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I have a HEPA filter air cleaner with 16 pounds of activated carbon. It's suppose to last up to years, but my experience is that after a few months (if not weeks) of operation under a light load like a bedroom, the air starts to exude a sweetish smell that is sometimes like honey and sometimes like citrus fruit. This has been explained by the vendor as possibly due to the fact that the activated carbon is derived from coconut hust, and I've verified from Wikipedia that this is typical.

I noticed repeatedly over the years, however, that as the temperature drops a few degrees (e.g., from 22 to 19 Celsius), the honey smell seems like an unpleasant sour smell. It only seems that way during downward transitions in temperature. Once the temperature is at its low, it starts to smell like fruit again.

What mechanisms in activated carbon could explain this?

2023-02-09 afternote: I wonder if the cause could be related to humidity. Small swings in temperature indoors are correlated with high swings outdoors, which correlate with relative humidity.

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The vendor is apparently handwaving to satisfy a customer with a natural reason. Activated carbon is usually prepared at 1000 C! No fruity smells or odors should survive at this temperature from any natural sources whether it is coconut or even if it is made from bones. In short fresh activated carbons do not have any odor.

Activated carbons have very large surface areas, this means they can adsorb anything, and since the filter is designed to "trap" bacteria, fungi, mold and spores mechanically, these organisms can decompose/react with other adsorbed stuff collected over a period of time (other domestic odors) to generate volatile molecules. By luck, some of them might smell nice and some don't. Indoor air is full of man-made chemicals at very very low but measurable concentrations. They are called volatile organic carbons (VOCs). Imagine countless detergent/soap odors, flame retardants, creams, shampoos, perfumes, decomposing glues (on the flooring, carpets), and countless other things. So unfortunately, nobody can pin-point the exact identity of those fragrances and mal-odors until and unless the air sample is analyzed by professional services. What you are describing is not a repeatable scientific phenomenon, it must be very specific to your indoor air and building air circulation.

If this is a concern and if bad odors persist in indoor air, seek professional assistance for indoor air quality and/or medical advice.

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  • $\begingroup$ I appreciate your informative response. It confirms what I suspected. But I was wondering, would you be aware of an absorption/desorption mechanism that might explain why the exuded smell becomes more pronounced as the temperature decreases, but this does not persist after the temperature reaches its low? Yes, smell changes, but it is also less once the temperature stabilizes. I've seen this with replacement filters over the years. Thanks. $\endgroup$ Feb 6, 2023 at 6:14
  • $\begingroup$ OK, thanks again. It's just that the smell is noticeably reduced at slight higher temperatures. I hardly notice it at above 20 Celsius, but it's quite noticeable below that, even though it's greatest during the downward transition rather than at the low end. $\endgroup$ Feb 7, 2023 at 7:07

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