Select a preset or enter custom values. Presets fill the contaminant name, allowable limit, and background level. The CO2 preset also loads the classroom generation rate; for every other contaminant, enter your own generation rate.
K accounts for imperfect air mixing. ACGIH recommends K = 3 to 10 for industrial spaces. Use K = 1 only if perfect mixing is confirmed.
Purge Time After Source Stops
Time for the concentration excess above background to decay after the source stops, using the effective flow (design flow divided by K).
Full Results Table
| Parameter | Value | Notes |
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How to Use This Calculator
Presets fill the contaminant name, allowable concentration, and background level, and the CO2 preset also loads the classroom generation rate. For everything else, enter the generation rate in L/s (volume of contaminant vapour released per second), the allowable indoor concentration, and the inlet background concentration.
Room volume determines the time constant, which is how quickly the space responds to ventilation changes. If the initial concentration is above the allowable limit, the results show how long it takes to fall to the limit once the source stops, which is useful after a spill.
K = 3 is appropriate for most well-designed commercial HVAC systems with good supply and return distribution. Use K = 6 for spaces with ceiling supply air and floor-level sources. Use K = 10 for poor distribution, large spaces, or sources remote from the supply air.
The results show the theoretical minimum airflow (K = 1) and the design airflow with your K factor applied. The decay table shows how long the space takes to purge after the source stops, using the effective flow (design flow divided by K). Use these values to size supply fans, specify outdoor air damper capacity, and set ventilation schedules.
Dilution Ventilation: Theory, Practice, and Canadian Applications
Dilution ventilation is the simplest ventilation control strategy: bring in enough clean outdoor air to reduce the concentration of a contaminant to an acceptable level. It's the basis of ASHRAE 62.1 for occupancy-generated pollutants and forms the backstop for more complex IAQ strategies. Understanding the math behind it lets you size systems correctly and know when dilution alone isn't enough.
The Steady-State Dilution Formula
At steady state, the rate at which outdoor air removes the contaminant equals the rate at which it's generated. The required airflow Q in m³/s equals the generation rate G in m³/s of contaminant divided by the allowable concentration C minus the background concentration C0. Since contaminant concentrations are usually in ppm, the formula becomes Q = G / (C - C0) × 10⁶, where G is in m³/s of contaminant vapour. For example, 30 students producing 0.005 L/s of CO2 each (0.15 L/s in total), with 425 ppm outdoors and a 1,000 ppm limit, need about 0.26 m³/s, or 261 L/s, at K = 1. Before steady state is reached, the space follows an exponential approach curve with a time constant of room volume divided by the effective airflow, which is the design airflow divided by K. The pollutant concentration calculator models this transient behaviour in detail.
The Mixing Factor K
Perfect mixing is a theoretical construct. In real spaces, pockets of stagnant air form near walls, corners, and under furniture. A contaminant source in the middle of a large floor area may not mix uniformly with supply air entering from ceiling diffusers 4 metres above. The mixing factor K multiplies the theoretical minimum airflow so that even in the worst-mixed portions of the space, the contaminant stays below the limit. ACGIH's Industrial Ventilation manual commonly cites K values of 3 to 10, and this calculator applies K as a multiplier on the design airflow while using the design flow divided by K for the purge time constant. Supply air placement is the main lever on K: outlets close to the source and well-chosen throw let you justify a lower value, and the register grille sizing calculator covers sizing the supply outlets. For residential and commercial comfort ventilation per ASHRAE 62.1, the equivalent is the zone air distribution effectiveness factor Ez, which appears in the ventilation effectiveness calculator.
When Dilution Isn't the Right Approach
CCOHS's industrial ventilation guidance limits dilution to situations where the amounts of pollutant generated are not very high, and notes that it cannot be used for highly toxic products, is not effective for dusts, metal fumes, or large amounts of gases and vapours, and requires large amounts of makeup air to be heated or cooled. A common rule of thumb is to reserve dilution for contaminants with limits of about 100 ppm or higher, which is why the toluene preset, with an ACGIH TLV of 20 ppm, should be read as a screening number rather than a design basis. Where the contaminant is highly toxic, a carcinogen, or released at a concentrated point, local exhaust ventilation capturing it at the source is the better control, with dilution as a supplement. The calculator adds that caution to the recommendation whenever the allowable limit is below 100 ppm.
