Indoor air quality often receives less attention than outdoor pollution, yet the air inside homes often contains higher pollutant concentrations. Understanding the various sources of these indoor contaminants is important for a healthy living environment. Many common household activities and materials can release substances into the air we breathe daily. Identifying these sources allows for informed decisions about improving indoor air quality.
Pollutants from Combustion Activities
Combustion activities within the home release gases and fine particulate matter from incomplete fuel burning, affecting indoor air quality. Common sources include gas stoves and ovens, which can emit nitrogen dioxide (NO2), carbon monoxide (CO), and particulate matter. Studies indicate that hourly average NO2 concentrations in kitchens with gas stoves can exceed World Health Organization (WHO) guidelines, sometimes reaching over 200 µg/m³.
Unvented fuel-burning space heaters, such as those powered by kerosene or gas, also release carbon monoxide, nitrogen dioxide, and various particles. Wood-burning fireplaces and stoves, particularly if not properly vented or maintained, can introduce particulate matter, unburned hydrocarbons, and sulfur dioxide into living spaces. Tobacco smoke is another major source, producing harmful gases and fine particles. Even seemingly innocuous items like burning candles and incense sticks can release particulate matter and volatile organic compounds into the indoor atmosphere.
Biological Contaminants in the Home
Living organisms and their byproducts are common biological contaminants in household air. These microscopic entities and organic materials become airborne and can trigger various health responses.
Mold and mildew, types of fungi, thrive in damp, warm environments such as bathrooms, basements, or areas with water leaks. They reproduce by releasing spores that float through the air and settle on surfaces, growing when moisture is present. These spores, along with fragments of the mold itself, can cause allergic reactions and respiratory irritation when inhaled.
Dust mites are microscopic arachnids that feed on dead human skin cells, preferring areas like bedding, upholstered furniture, and carpets. These tiny creatures absorb moisture directly from the air, flourishing in humid conditions. It is not the mites themselves but rather proteins in their feces, urine, or decaying bodies that act as potent allergens, becoming airborne and triggering symptoms.
Pet dander consists of tiny flecks of skin shed by animals with fur or feathers, such as cats, dogs, and birds. These microscopic particles contain proteins that can cause allergic reactions in sensitive individuals. Its microscopic and jagged shape allows it to remain airborne for extended periods and cling to fabrics and furniture, spreading throughout the home. Other biological contaminants that can circulate indoors include bacteria and viruses, which are carried by people, animals, and even soil or plant debris.
Chemicals from Building Materials and Household Products
Synthetic chemicals in building materials and household items can release gases into the indoor environment, a process known as “off-gassing.” These are Volatile Organic Compounds (VOCs), carbon-containing chemicals that evaporate at room temperature. VOCs contribute to indoor air pollution, with concentrations often higher indoors than outdoors.
Building materials and furnishings are sources of VOCs. Pressed wood products, including furniture and cabinetry, commonly emit formaldehyde, a known irritant. New carpets, vinyl flooring, and paints can also release a variety of VOCs, such as benzene and toluene, from their adhesives, dyes, and synthetic components. While some off-gassing may dissipate within days or weeks, certain materials can continue to emit chemicals for months or even years.
Household and cleaning products are another source of indoor chemical pollutants. Cleaning agents, disinfectants, aerosol sprays, and air fresheners often contain VOCs. These chemicals are released into the air during use, through spraying, wiping, or simply evaporating from the product. Solvents, commonly found in various household products, also contribute to these emissions.
Personal care products, including cosmetics, hairsprays, and deodorants, also release VOCs into indoor air. Fragrances, for instance, contain many VOCs. When these VOCs interact with other indoor air components, such as ozone, they can form new compounds and ultrafine particles that may be inhaled deeply into the lungs.
Radon Gas from the Ground
Radon is a naturally occurring radioactive gas, colorless, odorless, and tasteless, making it undetectable without specialized testing. It originates from the natural decay of uranium in soils and rocks beneath the Earth’s surface. As uranium breaks down into radium and then into radon, this gas seeps upwards through the ground.
Radon enters homes through foundation openings like cracks in concrete slabs, floor-wall joints, gaps around pipes, or through exposed soil in crawl spaces or sumps. The natural negative pressure often present inside a house, particularly during colder months, can draw radon gas from the soil into the dwelling. Once inside, radon can accumulate, especially in lower levels like basements or crawl spaces.
Breathing radon-contaminated air is a health concern. As radon decays, it produces radioactive particles that can become trapped in the lungs when inhaled. These particles release small bursts of energy, which can damage lung tissue and DNA over time, potentially leading to lung cancer. Radon exposure is the second leading cause of lung cancer overall and the primary cause among non-smokers in the United States, accounting for an estimated 21,000 lung cancer deaths annually.