[gumby]

The cigarette is made of (chemically treated) plant matter and paper, but the filter is not. Its main purpose is to turn brown when heated so the smoker thinks the filter has trapped something bad for them. A "marketing function" as the article says.

[J_cst]

If cigarette smoke is blown through a paper tissue it will get stained (dark brown spot)[0]. I don't think paper tissue is treated in order to become brown with cigarette smoke. Perhaps it absorbs something? On the other side, obviously, the cigarette filter (butt) does not seem effective in protecting the lungs from cancer or other smoking generated doseases. From that point of view it has only a "marketing" function.

[0]source: personal experience

[gumby]

"One chemist discovered that if you adjust the pH in cellulose-acetate filters, you can get them to change color during the smoking process, making it look like some really bad stuff is being screened out."

https://www.washingtoncitypaper.com/columns/straight-dope/ar...

Or you might prefer this: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4688990/

[J_cst]

Thank you for your references. The second one has little to do with the subject though. The first one confirms that "This did, indeed, block a little tar and toxic gas, but smokers, ever resourceful, responded by changing their behavior—smoking more, taking deeper puffs, etc—thereby making the practical effect of the cellulose-acetate filter approximately nil". This by the way, is what happens in the napkin "experiment" I was referring to. By the way, I just learnt that what I was calling a "filter", it is not a filter, but a mixer.[0]

TL;DR: It's a device whose purpose is to mix the smoke of the cigarette with air which is intaken via the micro holes found on the body of the mixer (a.k.a. "filter). The different air/smoke mix rates give the smoker a different perception of the smoothness / lightness of the cigarette. As a side note, as your first reference mentions, this "filter novelty" may have made things worst as the smoker will make deeper breaths and keep the smoke longer into the lungs when inhaling a more diluted smoke puff.

[0] https://www.theatlantic.com/health/archive/2017/07/cigarette...

[gumby]

The second ref was as support that the "filter"s are also bad for you in their own right but provide a "benefit halo" that helps support smoking, just as the tobacco companies intended.

[J_cst]

Thank you