Indoor Air Quality and Indoor Environmental Quality: CIH Reference
An IAQ investigation asks: What is being released, how does it reach people, and what building conditions allow exposure? A useful investigation connects occupant reports with sources, airflow, moisture, and building operation.
How to use this page
This reference organizes the subject around investigation decisions, adds calculations, and corrects several common oversimplifications.
- Sections 1 to 3 cover the people, the complaint and the contaminants; 4 and 5 the building (HVAC, moisture, comfort); 6 to 9 the method, the numbers, the standards and the controls.
- Original wording, not a copy of any course text, BGC material or standard. Source links are in the text and listed on the CIH References page. Radon, ASHRAE and formaldehyde values were checked against their sources on October 4, 2026; confirm the edition or limit your jurisdiction adopts.
01 The Foundation: Source, Pathway, Occupant
An IAQ investigation connects occupant reports with sources, airflow, moisture and building operation.
| Concept | What it means | Why it matters to an IH | Memory aid |
|---|---|---|---|
| Indoor air quality—IAQ | Conditions involving airborne chemical, biological, and particulate contaminants inside buildings. | Addresses inhalation exposure and air-related complaints. | What is in the air? |
| Indoor environmental quality—IEQ | Broader than IAQ: includes thermal comfort, lighting, noise, vibration, ergonomics, and other environmental conditions. | An “air-quality complaint” may have several contributing causes. | Air + the surrounding environment. |
| Source | Something that releases a contaminant. | Examples: cleaning products, damp materials, combustion appliances, renovation work, outdoor traffic. | Where does it come from? |
| Pathway | The route connecting source and occupant. | Doors, ductwork, cracks, ceiling plenums, pressure differences, and outdoor-air intakes can transport contaminants. | How does it travel? |
| Occupant | The person exposed, with a particular location, activity, exposure duration, and susceptibility. | The same building conditions may affect occupants differently. | Who receives the exposure? |
| Building operation | HVAC schedules, maintenance, occupancy, housekeeping, and space use. | A ventilation system suitable for yesterday’s occupancy may be inadequate after a room changes use. | Buildings change; operating plans must follow. |
The EPA/NIOSH building-air-quality framework connects contaminant sources, HVAC operation, transport pathways, and occupants. Building Air Quality Guide
Historical context: Energy-conservation changes during the 1970s helped bring IAQ concerns into focus. However, “too little outdoor air” is only one possible cause; moisture, emissions, outdoor pollution, and poor distribution also matter.
02 Understand the Complaint Before Selecting a Test
Sick building syndrome, building-related illness, comfort and odor complaints, and when to treat it as an emergency.
| Situation | Key distinction | IH response | Interpretation trap |
|---|---|---|---|
| Building-related symptoms, historically called sick building syndrome—SBS | Symptoms appear associated with time in a building, but no specific illness or cause has been established. | Map symptoms by location, time, activity, and HVAC operation. | Unexplained symptoms are not proof of a psychological cause. |
| Building-related illness—BRI | A diagnosable illness is linked to a building-related exposure. | Coordinate environmental findings with qualified clinicians and, when appropriate, public health. | A diagnosis alone does not establish which building caused it. |
| Thermal discomfort | Feeling too hot, cold, drafty, or exposed to uneven radiant temperatures. | Evaluate the thermal environment and personal factors. | Air temperature alone cannot explain all comfort complaints. |
| Odor complaint | An odor is present or perceived, sometimes intermittently. | Identify the source and timing; investigate potential hazards. | Odor strength is not a toxicity meter. |
| Possible acute hazard | Suspected CO exposure, significant chemical release, or severe acute symptoms. | Initiate the appropriate emergency response before routine investigation. | Do not delay protective action to complete a survey. |
Headache, fatigue, irritation, and difficulty concentrating are nonspecific. Improvement away from the building is useful evidence, but not proof of one particular contaminant. EPA health-effects overview
Interview questions worth remembering:
Who? Where? When? What changed? Better away? Others affected?
