulate prevalence, even if the amount of particulate matter being /put into the air/ is not changing vs. a dry day?

Is this a question? Click here to post it to the Questions page.

Reply to this comment...

Log in to comment