Second-hand smoke is a special health concern, especially with children. It also harbors dust mites, and animal dander. Paints and solvents are a source of many volatile chemicals which vaporize easily. Stored gasoline, kerosene and other fuels release volatile chemicals into household air. Exhaust fumes enter the home from the garage of nearby traffic.
Carbon monoxide is a primary danger.
It has a cumulative effect in the human system. Asbestos insulation on pipe cladding can release carcinogenic particles into the air, especially during house repairs. Asbestos fibers can cause emphysema and lung cancer. Radon gas is a natural emission where uranium exists underground. Radon carries radioactive particles that can cause lung cancer.