In my experience, there is no such thing as "sealed optics". All it takes is a tiny crack somewhere, anywhere, for smoke and dust to get in. The imaging chamber of a DLP is supposed to be sealed, yet with the right airflow, say, an overenthusiastic user trying to blow dust out of the lamp chamber with a can of air, dust can be forced in. I've seen it happen time and again.
When you consider that projectors have multiple fans, then ANY opening, however slight, is a pathway for dust and smoke. The case seams, the space around the lens and even the input ports. I had a projector on my bench one time that came out of a smoking environment and had not seen ANY cleaning or maintenance. The accumulation of dust on the OUTSIDE of the projector was so bad that the USB port was completely covered.
Bottom line, if you smoke, it WILL accumulate on the optics. No question. Been there.
Added...banning smoking in that room will help, BUT, smoking anywhere in the house will be picked up through the return air vents of the HVAC and be redistributed throughout the house. It won't be as bad, but it will still be there, just spread out throughout the house.
Unless, of course, you have HEPA filtration on your HVAC.