I have a very common air purifier: HEPA filter with a heavy duty helping of activated carbon and zeolite. My particular filter is designed for 5 years of operation under typical home conditions. In reality, I expect a few years, if not 5. I found this to be the case in the past.

I put an air purifier in the bedroom, which I keep colder than the rest of the apartment unit. After using a new filter for about 6 months, I noticed a sign that the filter was done: A sour smell. Very odd, since another purifier was working in the living room and kitchen for 1.5 years with no such sign.

Strange also that the smell was greater when the thermostat was turned down for the night. I would have expected higher temperature equals more energetic gases, and lesser binding with the filter material. That points to less effective filtration with higher temperature, which was the opposite of what I observed.

The only other explanation I could fathom was relative humidity (RH). Colder air has less vapour capacity, so RH goes up for the same amount of water in the air. Maybe that was the cause (somehow). I got some evidence that this was the case. When I moved the dehumidifier into the bedroom, I could hardly notice the smell from the air purifier anymore.

The dehumidifier was dialed to 35%, but I actually had to make it operate continuously (unregulated) in order for it to run. Without continuous operation, it would only regulate to down to 35% RH (plus/minus 3%). With continuous operation, the readout showed 30%. For all I know, this could be the lower limit of the sensor, but it does show that the RH is lower than under regulated operation.

I know that reliable RH measuring is tough, so I view the dehumidifier readout as a rough indicator only. But it does seem to show that higher RH yields smellier filter output, possibly indicating that captured VOCs are being released.

I'm not a chemist, but was wondering whether those with expertise in the field can comment on whether retention of trapped "volatile organic compounds" (VOCs) actually lessens with higher RH? Or perhaps there is a 3rd causal factor, leading to correlatino of RH with less filter retention? Or maybe it's not even that -- maybe the colder temperature or higher RH simply makes one's sense of smell more sensitive.

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    $\begingroup$ For those who might have been wondering: en.wikipedia.org/wiki/Volatile_organic_compound $\endgroup$ Jan 16, 2022 at 15:52
  • $\begingroup$ @Martin: Thanks. I added that to my question. I have had a year to observe the problem. It seems that if I run the machine 24/7, the smell is much abated. I know that it's not just me acclimatizing to the smell because I test the smell after coming into the dwelling, before I become acclimatized. However, I can't run the machine 24/7 in the summer because I keep windows open. Running the machine would be like trying to clean all of the outside air, and I live close to a major traffic intersection and a diesel bus transit hub. $\endgroup$ Jan 16, 2022 at 18:29
  • $\begingroup$ I should add that it doesn't seem to make sense that running the unit 24/7 eliminates the smell most of the time and makes it barely noticeable the rest of the time. One explanation that was provided elsewhere was that the filter was contaminated with bacteria, which feeds off the impurities caught by the filter. I'm not necessarily dismissing this, but I would expect this to be worsened by that 24/7 operation. $\endgroup$ Jan 18, 2022 at 4:04


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