APES 7.3-7.4: Indoor Air Pollution and Reducing Air Pollutants

Indoor air quality (IAQ)

The quality of air within buildings and other enclosed structures, including its effects on occupants’ health, comfort, and ability to function.

How does indoor air quality differ from indoor environmental quality?

IAQ focuses on airborne pollutants and ventilation, whereas indoor environmental quality (IEQ) also includes lighting, acoustics, visual quality, and thermal comfort.

Approximately what fraction of their time do people spend indoors, and why is this important?

People spend roughly 90% of their lives indoors, so even relatively low indoor pollutant concentrations can produce substantial exposure over time.

Why can indoor pollutant concentrations exceed outdoor concentrations?

Pollutants may be generated indoors by combustion, furnishings, cleaning, cooking, and occupants while ventilation is limited. Some pollutants, especially VOCs, can reach concentrations several times higher indoors than outdoors.

How can poor indoor air quality affect health and performance?

It can irritate the eyes and airways, worsen asthma and cardiovascular disease, and contribute to cancer or other chronic effects. It is also associated with sick building syndrome, reduced productivity, and impaired learning.

Household air pollution

Indoor air pollution associated especially with cooking or heating using solid fuels such as wood, charcoal, dung, or crop residues, often in poorly ventilated homes.

Why is indoor combustion a major source of air pollution?

Burning biomass or fossil fuels indoors can release carbon monoxide, particulate matter, nitrogen oxides, sulfur oxides, black carbon, and other toxic products. Poor ventilation allows these substances to accumulate.

Carbon monoxide (CO)

A colorless, odorless, acutely toxic gas produced by incomplete combustion. It impairs oxygen delivery and use in the body, particularly affecting the brain, heart, and respiratory system.

How does incomplete combustion produce carbon monoxide?

When a carbon-containing fuel burns with insufficient oxygen, carbon is only partially oxidized, producing CO rather than exclusively CO₂. In simplified form, $2C + O_2 \rightarrow 2CO$.

Which indoor sources commonly emit carbon monoxide?

Faulty or poorly maintained fuel-burning stoves, heaters, and furnaces; blocked flues; biomass combustion; and tobacco smoke are important sources.

Why is carbon monoxide especially dangerous even though it is not a reactive irritant like many pollutants?

CO is difficult to detect with human senses and interferes with oxygen transport, so dangerous exposure can occur before occupants realize a problem exists.

What health effects can result from acute carbon monoxide exposure?

Headache, nausea, dizziness, weakness, altered consciousness, shortness of breath, myocardial infarction, respiratory failure, coma, and death can occur. Severe exposure may also cause lasting neurological changes.

What health effects are associated with chronic exposure to low carbon monoxide concentrations?

Possible effects include lethargy, headaches, nausea, flu-like symptoms, and neurological or cardiovascular problems.

Secondhand tobacco smoke

A mixture of exhaled smoke and smoke released from the burning end of a cigarette or other tobacco product. It contains both gaseous pollutants and particulate matter, including fine particles that can reach deep into the lungs.

Why can sidestream smoke be a major component of secondhand tobacco smoke?

Secondhand smoke consists mostly of smoke from the burning end of the cigarette, called sidestream smoke, rather than smoke exhaled by the smoker.

What is thirdhand smoke, and why can it remain hazardous after smoking stops?

Thirdhand smoke consists of chemicals deposited on indoor surfaces, objects, and bodies. These residues can later re-enter the air or react with other chemicals to form additional toxic substances.

What is the most reliable way to eliminate indoor exposure to secondhand tobacco smoke?

Smoking must be eliminated from the indoor environment. Ventilation and filtration can reduce concentrations but do not reliably remove all harmful components.

Particulate matter (PM)

A mixture of solid particles and liquid droplets suspended in air. Indoor particles may enter from outdoors, be emitted directly by combustion or activities, or form through gas-to-particle chemical reactions.

How can particle size influence health effects?

Very small particles, including ultrafine particles and PM2.5, can penetrate deeply into the lungs and may enter the bloodstream. Exposure is associated with cardiovascular and pulmonary disease, cancer, and adverse pregnancy outcomes.

Why can cooking substantially increase indoor particulate matter?

Heating food and oils generates particles and gaseous precursors that can form secondary aerosols. Frying generally produces more particulate matter than boiling or grilling, and cooking meat can produce more than cooking vegetables.

Volatile organic compounds (VOCs)

Carbon-containing compounds that readily evaporate into indoor air. Sources include paints, solvents, cleaners, pesticides, furnishings, building materials, adhesives, personal-care products, printers, and cooking.

