Additional information about formaldehyde:
1. Off-gassing. Research has shown that it can take 10 years or longer to off-gas formaldehyde from building products, cabinets, etc. One website says it can sometimes take 20 years for off-gassing, as follows:
"Some of the more solid materials such as modern engineered floor joists and particle board products used in sub-flooring, countertops and furniture, can take as long as 20 years to fully off-gas." (Source: http://indoorair.net/id52.htm
2. Tobacco contains formaldehyde. If you are a smoker, this can add to the levels of formaldehyde in your home.
3. Air filters don't help. Air filters generally don’t help lower levels of formaldehyde in your home. Overheating your home to “bake” out the formaldehyde also doesn’t work and may even raise formaldehyde levels.
4. Formaldehyde levels increase with increases in temperature and humidity.
From the Consumer Products Safety Commission (CPSC):
Formaldehyde levels in indoor air can vary depending on temperature, humidity, and air exchange rate within the indoor space. In addition, several studies have shown that, in the presence of ozone, formaldehyde levels increase; therefore, the outdoor and indoor ozone levels are also relevant. Formaldehyde levels in a residence may change with the season, day-to-day, and day-to-night. Levels may be high on a hot and humid day and low on a cool, dry day. Understanding these factors is important when one is considering measuring formaldehyde levels.
The CPSC has a good paper
on this topic including a helpful table of information on page 3 that discusses outdoor and indoor air exchange rates and ozone.
5. Sources of formaldehyde:
Formaldehyde is a component of automobile exhaust and smokes from cigarettes, other tobacco products, gas cookers, wood stoves, and open fireplaces. Formaldehyde is also found in many everyday products such as paint, antiseptics, medicines, cosmetics, dish‐washing liquids, fabric softeners, shoe‐care agents, carpet cleaners, glues and adhesives, lacquers, paper, plastics, some types of wood products, plywood, particle board, unvented gas or kerosene heaters, latex paint, fingernail hardener, fingernail polish, fiberglass products, new carpets, decorative laminates, permanent press fabrics, paper products, grocery bags, paper towels, new clothing, bedding, etc.