Classroom Example: Measuring Ambient CO₂ to Estimate Ventilation

Purpose:

To estimate the outdoor air exchange in an occupied classroom using a CO₂ sensor (Option D in the Harvard 5-Step Guide).

In an occupied classroom, CO₂ sensors can be used to approximate how well ventilated the space is. For example, a 500 square foot, 10-ft ceiling classroom with 15 teenage students, and a ventilation rate of 4 ACH should have a steady state CO₂ concentration around 800 ppm. That steady state concentration drops to approximately 700 ppm and 650 ppm for 5 and 6 ACH, respectively.

Materials & Methods:

Measure the outdoor CO₂ concentration with the Aranet4 CO₂ sensor. The outdoor ambient CO₂ was 415 ppm at the time of this evaluation.

Location: Kaimuki High School

• Room 1: Student dining area (greater than 45,000 cubic feet), windows open, box fans in several windows, occupancy of 20 students.

• Room 2: Classroom (about 8,400 cubic feet), windows open, occupancy of 15 students.

Estimate the CO₂ generation rate by multiplying the number of occupants in the classroom by their rate of CO₂ exhalation (in CFM). The per person CO₂ generation rate is dependent on age, gender, weight, and metabolic activity. A CO₂ calculator was used to estimate ACH.

Note that this method only considers the dilution effect due to outdoor air flow. If part of your air supply is filtered, you may use a lower target air exchange rate for this calculation as long as the total ACH of clean air still meets the recommended target. For example, if your mechanical ventilation can provide 3 ACH, estimate the steady state concentration at 3 ACH considering that you could add 2 ACH with portable air cleaners to meet the 4 to 5 ACH target.