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CO2 as an Indoor Air Quality Indicator
People are the main CO2 source in most occupied rooms, and CO2 rises alongside the other contaminants they release, so indoor CO2 shows how well those are being diluted. At the levels found indoors it is not a direct health risk. ASHRAE's technical FAQ on CO2 gives no fixed limit and treats it as an indicator of body odour and ventilation, and it warns that CO2 is not a reliable indicator of overall air quality.
Two rules of thumb come from that FAQ. A steady rise of about 700 ppm over outdoor air corresponds to roughly 7.5 L/s of outdoor air per person at typical office activity. Indoor levels of 1,000 to 1,200 ppm in sedentary spaces suggest most visitors will find body odour acceptable. With outdoor air near 425 ppm, the first rule lands at about 1,125 ppm indoors. Both are screening values, not pass or fail limits.
The Steady-State CO2 Mass Balance
At steady state, the CO2 people produce equals the CO2 ventilation removes: Cin = Cout + 1,000,000 × N × G ÷ Q, with G and Q both in L/s. For 25 seated people (G = 0.0052 L/s each) and 250 L/s of outdoor air, the rise is 520 ppm, so the room settles at 945 ppm with a 425 ppm background. The default background of 425 ppm follows NOAA's reading of 426 ppm at Mauna Loa for September 2026, a seasonal low, so enter a measured outdoor value if you have one.
A room approaches that value gradually, with a time constant equal to one air change. It reaches about 95% of the final value after three time constants, roughly 90 minutes at 2 air changes per hour and 30 minutes at 6. A room used for less time than that will peak below the calculated value.
Activity Level and CO2 Generation
CO2 production scales with metabolic rate. The calculator uses 0.0052 L/s per person at 1.2 met, the ASTM D6245 value, and scales it linearly: 0.0087 at 2 met, 0.0173 at 4 met and 0.026 at 6 met. Measured data compiled by NIST put adults at 1.2 met a little lower, near 0.0035 to 0.0048 L/s, so the seated result leans slightly high. The same metabolic rate drives the heat people add to a room, which the heat gain from people calculator covers.
Demand-Controlled Ventilation Using CO2 Sensors
Many Canadian commercial buildings use CO2 sensors to modulate outdoor air dampers, raising ventilation when occupancy rises and cutting it when rooms empty to save energy. This calculator helps set a CO2 setpoint and check that the system still delivers enough air at peak occupancy. Use the ventilation rate calculator to establish the code minimum as a baseline before applying demand control logic. For a second CO2 estimate from the air quality side, see the CO2 level calculator.
Frequently Asked Questions
Cin = Cout + 1,000,000 x N x G / Q, where Cin is indoor CO2 in ppm, Cout is the outdoor background (about 425 ppm), N is the number of occupants, G is the CO2 generation rate per person in L/s (0.0052 for a seated adult) and Q is the outdoor air rate in L/s. For 25 students with 250 L/s of outdoor air: Cin = 425 + 1,000,000 x 25 x 0.0052 / 250 = 945 ppm. This value assumes the room has reached equilibrium, which takes roughly three air-change time constants, about 90 minutes at 2 air changes per hour and 30 minutes at 6.
There is no single level. ASHRAE's technical FAQ on CO2 treats it as an indicator of body odour and ventilation, not a direct health risk at normal indoor levels. A steady rise of about 700 ppm over outdoor air corresponds to roughly 7.5 L/s of outdoor air per person at typical office activity, and indoor levels of 1,000 to 1,200 ppm in sedentary spaces suggest most visitors will find body odour acceptable. Health Canada's long-term residential guideline is 1,000 ppm as a 24-hour average. Use these as screening values, not pass or fail limits.
CO2 production scales with metabolic rate. This calculator uses 0.0052 L/s per person at 1.2 met, the figure from ASTM D6245, and scales it by met: 0.0087 at 2 met, 0.0173 at 4 met and 0.026 at 6 met. NIST's review of measured data puts adults at 1.2 met nearer 0.0035 to 0.0048 L/s, so seated results here lean slightly high. Doubling the activity doubles the CO2 rise for the same ventilation rate.
No. ASHRAE states that CO2 is not a reliable indicator of overall building air quality. It tracks what people exhale, and the odours that come with them, but not volatile organic compounds from materials, particles or other sources. Use CO2 to check ventilation per person, and use the ventilation rate calculator for the code-based minimum.