You're thinking third-hand smoke, which is the residue of the smoke that remains on furniture/walls and such in enclosed places.

The burning end of the cigarette? That smoke trail is SHS, as is the smoke that the smoker exhales. The exhaled smoke isn't as harmful as the burning cigarette.

People are exposed to larger amounts of SHS in enclosed places that traps the SHS inside. You don't have to smoke to create SHS, you merely have to light a cigarette.

How much and how concentrated SHS needs to be to be considered harmful is another story. Are designated smoking areas enough? How much distance does there need to be to let the smoke dissipate into smaller amounts and be less harmful?

I don't know the answers to the above questions - and I'm not going to trust any study funded by a tobacco company.