There are different models available, perhaps most famously the TarGard used by Hunter S. Thompson. He said that, after seeing how much tar it removed, he could never smoke without it. He also found the experience too harsh and unpalatable in any case.
I can see how the disposable models might not be economically efficient, but the permanent models are indeed permanent. They only require cleaning.
And yet, one never sees this sort of thing in use. How come?
