The International Agency for Research on Cancer (IARC) released an extensive report on formaldehyde, considered a Group 1 carcinogen.  This means that there is enough evidence to conclude that formaldehyde can cause cancer in humans.

Formaldehyde is found as a natural product in most living systems and in the environment. It occurs naturally in fruits and some foods, and it is formed in mammals, including humans. In addition to these natural sources, common non-occupational sources of exposure to formaldehyde include combustion processes, e.g. through emissions from motor vehicles, power plants, incinerators, refineries, wood stoves, and kerosene heaters. Formaldehyde may be released from particle boards and similar building materials, carpets, paints and varnishes, during cooking of some foods, and during its use as a disinfectant. It is also present in tobacco smoke.