Formaldehyde is a chemical used for certain treatments of wood. Usually, after treatment most of the formaldehyde is removed and the wood is safe to use. However, occasionally there are complications, such as allergic reactions or when the wood still contains an unusually high quantity of the chemical after processing. One such example is a statement put out by Weyerhaeuser where certain floor joists produced after December 1st, 2016 which were used in about 2,200 new homes contained dangerously high quantities of formaldehyde. More information on this can be found in this article. High levels of formaldehyde or even low levels if a person is sensitive can be dangerous to the health of those who are exposed. If you have had construction recently, it can be a good idea to have the area tested for high levels of lingering chemicals to ensure to health hazards are present.