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.