Things You'll Need
Duct tape or foam weather stripping
Second-hand smoke is extremely dangerous, containing more than 40 cancer-causing agents. In fact, the U.S. Environmental Protection Agency places second-hand smoke in the same danger category as asbestos and radon. Unfortunately, many nonsmokers, especially those who live in dwellings with multiple units, are subjected to second-hand smoke that seeps through vents and other openings within the home. The only way to stop smoke from entering through a vent is by blocking the vent. This is an easy task; however, it may cause some inconveniences as you will lose use of that specific vent.
Remove the vent grate, using a screwdriver.
Place a vent cover over the opening of the vent. Many are magnetic and will stick directly to the inside of the vent opening.
Seal the edges of the vent cover with duct tape or adhesive-foam weather stripping (if not using a magnetic vent cover or if the magnetic cover does not sufficiently block the smoke).
Compensate for the loss of the vent's function. If it was for heating and/or cooling, add a fan, air conditioner and/or heater to the room. If air circulation feels reduced, add a window fan.
As an alternative to purchasing a vent cover, you could remove the vent grate, cover the inside of the grate with duct tape, plastic, cardboard or any other nonporous material, screw the grate back on and seal the edges with adhesive-foam weather stripping.