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Indoor air quality (IAQ) can affect health, comfort, concentration and productivity. The U.S. Environmental Protection Agency (EPA) says people spend about 90% of their time indoors, where some pollutant concentrations are often several times higher than outdoors. The practical answer is not to choose between ventilation and an air purifier: remove or isolate the source first, provide and distribute appropriate outdoor air second, and use filtration or air cleaning as a supplement.
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For architects, engineers, contractors and painting teams, that means treating air quality as a design-and-operations problem. Materials, moisture, combustion, construction dust, pressure relationships, filter maintenance and commissioning all determine what occupants actually breathe.
What indoor air quality means and why it matters
EPA defines IAQ as “characteristics of the air in buildings and structures that can impact the health, comfort, performance and productivity of the people inside.” That definition is broader than a single pollutant or sensor number. Exposure depends on what is present, how much reaches a person, for how long, and the person’s susceptibility.
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EPA’s 2025 guidance notes that people spend about 90% of their time indoors in the United States. Indoor levels of some pollutants are often several times outdoor levels. ASHRAE, summarizing EPA exposure studies in 2010, reported that indoor pollutant concentrations may be two to five times outdoor concentrations and, in some circumstances, more than 100 times higher. Those figures are context, not a prediction for every building; concentrations vary with climate, construction, occupancy, activities and ventilation.
How exposure can affect occupants
- Combustion by-products, smoke and fine particles can irritate the eyes and airways and are especially concerning for people with asthma or cardiopulmonary disease.
- Volatile chemicals from products, furnishings and some building materials can cause odor, irritation or other symptoms; risk depends on the chemical, concentration and exposure duration.
- Moisture and condensation can support biological contamination. Damp materials, leaks and poorly maintained HVAC components therefore become IAQ concerns, not merely cosmetic defects.
- Radon and other soil gases can enter through foundations and slabs. They require pollutant-specific testing rather than inference from a general IAQ monitor.
- Poor ventilation can allow occupant-generated carbon dioxide, odors and other contaminants to accumulate. CO2 is useful as an indicator of outdoor-air delivery in occupied spaces, but it is not a complete health test.
Health outcomes differ by pollutant, dose, duration and occupant vulnerability. A complaint, odor or elevated reading warrants investigation, not an automatic diagnosis.
Where indoor pollutants come from
A useful assessment starts with sources and pathways. EPA and CDC identify several recurring categories:
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- Products and furnishings: paints, coatings, adhesives, flooring, pressed-wood products, textiles and new furniture can emit chemicals, especially after installation.
- Building materials and construction: dust, wet materials, curing products and stored chemicals can contaminate a space before handover or during renovation.
- Moisture: roof or plumbing leaks, ground moisture, condensation and inadequate drying can create conditions for microbial growth.
- HVAC systems: dirty filters, standing water, contaminated insulation, inadequate outdoor air or poorly located intakes can distribute pollutants.
- Soil gases: radon and other gases may enter through cracks, drains, sumps and utility penetrations.
- Outdoor air: traffic pollution, wildfire smoke, pollen and industrial emissions can enter through open windows, leaks and mechanical intakes.
Building size, age, envelope tightness, room layout and pressure relationships change exposure. A pollutant released in a garage, workshop or kitchen can migrate to cleaner zones if exhaust, make-up air and door pressures are not coordinated.
The control hierarchy: fix the source before treating the air
CDC states that controlling pollutants at the source is the most effective way to maintain clean air. EPA describes three basic strategies—source control, improved ventilation, and air cleaners or filtration—in that order.
1. Control, eliminate or isolate the source
Specify materials with credible emissions documentation, keep combustion appliances properly vented, use local exhaust for cooking and wet processes, repair leaks, and isolate dusty or chemical-intensive work. During painting, select an appropriate low-emitting product where performance and durability allow, keep containers closed, prevent overspray and sanding dust from spreading, and separate the work area from occupied rooms. “Low-VOC” on a label does not by itself prove that every emission or odor concern is solved; product documentation, application conditions and curing time still matter.
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2. Ventilate and exhaust deliberately
Ventilation dilutes contaminants only when outdoor air is reasonably clean, delivered at an adequate rate and distributed to the occupied breathing zone. Exhaust should capture pollutants near the source. Intake and exhaust locations must prevent re-entrainment, and pressure relationships should keep contaminants from moving into clean areas. Natural ventilation can be useful in suitable weather, but opening windows during wildfire smoke or high outdoor pollution can worsen IAQ.
