What is IAQ?

From Indoor Air Quality Wiki


THEORETICAL FOUNDATIONS · INTRODUCTION TO IAQ
What is Indoor Air Quality?
An introduction to the physical, chemical, and biological dimensions of indoor air, defining how ventilation, building design, and source control govern the air we breathe indoors.
Quick Overview

Subject: Indoor Environmental Quality (IEQ)
Focus: Health, thermal comfort, ventilation
Core Standard: EN 16798 / ISO 16000
Guiding Goal: Dilution, filtration, and source control

90%
Time Spent Indoors
People spend approximately 90% of their lives inside enclosed spaces such as homes, schools, and offices.[1]
80%
Occupant Satisfaction
Under standard definitions, acceptable IAQ requires at least 80% of occupants to express no dissatisfaction with perceived air quality.[2]
2x to 5x
Pollutant Concentration
Concentrations of several common chemical pollutants can be 2 to 5 times higher indoors than outdoors due to internal emission sources.[3]

Understanding Indoor Air Quality

Indoor Air Quality (IAQ) refers to the physical, chemical, and biological condition of air within and around buildings, specifically regarding the health, comfort, and well-being of the occupants.[1]

Air itself is a mixture of gases, primarily nitrogen (approximately 78%) and oxygen (approximately 21%), alongside argon, water vapor, and carbon dioxide. In an enclosed room, this base composition is continuously altered by the respiration of occupants, emissions from building components and furnishings, domestic and workplace activities, and the entry of pollutants from the outdoor atmosphere.[2]

From an engineering and regulatory perspective, acceptable indoor air quality is achieved when:

  1. Known hazardous contaminants do not exceed health-based limit values established by authorities such as the World Health Organization (WHO) and public health agencies.[4]
  2. A large majority of occupants (typically 80% or more) express satisfaction with the perceived freshness, odor, and thermal neutrality of the air.[2]

The Five Pillars of Indoor Air Quality

To build a thorough understanding of the indoor environment, theoretical concepts are divided across five interconnected areas:

What is IAQ?
The baseline definitions, building dynamics, and physical principles of indoor air.

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Particulate matter, chemical compounds (VOCs, formaldehydes), and biological agents.

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Building materials, combustion appliances, outdoor traffic, and occupant activities.

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Acute respiratory symptoms, chronic diseases, allergies, and vulnerable populations.

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Workplace productivity, cognitive performance, healthcare costs, and building value.

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Two Perspectives: Discomfort vs Health Risk

Evaluating indoor air involves distinguishing between sensory perception and physiological toxicity:[2]

  • Sensory Discomfort and Perceived Air Quality:
    • Driven by body odors, bioeffluents, elevated carbon dioxide (CO2), and moderate thermal-hygrometric shifts.
    • Manifests as sensations of stuffiness, fatigue, minor eye dryness, or lack of freshness.
    • Typically addressed through standard ventilation rates designed to dilute human bioeffluents.
  • Toxicological and Health Risks:
    • Driven by harmful chemical, radiological, or physical agents such as fine particulate matter (PM2.5), radon, volatile organic compounds (e.g. benzene, formaldehyde), and mold spores.[4]
    • Many hazardous pollutants are odorless and colorless, making them imperceptible to human senses even when exceeding protective health benchmarks.[3]
    • Addressed primarily through source control, structural moisture remediation, and high-efficiency filtration.

Key Factors Influencing IAQ

Indoor air quality is dynamic, shaped by continuous physical and chemical processes inside the building envelope:

Outdoor Air Quality and Infiltration

Ambient air conditions determine the baseline quality of air entering an enclosure. Particulate matter from vehicular traffic, sulfur and nitrogen oxides from combustion, and seasonal biological allergens (such as tree and grass pollen) penetrate buildings via open windows, supply air intakes, and unintended cracks in the building envelope.[5]

Building Materials and Interior Furnishings

Paints, adhesives, pressed wood products, textiles, and vinyl flooring continuously off-gas chemical compounds into the indoor atmosphere. While emissions typically peak shortly after installation or renovation, secondary emissions and chemical degradation can release volatile compounds over many years.[3]

Ventilation Rates and Air Distribution

Ventilation supplies clean outdoor air to dilute and remove indoor contaminants and humidity. The effectiveness of ventilation depends not only on the nominal volumetric airflow rate (air changes per hour), but also on airflow distribution patterns within the space, preventing stagnant pockets and short-circuiting between supply and extract diffusers.[6]

Temperature and Relative Humidity

Thermal and hygrometric conditions affect both pollutant emission kinetics and biological viability:

  • Relative Humidity (RH): Low humidity (below 30%) dries mucosal membranes and impairs the body's natural airway clearance mechanisms. Excessive humidity (consistently above 60-70%) promotes microbial proliferation, condensation on building assemblies, and fungal growth.[7]
  • Temperature: Higher indoor temperatures accelerate the vaporization and emission rates of volatile chemicals from interior finishes and furnishings.[3]

Occupancy and Daily Routines

Occupants act as continuous emission sources through metabolic respiration, moisture generation, and shedding of skin particles and clothing fibers. In addition, routine activities such as unvented gas cooking, frying, burning candles or incense, vacuum cleaning without HEPA filtration, and using solvent-based cleaning agents generate high, short-term concentration peaks.[4]

The Principle of "One Air" Continuum

Indoor air quality cannot be evaluated in isolation from the outdoor environment. Modern building science treats air as a single continuous fluid moving through urban topography, building envelopes, heating and ventilation systems, and occupied spaces.[6]

Effective management of indoor environments requires a combined strategy:

  • Prescriptive ventilation approaches provide baseline fresh air exchange to support basic comfort and oxygen needs.
  • Performance-based approaches continuously verify that contaminant concentrations remain below health thresholds, integrating high-efficiency air filtration when outdoor air is polluted.[2]

Related Wiki Pages

Air Pollutants
Detailed examination of major chemical and physical contaminants.
Pollution Sources
Origin and generation pathways of indoor pollution.
Health and IAQ
Physiological effects and vulnerable groups.
Economics of IAQ
Productivity, building value, and societal impacts.

References

  1. ↑ 1.0 1.1
    Cincinelli, A., & Martellini, T. (2017). Indoor air quality and health. International Journal of Environmental Research and Public Health, 14(11), 1286.
  2. ↑ 2.0 2.1 2.2 2.3 2.4
    Settimo, G., Manigrasso, M., & Avino, P. (2020). Indoor air quality: A focus on the European legislation and state-of-the-art research in Italy. Atmosphere, 11(4), 370.
  3. ↑ 3.0 3.1 3.2 3.3
    Halios, C. H., Landeg-Cox, C., Lowther, S. D., Middleton, A., Marczylo, T., & Dimitroulopoulou, S. (2022). Chemicals in European residences: Part I: A review of emissions, concentrations and health effects of volatile organic compounds (VOCs). Science of the Total Environment, 839, 156201.
  4. ↑ 4.0 4.1 4.2
  5. ↑
    Cheek, E., Guercio, V., Shrubsole, C., & Dimitroulopoulou, S. (2021). Portable air purification: Review of impacts on indoor air quality and health. Science of The Total Environment, 766, 142585.
  6. ↑ 6.0 6.1
  7. ↑
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