Formaldehyde is one of the best known of the volatile organic compounds (VOCs), a group of carbon-based chemicals which readily evaporate into the air. Most living organisms emit tiny amounts of formaldehyde, and the levels in the environment from natural sources is around 0.03 parts per million (ppm). Formaldehyde is also found in many everyday products and may contribute to your indoor air pollution by a slow leakage process called outgassing. Formaldehyde is a colourless, flammable gas with a characteristic pungent odour.

There are many products used in the home which could be slowly leaking formaldehyde into the air. They include:

• The glue used in board-based wood (plywood, particleboard, fibreboard including medium density fibreboard) furniture (especially flat-pack furniture), flooring, and wall panelling
• Foam backing on carpets
• Paint preservatives
• Fabric treatment in 'easy care', 'crease-resistant' or 'non-iron' polyester or in polycotton textile products
• Urea-formaldehyde insulation (widely used in the 1970s but banned in many countries because of concern about its potential carcinogenic effects)
• Cigarette smoke and incompletely vented fuel-burning appliances (gas stoves, kerosene heaters, wood-burning stoves)

Formaldehyde is also used as an aqueous solution known as formalin, as a fungicide, germicide and disinfectant in certain workplaces (e.g. mortuaries, medical laboratories). There are also hundreds of industrial processes which use formaldehyde, including the manufacture of paints, plastics, paper, textiles, carpets, woods and furniture, glues and resins.

Medium density fibreboard (MDF) is a widely used and versatile low-cost alternative to solid wood. It consists of wood fibres bonded together with a synthetic resin alternative, commonly urea-formaldehyde (although non-formaldehyde containing resins, such as phenol resins, may also be used). Machining MDF produces dust particles that may be coated with formaldehyde, which may be inhaled. Therefore working with MDF may potentially pose a health hazard.

Formaldehyde can outgas from household products for several months, the amount decreasing with time.

Exposure to formaldehyde comes through inhaling the vapour or exposing the skin to formalin. Formaldehyde can produce symptoms at levels higher than 0.1 parts per million (ppm) and some people are very sensitive to it. Exposure can cause:

- Sneezing, coughing 
- Eye, nose and throat irritation 
- Longer-term exposure causes impaired lung function, skin problems including eczema and dermatitis

A review of the evidence has also suggested that formaldehyde exposure may cause childhood asthma although studies taken on their own are conflicting.

The International Agency for Research on Cancer cites formaldehyde as a human carcinogen and studies suggest it is a cause of throat and nasal cancer and leukaemia. Formaldehyde is also classed as a carcinogen in the United Kingdom and in the European Union.

People with asthma, rhinitis and multiple chemical sensitivity need to avoid formaldehyde because of its irritant properties. Those with existing skin complaints and the elderly, the very young, and pregnant women are also more at risk of the harmful effects of formaldehyde.

In the UK, the Control of Substances Hazardous to Human Health legislation (COSHH) sets the current workplace exposure limit (WEL) for formaldehyde at two parts per million (2ppm), time-weighted average over eight hours. The short-term limit (averaged over ten minutes) is also 2ppm. The HSE's own research shows that eye irritation can occur by exposures of 0.01 ppm, which is 200 times less than the WEL. The WEL for formaldehyde is currently under review and is therefore subject to possible change.

A worker's exposure to formaldehyde should not exceed this WEL and every effort should be made to reduce the exposure as low as is reasonably practicable, and in any case below the WEL.

In the USA, the Occupational Health and Safety Administration has set a lower level of exposure to formaldehyde, at 0.75 ppm. 

There is European legislation controlling the emission of formaldehyde from MDF manufactured in the European Union, although MDF is not banned anywhere in the world at the current time. The use of formaldehyde in preserving tissue and as a biocide is also restricted in the European Union.

These precautions would apply to people doing DIY or contractors involved in the renovation or refurbishment of buildings. The safest approach is to avoid products containing formaldehyde altogether. For instance:

• Consider solid wood rather than board-based wood or look for 'low gas' or 'zero gas' products
• When using MDF, look for 'low emission' or 'no added formaldehyde' brands
• When clearing up dust from machining MDF, the Health and Safety Executive recommends fitting a hose to the local exhaust ventilation system or using a vacuum fitted with a HEPA filter. Do not use a brush to clear up this kind of dust
• Metal or glass can be used instead of board-based wood for shelving
• If you have to work with board-based wood, leave it outside, covered over, for a week to get rid of the worst of the formaldehyde. Use a face mask when you are carrying out the work. Then use paint, or varnish, to seal surfaces of board-based wood products to prevent the formaldehyde from outgassing
• When choosing carpet, go for one with a hessian or felt, rather than foam, backing