Formaldehyde (HCHO) is a colorless, pungent-smelling gas classified as a Volatile Organic Compound (VOC). This chemical is commonly used in manufacturing and industrial processes, making its presence in indoor air nearly universal. Because it easily becomes a gas at room temperature, it is a persistent indoor air pollutant that affects health and air quality. Understanding the sources of HCHO and safe levels is essential, especially since indoor concentrations are often much higher than outdoor air levels.
Common Sources of Indoor Formaldehyde
The primary source of indoor formaldehyde is a process called off-gassing, where products slowly release the chemical into the surrounding air. The most significant contributors are pressed wood products, which utilize adhesives made with urea-formaldehyde (UF) resins. These materials include particleboard, hardwood plywood paneling, and medium-density fiberboard (MDF). MDF typically has the highest emission rate due to its greater resin-to-wood ratio.
Formaldehyde is also present in numerous household goods, including permanent-press fabrics, certain glues, paints, and varnishes. Insulation materials, such as urea-formaldehyde foam insulation (UFFI), can also be a source. The rate of off-gassing tends to decrease over time, but the release can continue for weeks, months, or even years. This means levels can be higher in newly built or recently renovated homes.
Combustion is another major source of indoor formaldehyde, contributing to the overall concentration in the air. This includes tobacco smoke, emissions from unvented fuel-burning appliances, gas stoves, kerosene space heaters, and smoke from fireplaces or wood-burning stoves. Even the chemical reaction between ozone and VOCs from electronics can form HCHO indoors. High indoor temperatures and elevated humidity levels accelerate the chemical reaction that causes off-gassing from materials, often leading to higher HCHO concentrations during warmer months.
Health Impacts of Elevated HCHO
Exposure to elevated levels of formaldehyde can cause both immediate and long-term health problems, with sensitivity varying widely among individuals. Acutely, the chemical is a known irritant that affects the mucous membranes of the eyes, nose, and throat. People exposed to concentrations above 0.1 parts per million (ppm) may experience watery eyes, burning sensations, coughing, and nausea.
For individuals with pre-existing respiratory conditions, such as asthma, even moderate concentrations can trigger or worsen symptoms. In more severe cases of short-term exposure, high levels can lead to inflammation of the lower respiratory tract, resulting in shortness of breath and wheezing. Some people can become chemically sensitized to HCHO, developing severe asthma or contact dermatitis at very low concentrations.
Formaldehyde poses a serious long-term health risk and is classified as a known human carcinogen by the International Agency for Research on Cancer (IARC). This classification is based on sufficient evidence linking chronic exposure to the development of nasopharyngeal cancer. The U.S. Environmental Protection Agency (EPA) also classifies it as carcinogenic to humans by the inhalation route, linking it to nasopharyngeal cancer, nasal sinus cancer, and myeloid leukemia. The risk of these chronic effects is tied to continuous, long-term exposure, even at levels that may not cause immediate irritation.
Regulatory Limits and Safe Exposure Levels
Defining a safe level for formaldehyde involves balancing the prevention of immediate irritation with the reduction of long-term cancer risk. Since there are no universally enforceable indoor air quality regulations for homes in the U.S., various agencies provide guidelines and reference concentrations. These levels are typically expressed in parts per million (ppm) or micrograms per cubic meter (\(\mu\)g/m\(^3\)).
Occupational agencies, such as the Occupational Safety and Health Administration (OSHA), set standards for workers. These include a Permissible Exposure Limit (PEL) of 0.75 ppm averaged over an eight-hour workday and a Short-Term Exposure Limit (STEL) of 2 ppm for a 15-minute period. These occupational limits are designed to prevent immediate irritation in a workplace setting and are generally much higher than recommended levels for residential spaces.
For general indoor air quality, the World Health Organization (WHO) recommends a maximum level of 0.08 ppm (100 ppb) for a 30-minute exposure to protect the public from sensory irritation. The U.S. EPA has established a long-term reference concentration of 7 \(\mu\)g/m\(^3\) (approximately 0.006 ppm) for continuous inhalation exposure, intended to protect sensitive groups from non-cancer effects over a lifetime. Typical indoor concentrations in homes without significant sources generally range from 10 to 40 \(\mu\)g/m\(^3\) (approximately 0.008 to 0.032 ppm), though levels in newly constructed homes can sometimes exceed 0.3 ppm.
Practical Steps for Reducing HCHO
The most effective strategy for reducing indoor formaldehyde levels is to control the source of the emissions. When purchasing new furniture or building materials, look for products certified as compliant with the Toxic Substances Control Act (TSCA) Title VI. Also look for those labeled as Ultra-Low-Emitting Formaldehyde (ULEF) or No Added Formaldehyde (NAF). Choosing solid wood products over pressed wood, which uses formaldehyde-based resins, can significantly limit new emissions.
Increasing ventilation is a simple way to dilute and remove airborne HCHO. Regularly opening windows for cross-ventilation flushes out accumulated pollutants. Using exhaust fans in kitchens and bathrooms, especially when cooking or after a renovation project, helps move indoor air outside and introduce fresh air. Managing the indoor environment’s temperature and humidity also plays a large role in mitigation.
Since higher heat and moisture accelerate the off-gassing process, maintaining a relative humidity level between 30% and 50% can help reduce emissions. Air purification can supplement source control and ventilation, but it requires the right technology to be effective against gaseous pollutants. Standard HEPA filters only capture particles and are not designed to remove gases like HCHO. For VOCs, look for air purifiers that contain a substantial amount of activated carbon or other adsorbent materials, as these are designed to chemically absorb the formaldehyde gas from the air.