Vapor intrusion is the movement of chemical vapors from contaminated soil, groundwater or sewer lines into nearby buildings. Vapors enter buildings primarily through openings, such as cracks and seams in the foundation or basement walls, gaps around utility lines, and sump pits. They can also enter homes from sewer lines due to chemicals disposed of in drains. Once inside the home or workplace, inhaled chemical vapors may pose health risks for occupants. In some cases, buildup of vapors, such as those from methane or gasoline, may cause explosive conditions.