03 Common Contaminants: Connect the Source to the Investigation
Each target has typical sources, a useful evaluation and a distinction a question may lean on.
| Target | Common sources / pathways | Useful evaluation | Key CIH distinction |
|---|---|---|---|
| Carbon monoxide—CO | Incomplete combustion; vehicle exhaust; malfunctioning or poorly vented combustion equipment. | Appropriate CO monitoring, combustion-system inspection, intake/pathway checks. | A combustion hazard, not an occupancy indicator. |
| Carbon dioxide—CO₂ | Occupant respiration; combustion or process sources in some settings. | Time-series indoor/outdoor measurements with occupancy and HVAC information. | Can help assess ventilation relative to occupancy; does not measure all IAQ hazards. |
| VOCs | Paints, adhesives, cleaners, furnishings, stored chemicals, outdoor emissions. | Product/source inventory; targeted compound analysis; suitable direct-reading screening. | A total-VOC number does not reveal the toxicity of the mixture. |
| Formaldehyde | Certain composite-wood products, resins, furnishings, and other sources. | Analyte-specific sampling, such as a validated derivatization/HPLC method. | A general VOC instrument may not measure formaldehyde adequately. |
| Particles / PM₂.₅ | Outdoor pollution, wildfire smoke, cooking, combustion, dust-generating activities. | Indoor/outdoor trends; source inspection; appropriate particle or mass measurements. | Optical readings depend on particle properties and do not establish composition. |
| Mold and dampness-related agents | Leaks, condensation, wet materials, poor drainage. | Moisture assessment, visual inspection, musty-odor investigation, building history. | Find and correct the moisture problem; routine mold air testing is usually not the starting point. |
| Allergens | Dust mites, pests, animals, pollen entering from outdoors. | Source and housekeeping assessment; targeted testing when it answers a defined question. | An allergic response is not necessarily associated with a high total particle count. |
| Legionella | Building water systems, cooling towers, showers, hot tubs, decorative fountains. | Water-system risk assessment and a water-management program. | Water reservoir → aerosol or aspiration exposure, not simply “dirty air ducts.” |
| Radon | Radioactive decay in soil/rock; entry through foundations; sometimes water. | Appropriate radon testing over a representative period. | Cannot be detected by smell or a standard VOC meter. |
| Asbestos / lead | Disturbance of relevant building materials or coatings. | Material assessment and task-specific air/dust sampling where appropriate. | A general IAQ meter does not exclude these hazards. |
Legionella can grow in water systems that support it and spread through contaminated water droplets. Ordinary home air-conditioning units do not use water in the same way as cooling towers. CDC exposure-source guidance
04 HVAC: Ventilation, Circulation, Filtration and Pressure
Four jobs that are easy to confuse, plus what carbon dioxide can and cannot tell you.
| Concept | What it does | Why it matters | Memory aid |
|---|---|---|---|
| Outdoor-air ventilation | Introduces outdoor air and replaces indoor air. | Can dilute indoor-generated contaminants, provided incoming air is suitable. | Replacement air. |
| Recirculation | Returns indoor air to the supply system. | Moves air and may support filtration, but does not inherently provide outdoor air. | Moving air ≠ fresh-air delivery. |
| Distribution effectiveness | Determines whether delivered air reaches occupied zones. | Blocked diffusers, partitions, or supply-to-return short-circuiting can leave poorly served areas. | Air must reach people. |
| Local exhaust | Captures contaminants near their source. | Useful for toilets, laboratories, printing areas, and other localized sources. | Capture before spreading. |
| Particle filtration | Removes particles from air passing through a filter. | Performance depends on efficiency, airflow, fit, and bypass. | Particle filters do not normally remove gases. |
| Gas-phase cleaning | Uses suitable sorbents or reactive media for particular gases/vapors. | Media must match the target and have adequate capacity. | Select the chemistry; track capacity. |
| Pressure relationships | Drive air from higher pressure toward lower pressure through available openings. | Negative pressure can draw contaminants from adjacent rooms, garages, crawlspaces, or soil. | Pressure provides the push; openings provide the path. |
| HVAC controls | Determine when and how fans, dampers, heating, cooling, and ventilation operate. | A system may function correctly during inspection but fail during occupied startup or low-load periods. | Check operating schedules, not just equipment. |
Increasing outdoor air is not always the best first action—for example, during wildfire smoke or when the intake is near an exhaust source.