Formaldehyde Off-Gassing in New Canadian Construction
Formaldehyde is one of the most common low-level indoor contaminants in new construction. Pressed wood products, MDF cabinetry, laminate flooring, and furniture off-gas formaldehyde for months to years after installation. Health Canada's formaldehyde guidance recommends a long-term limit of 50 µg/m³ (40 ppb, based on at least an 8-hour average) and a short-term limit of 123 µg/m³ (100 ppb, one hour), and reports daily average levels in Canadian homes of roughly 10 to 40 µg/m³. Take a 200 m³ home with a source releasing 2 mg/hr of formaldehyde. That is about 4.5e-7 L/s of vapour, which needs about 11 L/s of outdoor air at K = 1 and about 34 L/s at K = 3 to hold 0.04 ppm. At 34 L/s the home would need roughly 0.6 ACH, which a well-balanced HRV can supply but infiltration alone will not in a tightly sealed modern home. Our formaldehyde calculator models new construction off-gassing in more detail.
Purge Ventilation After Events
After a contaminant source stops, whether from cooking, painting, or a spill, the space needs time to purge back to background levels. The purge follows an exponential decay with the same time constant as the buildup. A space needs approximately 3 time constants to fall to 5% of the peak excess and 5 time constants to reach about 0.7%. For a 300 m³ space with 50 L/s of effective ventilation, the time constant is 6,000 seconds (100 minutes), so the excess is down to 5% after 5 hours and about 0.7% after 8.3 hours. Doubling the flow halves the time constant, so a temporary purge fan shortens the wait considerably, and the exhaust fan sizing calculator covers sizing one. Keep ventilation at full flow until the concentration is back near background.
VOC Dilution in Retail and Commercial Spaces
New retail fit-outs and commercial renovations release VOCs from adhesives, paints, caulks, and flooring materials. Because required airflow is proportional to the emission rate, a source ten times stronger needs ten times the airflow, and emissions generally fall as materials cure, so ventilation in the first weeks does more work than later. A pre-occupancy flush-out applies the purge math above: run outdoor air at a high flow until the measured concentration is near background, then occupy. The VOC calculator sizes ventilation for specific VOC sources and concentrations.
Frequently Asked Questions
The steady-state dilution ventilation formula is Q equals G divided by C minus C0, where Q is the required airflow in cubic metres per second, G is the contaminant generation rate as a volume of gas or vapour per second, C is the allowable concentration as a fraction, and C0 is the background or inlet concentration. For example, to dilute a solvent generating 0.01 L/s of vapour to an allowable limit of 100 ppm with clean outdoor air, Q equals 0.00001 divided by 0.0001, which gives 0.1 m³/s, or 100 L/s. In practice a mixing factor K is applied to account for imperfect mixing, so the design airflow equals K times the theoretical minimum. ACGIH's Industrial Ventilation manual commonly cites K values of 3 to 10.
Dilution ventilation suits contaminants of low toxicity that are released at a modest, steady rate spread across the whole space, such as low-level CO2 from occupants or VOCs from new furniture. CCOHS notes that dilution cannot be used for highly toxic products, is not effective for dusts, metal fumes, or large amounts of gases and vapours, and needs large amounts of makeup air to be heated or cooled. Local exhaust ventilation (LEV) is the better control when the contaminant is highly toxic, is released at a concentrated point, or is a carcinogen. A common rule of thumb is to keep dilution for contaminants with limits of about 100 ppm or higher. For most residential and light commercial applications where the concern is new building materials, cooking odours, or occupancy, dilution ventilation per ASHRAE 62.1 or 62.2 is the appropriate approach. Industrial processes with solvents, acids, or metal fumes almost always call for LEV supplemented by general dilution ventilation.
The mixing factor K accounts for the fact that real spaces don't mix air perfectly. A K of 1 assumes perfect uniform mixing, which rarely occurs in practice. ACGIH's Industrial Ventilation manual commonly cites K values of 3 to 10, with 3 for spaces with good air distribution and the source near the supply air, and 10 for poor mixing, low ceiling spaces, or sources remote from supply air. For general occupancy comfort ventilation per ASHRAE 62.1, the zone air distribution effectiveness factor Ez serves a similar purpose. This calculator starts at K = 3 for good distribution, and offers 1, 6, and 10 for the other cases.
After a contaminant source stops, the concentration excess above the outdoor background decays exponentially. The time constant tau equals room volume divided by the effective ventilation rate, which is the airflow divided by K (with K = 1 it is simply the airflow). After one time constant, the excess has dropped to about 37% of its starting value. After three time constants it's at about 5%, and after five it's effectively at background. A 200 cubic metre room with 50 L/s of effective ventilation has a time constant of 4000 seconds, or about 67 minutes. Reaching background after five time constants takes about 5.6 hours. During this purge period, the ventilation system should continue operating at full design flow even though the source has stopped.
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