Why are indoor VOC concentrations often higher than outdoor concentrations?

Many products continuously emit VOCs indoors, where air exchange is limited. Indoor reactions can also create additional VOCs from precursor chemicals.

How can cooking and household activities generate VOCs?

Heating oil can release aldehydes and alkanes, while preparing spices can release terpenes. Cleaning, painting, using personal-care products, and leaks of natural gas can also emit VOCs.

Give examples of hazardous indoor VOCs and their possible effects.

Benzene, formaldehyde, tetrachloroethylene, and trichloroethylene are examples. VOC exposure can cause irritation, headaches, nausea, and damage to the liver, kidneys, or central nervous system; some VOCs are carcinogenic.

Why can indoor materials act as VOC sinks?

Materials such as carpet and gypsum board can absorb VOC vapors and later release them by outgassing. This can cause prolonged, low-level exposure even after the original product was used.

How can ozone create additional indoor pollutants?

Ozone reacts with VOCs, skin oils, and other indoor chemicals, sometimes producing irritating compounds such as formaldehyde and generating fine or ultrafine particles. Terpenes in some citrus-based cleaning products are particularly reactive with ozone.

Asbestos fibers

Microscopic mineral fibers formerly used in many building materials for insulation and fire resistance. Disturbing or damaging asbestos-containing materials can release fibers that are associated with lung cancer, mesothelioma, and asbestosis.

Why is disturbing asbestos-containing material hazardous?

Cutting, sanding, drilling, remodeling, or deterioration can disperse respirable fibers into the air. Intact material may be safer to manage in place than to remove, because removal itself can release fibers.

Why are friable asbestos materials especially dangerous?

Friable materials, such as sprayed coatings and some insulation, can crumble or release fibers more readily than intact, nonfriable materials.

Airborne microplastics

Small plastic particles or fibers suspended in air and classified as a type of particulate matter. Indoor sources include textiles, renovation, construction, and power-tool use.

How is indoor ozone produced, and how does its concentration usually compare with outdoor ozone?

Indoor ozone can be generated by high-voltage devices such as some air ionizers and can also enter through ventilation. Indoor levels are commonly lower than outdoor levels because ozone is rapidly consumed by reactions with indoor surfaces and chemicals.

What does a large indoor surface-to-volume ratio do to ozone?

It increases opportunities for ozone to react with walls, furnishings, skin oils, and deposited chemicals. Consequently, surface reactions often remove more indoor ozone than gas-phase reactions do.

Mold and indoor biological allergens

Indoor biological contaminants include fungal spores, fungal fragments, mycotoxins, animal dander, and plant pollen. Mold growth is favored by moisture, suitable temperature and pH, and available nutrients.

What building conditions can create excessive moisture and mold?

Leaks, rain or groundwater intrusion, plumbing failures, condensation, inadequate ventilation, poor heating, and drying clothes indoors can all increase moisture and mold risk.

How does indoor humidity affect mold growth?

High humidity and condensation promote mold growth. Keeping indoor relative humidity below about 60%, and surfaces away from condensation conditions, helps limit growth; a commonly recommended comfort range is about 40–60% relative humidity.

How does mold exposure affect health?

Mold can affect the airways and lungs, trigger or worsen asthma, and contribute to allergic rhinitis and nasal or sinus symptoms. Children, infants, and older adults may be especially vulnerable.

Legionella

A waterborne bacterium that grows especially well in warm, stagnant or slow-moving water. Infection risk arises when contaminated water is aerosolized, such as by showerheads or evaporative cooling towers, and the aerosol is inhaled.

Why can Legionella be difficult to control in building water systems?

The bacteria can live as parasites of protozoans and form biofilms that resist chemical and antimicrobial treatment. Warm stagnant water and poorly maintained cooling systems provide favorable conditions.

How can Legionella contamination be monitored or remediated?

Water samples and surface swabs can be cultured, with abundance reported as colony-forming units per liter ($\mathrm{cfu/L}$). Remediation may include hot-water flushing, sterilizing cooling basins, replacing showerheads, and eliminating stagnant water.

How can viruses contribute to indoor air-quality problems?

Viruses can be transported in respiratory or water-related aerosols. Poor ventilation, contaminated surfaces, and airflow pathways such as bathroom drains and exhaust systems can facilitate indoor spread.

Why is indoor carbon dioxide commonly used as a ventilation indicator?

Occupants are usually the main indoor source of CO₂, so its concentration reflects the balance between occupant generation and outdoor-air exchange. Elevated CO₂ often indicates insufficient ventilation relative to occupancy.

