Formaldehyde is a colorless convergent gas, that causes cancer.
It is a strong-smelling chemical used in building materials, in various household products, glues, and adhesives.
On an industrial scale, it is used as a fungicide, bactericide, and disinfectant.
Formaldehyde occurs naturally in the environment. As part of normal metabolic processes, it's produced in small amounts by most living organisms.
1997 report by the U.S. CPSC says: "formaldehyde is normally present in both indoor and outdoor air at low levels, usually less than 0.03 parts of formaldehyde per million parts of air (ppm)".