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Air cleaners range from portable room units to whole-building systems. Their benefit depends on collection efficiency, airflow through the filter and the strength of the pollutant source. Many particle filters do not remove gaseous pollutants such as solvent vapors. A purifier cannot compensate for an unvented combustion appliance, persistent moisture, a major leak or inadequate outdoor-air delivery.
What architects and engineers should put into the design
ASHRAE’s Indoor Air Quality Guide is intended for architects, design engineers, contractors and commissioning agents. It organizes 40 strategies around eight critical IAQ objectives and covers design, construction and commissioning. A project team can translate that framework into the following decisions.
Specify and protect materials
- Choose paints, adhesives, sealants, flooring and furnishings with documented emissions information suited to the project.
- Keep moisture-sensitive products dry and protected before installation.
- Sequence dusty cutting, sanding and demolition away from occupied areas; use barriers and negative pressure where appropriate.
- Plan flush-out or ventilation after installation according to product instructions, weather and the mechanical system’s capacity.
Make combustion and source isolation fail-safe
Combustion equipment needs safe venting, make-up air and carbon-monoxide protection appropriate to the building and jurisdiction. Kitchens, toilets, janitor rooms, laboratories, garages and workshops generally need exhaust and pressure strategies that prevent contaminants from flowing toward offices, classrooms or sleeping areas.
Locate intakes and exhausts to avoid re-entrainment
Review prevailing winds, roof geometry, traffic, loading areas, cooling towers, plumbing vents and neighboring exhausts. The distance and height rules that apply depend on the adopted code and system type; the design team should document the rationale rather than rely on a generic spacing rule.
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Occupancy, floor area, activity, diversity, room use and distribution effectiveness all affect the required airflow. Supply air that bypasses the breathing zone or short-circuits directly to an exhaust does not provide the same benefit as well-distributed air. Pressure relationships should be modeled and balanced so that pollutant migration is predictable.
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Select filtration and provide maintenance access
Match filter performance to the target pollutant, fan capacity and acceptable pressure drop. Provide access for inspection and replacement, seals that prevent bypass, and controls that indicate when service is needed. A high-efficiency filter that is overloaded, poorly fitted or starved of airflow may perform worse than its specification suggests.
Control moisture through the envelope and HVAC system
Detail drainage, flashing, vapor control, insulation continuity and condensation protection. Commission coils, drain pans, humidification and dehumidification controls. Investigate musty odors, staining or repeated condensation promptly; covering a damp surface with new paint does not remove the moisture source.
Commission and document the system
Before occupancy, verify outdoor-air quantities, balancing, control sequences, sensor locations, pressure relationships, filter installation and alarm operation. Give facilities staff filter specifications, replacement intervals, access instructions and a record of the final settings. Commissioning evidence is what connects an IAQ design intent to actual performance.
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The right choice depends on the pollutant and the building defect. Use this comparison rather than treating either option as universally superior.
| Option | Best use | What it cannot do | Checks before relying on it |
|---|---|---|---|
| Source control or isolation | Paint and solvent emissions, leaks, combustion, smoking, damp materials, dusty work | Cannot remove pollutants already dispersed without cleanup and ventilation | Identify the source, stop or contain it, verify safe curing or drying |
| Mechanical or natural ventilation | Diluting occupant-generated pollutants, odors and some vapors when outdoor air is suitable | Does not make polluted outdoor air clean; insufficient or poorly distributed air leaves exposure zones | Outdoor-air rate, intake location, exhaust capture, balancing, pressure and weather conditions |
| Particle filtration or air cleaning | Reducing airborne particles such as dust or smoke when airflow and filter selection are adequate | Many units do not remove gases, moisture or a continuing source | Target pollutant, clean-air delivery, room size, noise, filter fit, pressure drop and maintenance |
| Whole-building air cleaning | Supplementing central HVAC treatment across multiple zones | Cannot correct source migration, dampness or inadequate outdoor-air delivery by itself | System compatibility, bypass, controls, access and commissioning records |
For a painting project, source containment and safe exhaust normally come before purchasing portable units. A purifier may reduce residual particles or odors in an occupied room, but it should not be used to justify keeping people in an actively emitting or poorly ventilated work area.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Should you monitor CO2, particles or VOCs?
