Why CO₂ is the dashboard light for indoor air quality

Carbon dioxide is not the most dangerous contaminant in a building, yet it is remarkably useful as a diagnostic tool. Humans are constant sources of CO₂, so concentrations spike quickly in poorly ventilated rooms, signaling that other exhaled aerosols and pollutants are likely building up as well. During a long meeting, a congregation, or a classroom session, a handheld CO₂ monitor can reveal when a stuffy feeling is more than a subjective complaint. The challenge is translating those readings into actionable decisions: How much fresh air is enough? How long do you need to open a window or run an exhaust fan after the crowd leaves? This planner treats CO₂ as a measurable proxy for air exchange and gives you a roadmap to maintain healthier indoor conditions.

Unlike temperature, which you can feel instantly, CO₂ builds gradually based on room volume, the number of occupants, and the amount of outdoor air introduced over time. The United States ASHRAE standard for acceptable indoor air quality often targets indoor CO₂ not exceeding about 700 parts per million (ppm) above outdoor levels. That rule of thumb keeps drowsiness and complaint rates low while also correlating with lower concentrations of fine aerosols. But hitting that target requires more than guessing. You need to know the size of the space, how quickly people generate CO₂ at their activity level, and what your ventilation system can deliver. This calculator steps through the mass-balance math so you can dial in a sustainable ventilation strategy.

Modeling indoor CO₂ with a simple mass balance

We assume the air in the room is well mixed, meaning CO₂ concentration is uniform throughout the volume. Let V represent the room volume, G the total generation rate of pure CO₂ from occupants, Q the volumetric flow rate of outdoor air entering the room, C the indoor CO₂ concentration, and C_out the outdoor concentration. The change in indoor CO₂ over time obeys a first-order differential equation. Expressed in MathML, the balance looks like:

$$\frac{\text{d}C}{\text{d}}=\frac{G·106^{}}{V}+\frac{Q}{V}(C_{\mathrm{out}}-C)$$

The factor of one million converts the generation rate from cubic meters per minute of pure CO₂ into ppm. When the room is occupied continuously, the solution tends toward a steady state where the time derivative is zero. Solving for C at steady state yields C_ss = C_out + (G/Q) × 10⁶. That simple expression drives two critical insights: doubling the ventilation rate cuts the excess CO₂ in half, and each additional person increases the required fresh air in direct proportion to their generation rate.

Translating the math into actionable metrics

The planner converts your inputs into a unified unit system behind the scenes. Room dimensions in feet become cubic meters, while ventilation rates in cubic feet per minute (CFM) or cubic meters per hour both land in cubic meters per minute. A sedentary adult generates roughly 0.3 liters of CO₂ per minute, which equates to 0.0003 cubic meters per minute. Multiply that by the number of occupants to find G. If you have more active occupants, like a yoga class or a choir rehearsal, increase the generation rate accordingly. Once we compute G and know the target indoor concentration, we rearrange the steady-state equation to find the ventilation rate required to stay below your threshold. If the existing system falls short, the tool shows by how much.

Purging a room after it empties uses the same formula but treats Q as the boosted ventilation you can achieve by opening windows or turning on additional fans. The solution to the differential equation is exponential, so each additional air change reduces the remaining excess by the same fraction. The time constant is τ = V/Q. After one time constant, the excess CO₂ drops to about 37 percent of its initial value; after three time constants, it is down to 5 percent. The calculator solves for the precise time needed to reach your target concentration and warns you if your boost ventilation cannot achieve the goal because the steady-state with that ventilation is still above the target.

Worked example: preparing a conference room turnover

Imagine a conference room measuring 18 feet by 14 feet with an 8.5-foot ceiling. Six people will occupy the room during an hour-long meeting, and a CO₂ monitor shows the concentration has climbed to 1,500 ppm by the end. Outdoor levels hover around 420 ppm. The office HVAC supplies about 250 CFM continuously, and you can open windows to augment airflow up to roughly 800 CFM during breaks. Plugging those numbers into the planner yields a room volume of 2,142 cubic feet (60.7 cubic meters). The six sedentary occupants generate about 0.00192 cubic meters of CO₂ per minute. At 250 CFM, the steady-state concentration would settle near 1,190 ppm—higher than the 900 ppm target. To stay below 900 ppm during occupancy, the space would need around 450 CFM, meaning you should either upgrade the mechanical ventilation or reduce the number of people in the room.

After the meeting, you plan to open windows and run fans at the 800 CFM purge rate. The tool calculates a time constant of roughly 2.9 minutes. Solving the exponential decay shows that it will take about 8.1 minutes to drop from 1,500 ppm to 900 ppm. The comparison table reveals that keeping the windows cracked during occupancy would bring the steady-state down to 765 ppm, whereas relying solely on the existing ventilation leaves you above 1,100 ppm. With those numbers in hand, you can schedule a ten-minute buffer between meetings to guarantee fresher air for the next group.

Reading the scenario comparison table

The table summarizes three default scenarios: the existing ventilation, the boosted purge plan, and the calculated requirement to stay below the target during occupancy. Each row lists the ventilation rate in the units you selected, the corresponding air changes per hour, the steady-state CO₂, and the time needed to reach the target if the room starts at your current reading. When the existing ventilation cannot achieve the desired indoor concentration, the time-to-target entry shows a dash, reminding you that no amount of waiting will solve the issue without more airflow. Conversely, the boost scenario typically shows a short purge time, confirming that strategic window openings or temporary fans can deliver quick wins even if the permanent system is modest.

Experimenting with occupancy and activity

The planner invites experimentation so you can design policies that keep air fresh without overinvesting in equipment. Try increasing the occupant count to see how much ventilation each additional person demands. You might discover that capping a workshop at eight participants keeps CO₂ under 1,000 ppm with current ventilation, while ten attendees would breach the threshold unless you add an exhaust fan. Similarly, adjust the generation rate upward for singing or vigorous exercise, where CO₂ output may double. The results show that activity level matters almost as much as headcount, guiding decisions about which rooms to assign for different events.

Seasonal changes also affect your approach. In winter, you may prefer not to open windows wide. Use the calculator to determine how long a smaller boost—perhaps 500 CFM instead of 800—would take to restore good air. If that pushes the purge time from eight minutes to fifteen, you can weigh the comfort trade-off against the schedule. During mild weather, you might leave windows cracked throughout the meeting to keep the steady-state low, trading a slight temperature fluctuation for more consistent air quality.

Assumptions and responsible interpretation

This model treats the room as perfectly mixed, yet real spaces have stagnant corners and faster-moving zones near vents. The planner therefore offers a directional guide rather than a precise prediction. If occupants cluster in one area or if the air supply is unbalanced, local CO₂ readings may overshoot the average. The tool also assumes that ventilation air is 100 percent outdoor air; some systems recirculate a portion, which effectively reduces Q. You can compensate by entering the outdoor-air fraction of the total flow. Finally, human CO₂ output varies by age, activity, and metabolism. The provided defaults represent sedentary adults; adjust upward for active scenarios or downward for children. Pair these estimates with real-time CO₂ monitoring to validate your plan, and engage qualified HVAC professionals when commissioning permanent system upgrades.