CO₂: useful evidence, limited conclusions
| Finding | What it may suggest | What it does not prove |
|---|---|---|
| CO₂ rises as occupancy increases | Occupant emissions are accumulating relative to outdoor-air delivery. | That CO₂ is causing every symptom. |
| One occupied room is much higher than comparable rooms | A local occupancy or ventilation difference worth investigating. | The identity of another pollutant. |
| CO₂ falls after occupants leave | Dilution or removal of occupant-generated CO₂. | That all contaminants behave the same way. |
| CO₂ is relatively low | Low occupancy, substantial ventilation, or other favorable measurement conditions. | Absence of mold, formaldehyde, CO, radon, or particles. |
Do not teach “below 1,000 ppm = acceptable IAQ” as a universal ASHRAE rule. ASHRAE explicitly rejects that interpretation. ASHRAE position document on indoor CO₂
05 Moisture and Thermal Comfort
Follow the water, not just the mold; and the six factors behind comfort.
Moisture: follow the water, not just the mold
| Concept | Meaning | Why it matters |
|---|---|---|
| Bulk water | Leaks, rain penetration, flooding, or plumbing releases. | Can wet large areas quickly; repair the source. |
| Capillary movement | Water moves through small pores in materials. | Moisture may move through masonry or other porous materials. |
| Air leakage | Moving air transports water vapor through openings. | Moist air can reach cold surfaces inside an assembly and condense. |
| Vapor diffusion | Water vapor moves through materials because of vapor-pressure differences. | Vapor-retarder selection depends on climate and assembly design. |
| Dew point | Temperature at which air becomes saturated when cooled without changing its moisture content. | Condensation can occur when a surface is at or below the dew point. |
| Relative humidity—RH | Water-vapor pressure relative to saturation at the same temperature. | Cooling air without removing moisture raises RH; warming it lowers RH. |
| Air barrier vs. vapor retarder | An air barrier controls airflow; a vapor retarder limits diffusion. | They perform different jobs, even when one material performs both. |
| Thermal bridge | A location that conducts heat more readily than surrounding construction. | A locally cold surface can develop condensation despite acceptable room-average conditions. |
Example: Indoor air has a dew point of 12°C. A pipe surface is 8°C. Because the pipe is below the dew point, condensation can form.
EPA recommends keeping indoor RH below 60%, ideally 30–50% where feasible, and promptly drying water-damaged materials—often within 24–48 hours. These are moisture-management recommendations, not universal proof of a safe building. EPA moisture guidance
Mold-testing principle: There is no generally applicable health-based indoor mold-spore threshold. Short air samples can miss intermittent conditions and cannot certify that a building is safe. NIOSH mold assessment guidance
Thermal comfort: six factors to remember
| Environmental factors | Personal factors |
|---|---|
| Air temperature | Clothing insulation |
| Mean radiant temperature | Metabolic rate / activity |
| Air speed | |
| Humidity |
A cold window can make someone uncomfortable even when room air temperature is acceptable. ASHRAE Standard 55 evaluates combinations of these factors, rather than specifying one universally comfortable thermostat setting. ASHRAE Standard 55
06 Investigation Sequence and Instruments
Measure to test a hypothesis, and match the instrument to the question.