Measurement is most useful when it answers a defined question. EPA recommends considering measurements when occupants have symptoms or there are signs of poor ventilation and a specific source or pollutant is suspected. A sensor reading alone does not establish that a health risk exists.
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Start with the question
- Describe the symptom or observation: odors after painting, visible dust, condensation, smoke intrusion, headaches in a particular room or unusually stuffy conditions.
- Identify the likely pollutant: particles, combustion carbon monoxide, radon, a solvent or another VOC, moisture, or inadequate outdoor-air delivery.
- Choose an instrument designed for that pollutant and understand its detection range, cross-sensitivities, calibration needs and placement limits.
- Record occupancy, doors and windows, HVAC operating mode, outdoor weather and nearby activities while measuring.
- Interpret repeated patterns with a qualified professional when the result could affect health, building operation or a construction decision.
CO2 monitors
CO2 can reveal whether occupied rooms receive changing amounts of outdoor air and can help identify poorly ventilated periods. It is not a universal “safe versus unsafe” boundary, and one number does not account for particles, VOCs, radon, humidity or outdoor pollution. Use trends by room and occupancy, not a single alarm value presented as a health standard.
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Particle monitors
Particle sensors can help investigate smoke, sanding dust, cooking emissions or outdoor particulate intrusion. Optical readings vary with particle composition and humidity. They are useful for locating patterns and checking whether controls change concentrations, but they do not identify every particle type or determine toxicity.
VOC monitors
Consumer VOC devices often report a combined or estimated signal rather than identifying a specific chemical. A high reading may prompt investigation of paints, cleaners, furnishings or solvents, but it cannot tell you which compound is present or whether a particular health limit has been exceeded. Source identification may require product records or professional sampling.
Radon and carbon monoxide
Radon requires a test appropriate to the jurisdiction and the decision being made; a general IAQ monitor is not a substitute. Carbon monoxide from combustion requires a dedicated, properly located alarm and immediate attention to the appliance and venting if an alarm activates.
Standards, codes and project responsibilities
ASHRAE guidance directs designers to use the relevant building codes and the applicable edition of Standard 62.1 for commercial and institutional buildings or Standard 62.2 for residential buildings. Healthcare projects may use application-specific Standard 170. These standards are not interchangeable, and local adoption, amendments, climate and project type must be checked at the time of design.
The architect coordinates envelope, materials, room planning and specifications. Mechanical engineers calculate and distribute outdoor air, exhaust and filtration. Contractors protect materials and maintain separation during construction. Commissioning agents verify that controls, airflow, sensors and filters perform as designed. Owners and facility teams preserve that performance through inspections, cleaning, filter replacement and moisture response.
Quick Recap
Common IAQ mistakes and practical fixes
- Relying on a purifier to solve a source: Find and stop the emission, leak or combustion problem first.
- Treating one CO2 threshold as a health verdict: Use CO2 as a ventilation indicator and investigate other pollutants separately.
- Installing a tight envelope without a ventilation plan: Provide balanced, controlled outdoor air and commission it.
- Opening windows during smoke or high pollution: Check outdoor conditions and use filtration or a controlled ventilation mode when appropriate.
- Choosing a filter by efficiency alone: Confirm airflow, pressure drop, fit, noise and replacement access.
- Painting over dampness or odor: Repair the water source, dry the assembly and remove contaminated material as needed before coating.
- Ignoring construction-phase contamination: Isolate work, protect ducts and store materials correctly; clean and verify before occupancy.
- Skipping commissioning: Test outdoor-air delivery, pressure, controls and filter installation, then leave an operations record.
A decision sequence for a healthier building
- Define the exposure concern. Name the room, occupants, timing, activity and suspected pollutant.
- Inspect sources and pathways. Check materials, moisture, combustion, HVAC condition, outdoor air, pressure and construction practices.
- Apply source control. Remove, repair, isolate or substitute the source before adding equipment.
- Verify ventilation and exhaust. Confirm intake location, outdoor-air quantity, capture, distribution and pressure relationships.
- Add targeted filtration or cleaning. Select equipment for the pollutant, airflow and maintenance capability.
- Measure to verify the question. Use fit-for-purpose instruments, document conditions and avoid treating consumer thresholds as universal health limits.
- Commission and maintain. Record settings, train operators, replace filters and respond to moisture or new odors before they become chronic problems.
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