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Someone I know recently bought a set of small stylish and expensive cloth bags (roughly paperback book sized) filled with activated bamboo charcoal, that you place in a room and it purportedly purifies the air of the room and eliminates odors. Similar claims have been made for open boxes of baking soda in a refrigerator.

My initial reaction is skepticism, mostly from a possibly-wrong intuition that the physics don't make sense; I also don't see any studies of the effectiveness of either product, and web searches just reveal a lot of SEO affiliate pages, blog marketing and the like. And this seems the type of situation that would be rife with confirmation bias ("I definitely think I smell a difference").

So down to the physics of it - in, say, a 4 x 4 x 3 meter (48 m^3) room, with a small bag of charcoal (say 12 x 12 x 6 cm, or 864 cm^3), how long would it take for say 95% of the air to pass through the bag due to gas diffusion, typical air convection, etc.? Or looked at another way, how much air volume would the bag "process" per hour? The interior is little charcoal pieces with an apparently high surface area. If a stinky carpet covering the floor of the room were off-gassing a given volume of gas per minute, could the bag keep up?

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  • $\begingroup$ I am not a physicist, but I would be very surprised, if it turned out that a large percentage of the air would go through the bag at all without directing its flow with something like a fan. On the other hand, when you put eg, a bunch of flowers in the room, the odour does permeate the room, so there is quite some movement. $\endgroup$
    – Gnudiff
    Jun 14, 2019 at 4:55
  • $\begingroup$ I would imagine this wildly depends on air composition, altitude, temperature, how well room is sealed, physical/checmical properties of the odor particles... But you can do much better by testing. A box with a rug, a box with charcoal, a box with a similar rug and charcoal. Test the scent reduction in the box with both, and test the weight difference between the charcoal bags. Easy peasy. $\endgroup$
    – Tamir Daniely
    Jun 22, 2019 at 12:25
  • $\begingroup$ I was hoping it'd be simpler to just do ballpark calculations than setting up a whole experiment in my house that relies on the subjectivity of the nose etc. E.g. if charcoal magically absorbs every bad smell that passes through the bag, is it plausible that enough of the smelly-gasses would diffuse or convect through the bag & room? $\endgroup$
    – QuadrupleA
    Jun 23, 2019 at 16:12
  • $\begingroup$ FWIW: There's a limit to how much "stuff" the charcoal would be able to adsorb, and the rate at which it approached that limit would depend on how much "stuff" was in the air. $\endgroup$
    – Solomon Slow
    May 22, 2020 at 18:13
  • $\begingroup$ Typical air velocities in rooms are of order 0.1 m/s based on the link below. I guess you could use that and multiply by the cross sectional area of your filter to get volume of air/second. designingbuildings.co.uk/wiki/Indoor_air_velocity $\endgroup$
    – KF Gauss
    May 22, 2020 at 20:18

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This answer as far my knowledge goes , is from surface chemistry . This depends on the gases present in the room and the surface covered . The surface phenomena suggests that gases which can easily be liquefied can easily be adsorbed on its surface . Now try applying the equation ,

ΔG=H-TΔS

This suggests that adsorption of gas on black charcoal bag ( which in your case is a dirty smelling gas from the carpet) is an exothermic process . Applying Le-Chatelier's principle for an exothermic process you would require quite a constant cool temperature . Also ΔH of adsorption is negative and when a gas is adsorbed on adsorbent's surface it would cause to reduce its entropy ( ΔS) and take a negative sign .Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy .This would give the (TΔS) a positive sign which would reduce the negative(ΔH) . The gas would continue to adsorb on bag until all of its charcoal surface is occupied and until (ΔG) is zero . The process would stay spontaneous until (ΔG) is negative .

If you want to know how much mass of gas can be adsorbed on charcoal , it can be calculated from Freundlich's Adsorption Isotherm ( constant temperature ) .

This is all if you experiment it in a lab on a small sample . For a large room this may or may not be true . I would say this might work a 50-50.You need a very large amount of charcoal and more bags with all thermodynamic conditions satisfying the phenomenon .

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  • $\begingroup$ Thanks for the reply. This seems very broad, some possibly-related laws / theorems, I guess I'm looking for more specific quantified / empirical answer. Also I'm having trouble understanding parts of it, the langauge / grammar is a bit confusing. $\endgroup$
    – QuadrupleA
    Jun 23, 2019 at 16:07

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