What approximate indoor CO₂ concentration suggests inadequate ventilation according to NIOSH?

An indoor concentration above about $1000\ \mathrm{ppm}$ is considered a marker suggesting inadequate ventilation, although CO₂ itself may not be the only cause of complaints.

How are CO₂ concentration ranges commonly interpreted for indoor air quality?

Concentrations below about $1000\ \mathrm{ppm}$ are often considered good, $1000$–$1500\ \mathrm{ppm}$ moderate, and above $1500\ \mathrm{ppm}$ poor. These are practical indicators rather than universal measures of all pollutants.

What is the recommended indoor-to-outdoor CO₂ difference for minimizing ventilation-related complaints?

The indoor CO₂ level should generally be no more than about $700\ \mathrm{ppm}$ above the outdoor concentration.

Why should elevated CO₂ not automatically be interpreted as the direct cause of every indoor health effect?

At moderate concentrations, CO₂ often serves as a marker for occupant density and inadequate ventilation. Other pollutants and human bioeffluents that accumulate under the same conditions may be responsible for the symptoms.

How can indoor air quality be evaluated?

Methods include collecting air samples, monitoring occupants’ pollutant exposure, analyzing building surfaces, and modeling airflow computationally.

What is source control in indoor air-quality management?

Source control means removing, isolating, replacing, or reducing pollutant-generating activities and materials. Examples include repairing combustion appliances, eliminating indoor smoking, controlling moisture, and selecting low-emitting products.

How can outdoor pollution complicate the use of ventilation to improve IAQ?

Ventilation dilutes indoor contaminants only if the incoming air is cleaner. If outdoor air contains ozone, particulate matter, or other pollutants, filtration or other treatment may be needed.

Why are filtration and ventilation complementary IAQ strategies?

Ventilation reduces concentrations by exchanging indoor air with outdoor air, while filtration physically removes particles and sometimes other contaminants from recirculated or incoming air. Using both is useful when pollutant sources cannot be eliminated or outdoor air is polluted.

What are the three primary strategies for improving indoor air quality?

Control or eliminate pollutant sources, filter contaminated air, and ventilate with clean outdoor air to dilute pollutants. Ventilation alone may be insufficient when outdoor air is polluted or indoor sources are strong.

Radon

A naturally occurring, radioactive, colorless, and odorless gas produced by the decay of uranium in soil and rock. It can enter buildings through cracks and gaps in foundations and increase lung-cancer risk when inhaled over long periods.

Why can radon accumulate indoors?

Buildings can draw radon-containing soil gas through foundation cracks, sump pits, drains, and gaps around pipes. Because enclosed spaces have limited air exchange, radon may accumulate to higher concentrations indoors than outdoors.

How can indoor radon exposure be reduced?

Test buildings for radon and reduce entry by sealing foundation openings, improving sub-slab ventilation, and increasing appropriate ventilation. Testing is important because radon cannot be detected by human senses.

Formaldehyde

A highly reactive VOC used in or released by products such as pressed-wood materials, adhesives, furnishings, and some building materials. It can irritate the eyes and airways and is a recognized human carcinogen.

How can exposure to indoor formaldehyde be reduced?

Use low-emitting products, increase ventilation when products are installed, control indoor temperature and humidity, and prevent or remove sources when feasible.

How do scrubbers reduce air pollution?

Scrubbers remove gaseous pollutants or particles from industrial exhaust before release. Wet scrubbers use a liquid to capture pollutants, while dry scrubbers use a dry sorbent; they are commonly used to reduce sulfur dioxide and particulate emissions.

How do catalytic converters reduce vehicle air pollution?

Catalytic converters use catalysts to promote chemical reactions that convert carbon monoxide, nitrogen oxides, and unburned hydrocarbons in exhaust into less harmful substances such as carbon dioxide, nitrogen, and water.

What is an electrostatic precipitator?

An air-pollution control device that electrically charges particles in exhaust and collects them on oppositely charged plates. It is especially effective for removing particulate matter from industrial emissions.

How do regulations such as the Clean Air Act reduce air pollution?

They establish air-quality standards, limit emissions from industrial sources and vehicles, require pollution-control technology, and authorize monitoring and enforcement. These measures reduce emissions at their sources rather than relying only on cleanup after release.

How can fuel switching reduce air pollution?

Replacing high-pollution fuels such as coal or biomass with cleaner fuels or renewable energy can reduce emissions of sulfur dioxide, particulate matter, carbon monoxide, and other combustion pollutants.