This detailed explanation has been provided by a respected New Zealand academic authority on formaldehyde.
Formaldehyde (HCHO or CH2O) is a natural component of the environment and of the human body in trace amounts. It is required by our bodies to produce DNA, but it is not present at significant levels. Instead, folic acid carries formaldehyde round our body to create DNA, which is why pregnant women need larger amounts of folic acid, to create the additional DNA needed for the baby. This naturally occurring formaldehyde is rapidly metabolised through a dedicated metabolic pathway.
It also occurs naturally in some foods, such as cheeses and grains, and is added to kill pathogens.
When introduced to other substances (eg into clothing or into a body) the two bonds to oxygen are exchanged with other molecules, restricting their ability to move. This means that, for example, in clothing, the introduction of formaldehyde makes the threads hold together and hold their shape. For this reason, it is popular in wrinkle-free garments and also in permanent press. When added to the human body, it holds molecules together and prevents the entry of bacteria, which would begin to break the tissues down. When the body is buried, the formaldehyde molecules are gradually released, eventually breaking down and bacteria gradually take hold, decomposition of the body will occur over an indefinitely long period but eventually is likely to leave little or no trace of formaldehyde in the soil. Formaldehyde is harmful when present at significant levels, for example if not washed away from surfaces, as it is then in free form (ie not attached to other molecules) and can get into the air (or soil). This free form of formaldehyde is a significant chemical hazard.
Formaldehyde is also made industrially from methanol in a process discovered in 1868 and is widely used in making resins, particle board and plywood (in the wood adhesives), fertilisers, cosmetics, disinfectants, permanent press fabric treatments, tissue preservative for science laboratories and embalming fluids. At very high concentrated levels, formaldehyde fluid has a strong odour, is highly irritating and is a carcinogen. Inhaled formaldehyde can cause irritations in the upper respiratory tract. Skin contact results in severe irritation and burns.
There are strict OSH guidelines for people working with formaldehyde, restricting length of exposure and safety measures necessary. Formaldehyde has now been banned in Europe for use as embalming fluid because it is a Class 1 carcinogen.
Pure formaldehyde is rarely used. It is most often used (including for embalming) as a 37% solution in water, which is known as formalin. This solution contains a stabiliser, typically methanol (as 12%), to prevent polymerisation.
It biodegrades readily in air, water and soil under both aerobic and anaerobic conditions.
It is destroyed by incineration when burnt in a cremator and, in modern cremators, there should be few or no emissions.