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Steady-State Indoor Air Quality Formulae and Calculator

Civil Engineering

Steady-State Indoor Air Quality Concentrations, Review, Formulas and Calculator

Indoor pollutant concentrations depend on the strength of pollutant sources and the total rate of pollutant removal. Pollutant sources include outdoor air; indoor sources such as occupants, furnishings, and appliances; lack of cleanliness as well as use of cleaning and other products; soil adjacent to the building; and building materials themselves, especially when new. Pollutant removal processes include dilution with cleaner outdoor air, local exhaust ventilation, deposition on surfaces, chemical reactions, and air-cleaning processes.

If (1) general building ventilation is the only significant pollutant removal process, (2) indoor air is thoroughly mixed, and (3) pollutant source strength and ventilation rate have been stable for a sufficient period, then the steady-state indoor concentration of a specific pollutant is given by Equation 1.

Equation 1
Ci = Co + 106 · S / Qoa


Ci = steady-state indoor concentration, ppm
Co = outdoor concentration, ppm
S = total pollutant source strength, cfm
Qoa = ventilation rate for volume of consideration, cfm

ppm = parts per million = parts / million

Variation in pollutant source strengths, rather than variation in ventilation rate, is considered the largest cause of building-tobuilding variation in concentrations of pollutants that are not generated by occupants. Turk et al. (1989) found that a lack of correlation between average indoor respirable particle concentrations and wholebuilding outdoor ventilation rate indicated that source strength, high outdoor concentrations, building volume, and removal processes are important. Because pollutant source strengths are highly variable, maintaining minimum ventilation rates does not ensure acceptable indoor air quality in all situations. The lack of health-based concentration standards for many indoor air pollutants, primarily because of the lack of health data, makes the specification of minimum ventilation rates difficult.

Sources and classification of contaminants

  • Dusts, fumes, and smokes are mostly solid particulate matter, although smoke often contains liquid particles.
  • Mists, fogs, and smogs are mostly suspended liquid particles smaller than those in dusts, fumes, and smokes.
  • Bioaerosols include primarily intact and fragmentary viruses, bacteria, fungal spores, and plant and animal allergens; their primary effect is related to their biological origin. Common indoor particulate allergens (dust mite allergen, cat dander, house dust, etc.) and endotoxins are included in the bioaerosol class.
  • Particulate contaminants may be defined by their size, such as coarse, fine, or ultrafine; visible or invisible; or macroscopic, microscopic, or submicroscopic.
  • Particles may be described using terms that relate to their interaction with the human respiratory system, such as inhalable and respirable.
  • Gases, which are naturally gaseous under ambient indoor or outdoor conditions (i.e., their boiling point is less than ambient temperature at ambient pressure).
  • Vapors, which are normally solid or liquid under ambient indoor or outdoor conditions (i.e., their boiling point is greater than ambient temperature at ambient pressure), but which evaporate readily.
  • Outdoor air contaminants
  • Industrial air contaminants
  • Nonindustrial indoor air contaminants and indoor air quality
  • Flammable gases and vapors
  • Combustible dusts
  • Radioactive contaminants
  • Soil gases

Note: This webpage is not a intended to be guide for system design, specifiation nor should be treated as an authority for indoor air quality. Consult regulatory and local standards for your requirements.



  • ASHRAE Handbook of Fundamentals, 2021 Inch-Pound Edition