My car has 67k on it and I inspected the charcoals after changing out the regular filters. Mine looked new and not discolored. My thought is that people rarely turn them on unless pressed by a bad smell. Odds are they, like me, won't make the effort unless really offended. So I doubt that there is more than several hours on mine (can't prove it of course). I can say that as a physician, that there is not likely to be any bacteria in the filter at all. Mold spors should be mostly in the regularefilters since all goes through there first. Charcoal without moisture, does not promote bacterial growth at all. I therefore do not think the charcoal refills are a good investment unless used a lot and begin to fail. But WTHDIK? Just my opinion.

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hlg