Why do I get brown stains from smoking?

Tar, fungus, other. Assume you mean stains on fingers. It's from tar, a sticky toxic chemical in tobacco, or fungus of skin and nails, or a sign of lung disease caused yellow nail syndrome. Stains on teeth, clothes, furniture is from tar & chemicals in tobacco. When U.S. Stopped smoking on planes, airlines saved big $$$ in cleaning costs http://betobaccofree.Hhs.Gov/about-tobacco/smoked-tobacco-products/index.Html.