7.5 - Indoor Air Pollutants

Learning Objectives

I. Sources: Developed vs. Developing Nations

• Fuels: Easily gathered materials like wood, charcoal, dried plant matter, or animal manure. They’re economically beneficial because they’re cheap or free.

• Pollutants and impact: When combusted, these fuels release air pollutants including carbon monoxide, particulate matter, NOx, and VOCs. This is particularly dangerous because subsistence fuels are often burned indoors with poor ventilation, which leads to a buildup of toxic and deadly concentrations.

• Health crisis: Household air pollution from cooking and heating with polluting fuels (wood, dung, crop waste, charcoal, coal, and kerosene) caused about 2.9 million deaths in 2021, including over 309,000 children under 5 (WHO, December 2025). About 2.1 billion people, roughly a quarter of the world, still cook this way.

• Pollutants: Indoor pollutants generally come from industrial chemicals, like cleaners, deodorizers such as Febreze, paint, or residual lead in old homes.

II. Key Indoor Air Pollutants

• Source: Comes from the incomplete combustion of pretty much any fuel source, due to a lack of oxygen or low temperatures. In developed nations the major source is a furnace releasing CO into the home because of improper ventilation. In developing nations the source is biomass combustion indoors.

• Health impact: Classified as an asphyxiant, meaning it causes suffocation. CO binds extremely well with the hemoglobin in red blood cells, displacing oxygen and leading to suffocation, which can be fatal.
• CO binds to hemoglobin more than 200 times more tightly than oxygen does, so even a small amount in the air blocks a lot of oxygen transport. Early symptoms (headache, dizziness, nausea) feel like the flu, which is why people often don't notice until it's too late.
• Example: running a generator, grill, or car in a closed garage or house during a power outage is a common cause of CO deaths, so they always belong outside, away from windows.

• Mitigation: Because it’s odorless and colorless, residents should use a carbon monoxide detector to catch high levels.

• Definition: A natural silicate mineral made of long, thin fibers. It was widely used in insulation, fireproofing, and floor and ceiling tiles because it resists heat and fire. It remains in many older buildings.

• Health risk: Linked to lung cancer and asbestosis. It’s only dangerous when the insulation is disturbed or jostled, which lets particles enter the respiratory tract.
• Asbestos also causes mesothelioma, a rare cancer of the lining around the lungs. These diseases show up 20 to 50 years after exposure, because the sharp fibers stay lodged in lung tissue.

• Mitigation: Should be removed by trained professionals using proper ventilation and sealing off the room.

• Dust: a natural indoor particulate made of soil, pollen, pet dander, dust mite droppings, and skin flakes. It triggers allergies and asthma.

• Smoke: from tobacco, wood stoves, fireplaces, candles, and cooking. Smoke particles are mostly PM2.5, small enough to reach deep into the lungs (7.4).

• Mitigation: ventilation, HEPA air filters and vacuums, and never smoking indoors.

• Source: Chemicals used in a variety of home products that easily vaporize. Common sources include adhesives or sealants in carpets, particle board, and furniture, plus cleaners and deodorizers.

• Formaldehyde: A VOC (it's a gas at room temperature) released by building materials, pressed wood like particleboard and plywood, furniture, upholstery, and carpeting, plus some glues and sealants. It's classified as a known human carcinogen (linked to nose and throat cancer and leukemia), and it irritates the eyes, nose, and throat.
• Example: after Hurricane Katrina (2005), many families lived in FEMA trailers built with pressed wood. CDC testing in late 2007 and early 2008 found formaldehyde levels in many trailers several times higher than in typical homes.

• Health impact: VOCs can irritate eyes and inflame air passageways (bronchioles or lungs), causing respiratory problems.

• Source: A radioactive gas released by the decay of uranium naturally found in rocks underground, especially granite.
• Radon-222: uranium-238 decays step by step into radium-226, and radium-226 decays into radon-222. Radon has a half-life of only about 3.8 days, but it's constantly being made, and because it's a gas it can move up through cracks in soil and rock.

• Entry: It enters homes through cracks in the foundation or basement, and it can also seep into groundwater, so testing is needed if you use well water.

• Toxicity: A carcinogen and the second leading cause of lung cancer in the United States, after smoking (and the leading cause among people who have never smoked). The EPA estimates radon causes about 21,000 lung cancer deaths a year.
• How it causes cancer: when you breathe in radon, it and its radioactive decay products (like polonium) lodge in the lungs and give off alpha particles that damage the DNA of lung cells.

• Mitigation: The EPA recommends using an airborne tester to detect it. If detected, solutions include sealing the foundation cracks and establishing better ventilation to dilute the gas.
• Action level: the EPA recommends fixing a home if radon is 4 pCi/L (picocuries per liter) or higher, and about 1 in 15 U.S. homes is estimated to be at or above that level. The most common fix is a vent pipe and fan that pull radon from under the foundation and release it above the roof.

• Health impact: Black mold is particularly problematic because it releases spores that can enter the air and cause irritation in the lungs.

• Mitigation: Physically wipe out the mold with a cleaner, then fix the underlying water or moisture source. Ventilation or a dehumidifier can also help keep the area dry.

• Source: Formerly used as an additive in paint prior to 1978.

• Entry: Lead paint is the predominant way it enters the human body. Children often ingest the sweet tasting paint chips that flake off walls, especially window sills. It can also be inhaled as dust or contaminate water through lead pipes.

• Toxicity: A neurotoxicant that damages the developing nervous system in children, causing learning disabilities and brain development delay.

• Mitigation: Lead paint can be removed by scraping and vacuuming up the dust. Lead pipes should be dug up and replaced, often with copper.

• Tobacco smoke: contains more than 7,000 chemicals, including CO, particulates, formaldehyde, and benzene, and about 70 of them can cause cancer. Secondhand smoke harms people who don't smoke, which is why many states and cities ban smoking indoors in public places.

• Gas stoves, furnaces, and heaters: burning natural gas, propane, or kerosene indoors releases NOx (mostly $\text{NO}_2$), CO, and particulates, and kerosene heaters also release $\text{SO}_2$. Range hoods vented outside and properly vented furnaces keep these out of living spaces.

Big picture: Indoor air quality is defined by stark differences globally. Developing nations face fatal respiratory consequences from burning subsistence fuels indoors, which releases pollutants like CO and particulate matter. Developed nations instead grapple with exposure to industrial chemicals like formaldehyde and hazardous natural risks like radon gas, the second leading cause of lung cancer in the United States.