Today I Learned (TIL)

The other thing in that article is that the filter doesn't really filter anything of note, meaning the litter is actually just the result of a con make smokers think it was less damaging to their health..

Colour change
The tobacco industry determined that the illusion of filtration was more important than filtration itself. The pH of the cellulose acetate used is modified, so that its colour becomes darker when exposed to smoke. The industry wanted filters to be seen as effective for marketing reasons, despite not making cigarettes any less unhealthy.