| Parameter | Value | Note |
|---|
Diluting an Indoor Contaminant With Clean Air
When something in a room keeps releasing a gas, the concentration rises until the air leaving carries away as much as the source adds. Ventilation sets where that balance falls. This calculator finds the clean airflow that puts the balance at a target you choose, for a source that is spread out and steady, such as new cabinetry, flooring or a stored product.
The Mass Balance for a Room
At steady state the contaminant leaving equals the contaminant entering, so airflow equals generation divided by the allowed rise over the outdoor level. A source adding 3 mg/h of formaldehyde, held to 0.05 mg/m³ with none in the outdoor air, needs 3 ÷ 0.05 = 60 m³/h if the room mixes perfectly. A mixing factor of 3 raises that to 180 m³/h, which is 50 L/s or 106 CFM, and in a 150 m³ room it is 1.2 air changes an hour. The 3 mg/h is an illustration. Real emission rates come from a data sheet, a chamber test or a measurement, and they vary with temperature, humidity and the age of the material.
Picking a Target Concentration
For an occupied building the natural starting point is the Health Canada residential indoor air quality guidelines. They give formaldehyde as 50 µg/m³ over 8 hours and 123 µg/m³ over 1 hour, acetaldehyde as 280 µg/m³ over 24 hours, toluene as 2.3 mg/m³ over 24 hours, xylenes as 150 µg/m³ over 24 hours, naphthalene as 10 µg/m³ over 24 hours, carbon monoxide as 11.5 mg/m³ over 24 hours and nitrogen dioxide as 20 µg/m³ over 24 hours. The preset list in the calculator fills these in as mg/m³. Match the averaging time to the source. A short burst from a cleaning task suits a 1-hour value, and a slow release from furnishings suits an 8-hour or 24-hour one. These are residential guidelines. Workplace exposure limits are set for a different purpose and are far higher.
Mixing, and Why the Air Must Be Clean
The formula assumes the contaminant spreads evenly, and a real room has richer pockets near the source. The factor K inflates the airflow to cover that. Industrial ventilation practice commonly uses 3 to 10, and the choice is a judgment about how close people sit to the source and how toxic it is. The airflow also has to be clean. Outdoor air counts, and so does air that has had the contaminant removed. Plain recirculated air does not dilute a gas, because it returns the same gas to the room. For particles, a filter can supply clean air, and the air filtration calculator looks at filter performance.
How Long the Room Takes to Settle
Steady state is not instant. A well-mixed room moves toward it along an exponential curve with a time constant equal to the room volume divided by the airflow. The 150 m³ room at 180 m³/h has a time constant of 0.83 hours, and it reaches 95% of the steady value after three time constants, about 2.5 hours. The calculator shows both numbers when you enter a volume. The ACH calculator converts between airflow and air changes for the same room.
When Dilution Is the Wrong Tool
Dilution has limits. The Canadian Centre for Occupational Health and Safety lists them in its introduction to industrial ventilation: it cannot be used for highly toxic products, it is not effective for dusts, metal fumes or large amounts of gases and vapours, and it does not handle surges or irregular emissions well. It also needs a lot of makeup air to be heated or cooled. For a concentrated source, capture the contaminant at the source with local exhaust, and use dilution for the low, steady background that remains.
Formaldehyde and Other Off-Gassing Sources
Composite wood products, adhesives and some finishes release formaldehyde for months after installation, and the release rises with heat and humidity. Dilution lowers the concentration while the source runs down, but it does not remove the source. The formaldehyde calculator looks at this contaminant in more detail, and the best control for a known source is to remove it or choose a low-emitting material.
Frequently Asked Questions
Divide the contaminant generation rate by the concentration you will allow above the outdoor level: Q = G ÷ (C target − C outdoor), with G in mg/h and the concentrations in mg/m³. A source releasing 3 mg/h of formaldehyde, held to the 0.05 mg/m³ eight-hour guideline with none in the outdoor air, needs 60 m³/h at perfect mixing. With a mixing factor of 3 the design airflow is 180 m³/h, which is 50 L/s or 106 CFM. The 3 mg/h emission rate is an illustration, so use a measured or data-sheet figure for a real source.
For a home or other occupied building, start from Health Canada's residential indoor air quality guidelines. Formaldehyde is 50 µg/m³ (0.05 mg/m³) over 8 hours and 123 µg/m³ over 1 hour, toluene is 2.3 mg/m³ over 24 hours, and carbon monoxide is 11.5 mg/m³ over 24 hours. Choose the value whose averaging time matches how your source behaves, and note that workplace exposure limits are set for a different purpose and run much higher.
CCOHS says dilution ventilation cannot be used for highly toxic products and is not effective for dusts, metal fumes, large amounts of gases or vapours, or surges and irregular emissions. It suits low, steady emission rates with moderate toxicity and nobody working near the source. A concentrated or toxic source calls for local exhaust at the source, and a surface that off-gasses calls for removing or sealing the source where possible.
A well-mixed room approaches its steady concentration along an exponential curve with a time constant of room volume divided by airflow. A 150 m³ room with 180 m³/h of clean air has a time constant of 0.83 hours, and reaches 95% of the steady value in about 2.5 hours, which is three time constants. The airflow must be outdoor air or air that has had the contaminant removed, because recirculated air carries the contaminant back into the room.