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CO2 as an indoor air quality proxy in Canadian buildings
CO2 itself isn't typically harmful at the levels seen in occupied buildings — measurable cognitive effects don't usually appear until levels far exceed typical indoor readings. Its real value as a metric is as a proxy for overall ventilation adequacy: since people are essentially the only significant CO2 source in most occupied spaces, and CO2 generation correlates with other human bioeffluents and pollutants, CO2 concentration tells you how well diluted those other contaminants are.
ASHRAE's Indoor Air Quality Procedure in Standard 62.1 allows using CO2 as one input to a performance-based ventilation design, generally targeting indoor CO2 approximately 700 ppm above outdoor background. With current outdoor CO2 around 420 ppm, this translates to a design target near 1,000-1,100 ppm indoors at peak occupancy.
The steady-state CO2 mass balance equation
The governing equation is a simple mass balance: at steady state, the rate of CO2 generation by occupants equals the rate of CO2 removal by ventilation. Cin = Cout + (N × G) / Q, where G is the per-person generation rate (varies with metabolic activity), N is occupant count, and Q is outdoor air ventilation rate. This calculator lets you solve either direction: predict CO2 from a known ventilation rate, or find the ventilation rate needed to maintain a target CO2 level.
Steady state assumes the room has been continuously occupied long enough for CO2 concentration to stabilize, typically 30-60 minutes depending on room volume and air change rate. In rooms with intermittent or transient occupancy, actual peak CO2 may be lower than the calculated steady-state value.
Demand-controlled ventilation using CO2 sensors
Many Canadian commercial buildings now use CO2 sensors to modulate outdoor air dampers dynamically, increasing ventilation when occupancy (and CO2) rises and reducing it during low-occupancy periods to save energy. This calculator helps establish the CO2 setpoint for demand-controlled ventilation systems and verify the system will provide adequate ventilation at peak design occupancy. Use the ventilation rate calculator to establish the ASHRAE 62.1 minimum rate as a baseline before applying demand control logic.
Frequently Asked Questions
Cin = Cout + (N × G) / Q, where Cin is indoor CO2 (ppm), Cout is outdoor background (~420 ppm), N is occupant count, G is per-person generation rate (~0.0052 L/s seated/light activity), and Q is outdoor air rate (L/s). For 25 students with 250 L/s OA: Cin = 420 + (25×0.0052×1000)/250 = 940 ppm. This assumes steady-state equilibrium after 30-60 minutes of continuous occupancy. Use the calculator above to solve for either CO2 or required ventilation.
ASHRAE's IAQ Procedure targets roughly 700 ppm above outdoor background, suggesting about 1,000-1,100 ppm indoors as the design point with current ~420 ppm outdoor levels. CO2 above 1,000 ppm doesn't directly harm health but indicates insufficient dilution of other bioeffluents, correlating with stuffiness and reduced cognitive performance in studies. Many Canadian school boards use 1,000 ppm as an action threshold for ventilation assessment, with some adopting lower thresholds post-COVID.