Indoor Air Quality Parameters

Why This Matters

Indoor air quality (IAQ) sits at the intersection of environmental health, occupational safety, and building science—three areas that frequently overlap on exams. You're being tested on your ability to connect specific pollutants to their sources, health effects, and control strategies, not just recognize their names. Understanding IAQ parameters means grasping how ventilation systems, building materials, and human activities interact to create exposure risks in the places where people spend roughly 90% of their time.

The parameters covered here demonstrate core principles you'll see throughout environmental health: dose-response relationships, exposure pathways, threshold limit values, and the hierarchy of controls. When you encounter an FRQ about occupational hazards or a multiple-choice question on building-related illness, these are your go-to examples. Don't just memorize the numbers—know what each parameter reveals about ventilation adequacy, combustion safety, or material off-gassing, and you'll be able to tackle any IAQ scenario thrown at you.

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Physical Environment Parameters

The physical conditions of indoor spaces create the foundation for air quality. Temperature and humidity don't just affect comfort—they determine whether biological and chemical contaminants thrive or remain controlled.

Temperature

• Optimal range is 68°F–72°F (20°C–22°C)—this thermal comfort zone maximizes productivity and minimizes physiological stress
• Extreme heat triggers heat stress responses including fatigue, reduced cognitive function, and exacerbated cardiovascular strain
• Temperature fluctuations promote microbial growth—warm, variable conditions create ideal environments for mold and bacteria colonization

Relative Humidity

• Target range is 30%–50% RH—this sweet spot inhibits both biological growth and static electricity buildup
• High humidity (>60%) causes condensation on surfaces, creating moisture reservoirs where mold, mildew, and dust mites proliferate
• Low humidity (<30%) irritates mucous membranes—dry air damages the respiratory tract's natural defense mechanisms and worsens skin conditions

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Ventilation Indicators

These parameters tell you whether a building's air exchange system is doing its job. Carbon dioxide isn't toxic at typical indoor levels—it's a proxy measure for how well fresh air is replacing stale air.

Carbon Dioxide ($CO_2$)

• Levels above 1,000 ppm indicate inadequate ventilation—this threshold signals that exhaled air is accumulating faster than fresh air replaces it
• Normal indoor range is 400–1,000 ppm—outdoor air is approximately 400 ppm, so indoor levels reflect dilution effectiveness
• Elevated $CO_2$ correlates with drowsiness and cognitive decline—studies show decision-making performance drops measurably above 1,000 ppm

Ventilation Rate (Air Exchange Rate)

• Measured in air changes per hour (ACH) or CFM per person—ASHRAE Standard 62.1 provides minimum requirements based on occupancy and building type
• Inadequate ventilation concentrates all indoor pollutants—$CO_2$, VOCs, particulates, and biological contaminants all accumulate when fresh air supply is insufficient
• Demand-controlled ventilation uses $CO_2$ sensors to adjust airflow dynamically, balancing energy efficiency with air quality

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Combustion Byproducts

Incomplete combustion from fuel-burning appliances, vehicles, and tobacco creates some of the most acutely dangerous indoor pollutants. These contaminants share sources but differ dramatically in their toxicity mechanisms.

Carbon Monoxide ($CO$)

• Colorless and odorless with a safe threshold below 9 ppm—its undetectable nature makes it especially dangerous without proper monitoring
• Binds to hemoglobin 200× more readily than oxygen—this competitive binding creates carboxyhemoglobin, effectively suffocating tissues even when breathing normally
• Sources include gas appliances, attached garages, and tobacco smoke—any incomplete combustion in enclosed spaces creates exposure risk

Particulate Matter ($PM_{2.5}$ and $PM_{10}$)

• $PM_{2.5}$ (≤2.5 μm) penetrates deep into alveoli and enters the bloodstream—these fine particles bypass the respiratory system's filtration mechanisms entirely
• $PM_{10}$ (≤10 μm) deposits in upper airways—causes irritation, coughing, and aggravates asthma but doesn't reach systemic circulation
• Indoor sources include cooking, candles, tobacco smoke, and infiltration from outdoors—combustion activities generate the finest, most dangerous particles

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Chemical Off-Gassing Contaminants

Building materials, furnishings, and consumer products continuously release volatile chemicals into indoor air. This "off-gassing" is highest when products are new and decreases over time—a concept called sink effect reversal.

Volatile Organic Compounds (VOCs)

• Emitted from paints, adhesives, cleaning products, and pressed-wood furniture—nearly every manufactured product contributes to the indoor VOC burden
• Common examples include benzene, toluene, and xylene—these aromatic hydrocarbons are both irritants and, in some cases, carcinogens
• Health effects range from acute irritation to chronic neurological damage—short-term exposure causes headaches and dizziness; long-term exposure may impair cognitive function

Formaldehyde

• A specific VOC with a recommended limit below 0.1 ppm—its prevalence and toxicity earn it separate regulatory attention
• Primary sources are pressed-wood products, insulation, and permanent-press fabrics—urea-formaldehyde resins in particleboard and plywood are major contributors
• Classified as a known human carcinogen (Group 1)—long-term exposure increases risk of nasopharyngeal cancer and leukemia

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Radioactive and Biological Hazards

These contaminants arise from natural environmental processes rather than human products or activities. Their control strategies focus on source removal and environmental modification rather than product substitution.

Radon

• A naturally occurring radioactive gas with an action level of 4 pCi/L—this EPA threshold triggers mandatory mitigation in homes and workplaces
• Second leading cause of lung cancer after smoking—radon decay products lodge in lung tissue and emit alpha radiation that damages DNA
• Accumulates in basements and ground-floor spaces—enters through foundation cracks, sump pits, and construction joints from underlying soil and rock

Biological Contaminants (Mold, Bacteria, Dust Mites)

• Mold thrives at >60% RH and triggers allergic and asthmatic responses—Aspergillus, Penicillium, and Stachybotrys are common indoor genera
• Dust mites proliferate in bedding, carpets, and upholstery—their fecal particles are potent allergens that become airborne when disturbed
• Bacterial contamination often originates in HVAC systems—cooling towers, humidifiers, and drain pans provide moisture for Legionella and other pathogens

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Quick Reference Table

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Self-Check Questions

1. Which two IAQ parameters serve primarily as indicators of ventilation effectiveness rather than direct health hazards at typical indoor concentrations?

2. Compare and contrast the health mechanisms of $CO$ and $PM_{2.5}$—both are combustion byproducts, but how do their toxicity pathways differ?

3. A building investigation reveals elevated mold growth and dust mite populations. Which physical parameter is most likely out of range, and what is the target value?

4. An FRQ asks you to identify indoor air contaminants classified as known human carcinogens. Which three parameters from this list would you include, and what are their primary sources?

5. Why is formaldehyde tracked separately from general VOCs in indoor air quality assessments, despite being chemically classified as a volatile organic compound?