I really don't know of any other way to test. The dust filters pretty well keep any particulate material, pollens, etc out. As for charcoal, it is an adsorbent, not absorbent. It chemically binds to molecules, such as in odors, fumes, non-odorous toxins, gases. If you use it all the time, then you best change it. MB recommends every 60k, but more often I would suspect if you use in constantly.

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hlg