Complaint → immediate-risk check → history and walkthrough → hypothesis → targeted measurement → control → verification
| Step | Practical action | Question it answers |
|---|---|---|
| 1. Define the problem | Identify affected people, locations, times, activities, and recent changes. | Is there a repeatable pattern? |
| 2. Review records | Examine renovations, leaks, occupancy changes, maintenance, HVAC schedules, and previous reports. | What changed before the complaints? |
| 3. Walk through | Inspect affected and comparison areas, intakes, exhausts, damp materials, and HVAC components. | Are there plausible sources and pathways? |
| 4. Form hypotheses | Example: vehicle exhaust enters an intake during morning deliveries. | What explanation can be tested? |
| 5. Select measurements | Measure the suspected contaminant, airflow, moisture, or operating condition at relevant times. | Will the result change the next decision? |
| 6. Apply controls | Remove the source, repair moisture entry, correct airflow, or improve treatment. | What interrupts exposure? |
| 7. Verify performance | Recheck the relevant conditions and occupant pattern after correction. | Did the intervention solve the identified problem? |
This approach avoids collecting broad, expensive data with no interpretation plan. EPA office investigation guidance
Match the instrument to the question
| Question | Useful instrument / method | Limitation |
|---|---|---|
| Is outdoor-air delivery appropriate relative to occupancy? | CO₂ trend logging plus airflow and occupancy information. | CO₂ alone does not establish ventilation compliance. |
| Are combustion products entering? | Suitable CO monitoring and source inspection. | A brief spot reading may miss intermittent events. |
| Are VOC emissions linked to an activity? | PID screening followed by targeted sampling as needed. | PID response varies by compound; it is not a universal identifier. |
| Is moisture present? | Moisture meter, temperature/RH measurements; infrared imaging as a screening aid. | A thermal image shows temperature differences, not direct proof of moisture or mold. |
| Which direction does air move? | Differential-pressure measurement and suitable airflow visualization. | Conditions may change when doors or HVAC modes change. |
| How much air is delivered? | Appropriate flow hood, anemometer, or duct traverse. | Supply airflow is not necessarily outdoor airflow. |
| Is particulate entering from outdoors? | Indoor/outdoor particle monitoring with activity and HVAC logs. | Optical particle readings do not identify composition. |
07 Useful IAQ Calculations
Simplified models: the assumptions matter as much as the arithmetic.
These equations are simplified models; their assumptions matter as much as the arithmetic.
| Calculation | Formula | Meaning / key condition |
|---|---|---|
| Air changes per hour—ACH Equation card › | $ACH=Q/V$, with $Q$ in m³/h and room volume $V$ in m³. | State whether $Q$ is outdoor, total supply, or equivalent clean airflow. |
| ACH using cfm | $ACH=60Q/V$, with $Q$ in ft³/min and $V$ in ft³. | The factor 60 converts minutes to hours. |
| Outdoor-air fraction | $f_{\mathrm{OA}}=Q_{\mathrm{OA}}/Q_{\mathrm{supply}}$ | Fraction of supply that comes from outdoors. |
| Steady-state source balance Equation card › | $C_{\mathrm{in}}=C_{\mathrm{out}}+G/Q$ | Well-mixed space, constant generation and outdoor airflow, with no other removal. Units must match. |
| CO₂ estimate of outdoor airflow | $Q=G/(C_{\mathrm{in}}-C_{\mathrm{out}})$ | Use CO₂ fractions, not raw ppm numbers; requires appropriate steady-state assumptions. |
| Ideal dilution after a source stops Dilution decay card › Purge time card › | $\frac{C_t-C_{\mathrm{out}}}{C_0-C_{\mathrm{out}}}=e^{-Qt/V}$ | Assumes perfect mixing, constant outdoor concentration, and no continuing source. |
| Particle-cleaner equivalent ACH | $eACH=CADR/V$, with CADR in m³/h. | Applies to the particles represented by the clean-air delivery rate; does not imply CO₂ or gas removal. |
Example 1—supply ACH versus outdoor-air ACH
Room volume = 300 m³; supply airflow = 900 m³/h; outdoor-air fraction = 20%.
Three supply air changes do not mean three outdoor-air changes.
Example 2—CO₂ steady-state estimate
Assume total occupant CO₂ generation is 0.30 L/min, indoor CO₂ is 1,000 ppm, and outdoor CO₂ is 400 ppm:
This estimates total outdoor airflow under the stated assumptions. It is not a universal ventilation requirement.
