📊 IAQ Procedure Result
⚖ vs. VRP Result
Procedure Comparison
Full Calculation Table
| Procedure | Basis | Result |
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How to Use the IAQ Ventilation Rate Calculator
Enter the generation rate of the contaminant of concern in mg per minute, the target concentration in mg/m³ and the outdoor concentration of the same contaminant.
Enter the zone floor area, the population and the air distribution effectiveness so the calculator can also show the Ventilation Rate Procedure result for the same zone.
Click Calculate and Compare to see both results side by side in L/s or CFM.
Under the combined approach the larger of the two results governs the zone outdoor airflow, and the recommendation card says which one it is.
Sizing Outdoor Air From a Contaminant Source
Most spaces get their outdoor air from tabulated rates. A few have a source that the tables never anticipated, such as a room full of new flooring or a process that releases a particular chemical. For those, ASHRAE 62.1 offers a second route, and this calculator works the numbers for it.
Two Procedures in ASHRAE 62.1
The Ventilation Rate Procedure takes a per-person rate and a per-area rate from the occupancy table, adds them and divides by the air distribution effectiveness. The ASHRAE 62.1 calculator runs that procedure in full. The IAQ Procedure instead starts from a contaminant and a concentration limit. The current edition is ANSI/ASHRAE Standard 62.1-2025. The standard's text is behind a paywall, so the procedure details here come from published summaries and should be checked against the edition your jurisdiction has adopted.
The Mass Balance Behind the IAQ Procedure
At steady state the outdoor airflow equals the source strength divided by the allowed rise over the outdoor concentration, with the source in mg/min and the concentrations in mg/m³. Take a formaldehyde source of 0.05 mg/min, held to 0.009 mg/m³ with none in the outdoor air. The airflow is 0.05 ÷ 0.009 = 5.56 m³/min, which is 92.6 L/s or 196 CFM. For the same 100 m² zone with 15 people at 2.5 L/s per person and 0.3 L/s per m², the Ventilation Rate Procedure gives 37.5 + 30 = 67.5 L/s. The contaminant needs 1.37 times as much. The 0.05 mg/min source is an illustration, so use emission data for the real material. The VOC calculator looks at volatile organic compounds as a source in more detail, and the dilution ventilation calculator adds a mixing factor for workplace use.
Where the Target Concentration Comes From
The limit is not the designer's choice. It comes from a recognized authority, and recent editions of the standard list the compounds that must be considered. California's OEHHA publishes reference exposure levels in a summary table, and for formaldehyde it lists 9 µg/m³ for 8-hour and chronic exposure and 55 µg/m³ for an acute 1-hour exposure. Match the averaging time to the source, and keep in mind that the limit is only as good as the emission figure it is divided into. Source strengths vary with temperature, humidity and the age of the material.
Which Result Governs
Under the combined approach, the Ventilation Rate Procedure sets the minimum outdoor airflow for the zone and the IAQ Procedure adds outdoor air or air cleaning where a contaminant needs more. That is why this calculator shows the larger of the two as the design basis. Using the IAQ Procedure alone to go below the prescriptive rate needs the full process, including a list of the compounds of concern, a check that mixtures of compounds with similar effects sum to less than 1.0, and verification, and the authority having jurisdiction has to accept it. A code compliance check with the ASHRAE 62.1 compliance calculator is worth running on the prescriptive side.
Verification and Pitfalls
A design done this way is checked after construction. The standard asks for measurement of the contaminants of concern and a survey of occupants that shows acceptable air quality, with 80% satisfied as the target. The simple mass balance assumes the air in the zone is well mixed and the source is steady. A real room has pockets of higher concentration, sources that start and stop, and ventilation that varies through the day, so the number is a starting point for an engineer or industrial hygienist to validate and not a finished design.
Frequently Asked Questions
The IAQ Procedure is the performance-based route in ASHRAE 62.1. Instead of fixed per-person and per-area rates, it sets limits for the contaminants of concern and finds the outdoor airflow, or air cleaning, that keeps their calculated concentrations under those limits using a mass balance. It also asks for a check that occupants find the air acceptable, with 80% satisfied as the usual target. This calculator covers the mass balance for one contaminant.
Use it when one known source dominates and the prescriptive rates do not reflect it, such as strong off-gassing materials, a process that emits a specific chemical or a space with unusual contaminants. Most general spaces use the Ventilation Rate Procedure alone. For a 100 m² office with 15 people, the prescriptive rates give 67.5 L/s, and a formaldehyde source of 0.05 mg/min held to 0.009 mg/m³ needs 92.6 L/s, so the contaminant governs.
In the combined approach, the Ventilation Rate Procedure sets the minimum zone outdoor airflow and the IAQ Procedure adds outdoor air or air cleaning where a contaminant needs it, so the larger of the two governs. Using the IAQ Procedure on its own to go lower requires the full process of design compounds, mixture checks and verification, and the authority having jurisdiction has to accept it. This calculator treats the larger result as the design basis.
Target concentrations come from a recognized authority, not from the designer's preference. California's OEHHA publishes reference exposure levels, which for formaldehyde are 9 µg/m³ for 8-hour and chronic exposure and 55 µg/m³ for an acute 1-hour exposure. Source strength comes from emission data for the material or process, measured where possible, and it changes with temperature, humidity and age. Treat any single number as an estimate and document where it came from.