Example 3—ideal contaminant reduction
At 6 outdoor-air changes per hour, with the source stopped:
Real spaces can take longer because of imperfect mixing or continuing emissions.
08 Radon, Legionella and Standards
Distinctions worth learning, including the Canadian and U.S. radon values.
| Topic | CIH-oriented knowledge | Important distinction |
|---|---|---|
| Radon-222 | Radioactive gas in the uranium-238 decay chain, produced from radium-226; half-life approximately 3.8 days. Its short-lived decay products are important contributors to lung dose. | Often enters from the ground, but testing should cover relevant occupied areas—not assume risk exists only in basements. |
| Canadian radon guidance | Reference level: 200 Bq/m³, based on long-term exposure. Representative long-term testing is essential. | Do not compare a brief hourly peak directly with an annual-average guideline. |
| U.S. EPA radon action level | 4 pCi/L = 148 Bq/m³; EPA also advises considering reduction between 2 and 4 pCi/L. | Different jurisdiction, different action guidance; neither level means zero risk. |
| Legionella control | Manage water conditions: temperature, stagnation/water age, biofilm/sediment, and disinfectant residual. | Routine air-filter replacement is not a substitute for a water-management program. |
| ASHRAE 62.1 | Ventilation and acceptable IAQ for buildings within its scope; the current published edition, checked October 2026, is 2025. | Earlier editions, such as 2016, are historical. The edition adopted by a jurisdiction may differ. |
| ASHRAE 62.2 | Ventilation and acceptable IAQ for dwelling units within its residential scope. | Do not apply one building standard indiscriminately to every occupancy. |
| ASHRAE 55 | Thermal environmental conditions for human occupancy. | Thermal comfort and chemical exposure acceptability are separate questions. |
| Formaldehyde product-emission requirements | Limits apply to specified products under specified test conditions. | A product’s chamber-emission result is not a room-air exposure limit. |
| LEED / green-building certification | Can support better material selection, ventilation, and building practices. | Certification does not prove that present-day IAQ is acceptable under every operating condition. |
Sources: Health Canada radon guideline, EPA radon action level, CDC water-management programs, ASHRAE ventilation standards.
Formaldehyde limits are not an IAQ value. Do not quote an old “HUD plywood limit = 0.20 ppm” as a current universal requirement. Current HUD provisions reference EPA’s composite-wood requirements, and product categories have different limits. These are product-emission standards, not an IAQ pass/fail value. HUD provision, 24 CFR 3280.308, EPA product-emission explanation
09 Controls, Renovation and Communication
What interrupts exposure, and how to report findings honestly.
| Control | How to apply it | Why it works |
|---|---|---|
| Remove or reduce sources | Eliminate unnecessary chemicals; select appropriate lower-emitting products; control combustion emissions. | Prevents contaminants from entering the space. |
| Repair moisture entry | Correct leaks, drainage, condensation, and wet-material problems. | Removes conditions supporting recurring dampness and microbial growth. |
| Separate renovation work | Use appropriate barriers, controlled pressure, protected HVAC pathways, and dedicated exhaust where needed. | Limits movement of dust and vapors into occupied areas. |
| Schedule disruptive activities | Coordinate dusty or high-emission work with occupancy and ventilation operation. | Reduces overlap between emissions and occupant exposure. |
| Maintain HVAC and treatment systems | Check drainage, filters, seals, dampers, sensors, schedules, and airflow. | Preserves actual performance rather than relying on design intent. |
| Verify completion | Confirm cleanliness, dryness, restored HVAC operation, and any project-specific clearance criteria. | Establishes that controls achieved their intended purpose. |
| Communicate findings clearly | Explain what was observed, what remains uncertain, what will change, and when results will be reviewed. | Builds trust without dismissing complaints or promising zero risk. |
CIH interpretation rule: A result below an occupational exposure limit does not, by itself, establish acceptable IAQ for every occupant or explain a comfort complaint. Match the benchmark to the population, averaging period, contaminant, and purpose of the evaluation.