Ventilation in schools: CO2 limits, rules and monitoring

There is no single CO2 limit for ventilation in schools. The numbers that exist come from national guidance documents, they run from 800 ppm to 2,000 ppm depending on the country and the purpose, and most are guidance. Where a source does not say whether a number is binding, the text says so.

CO2 values for ventilation in schools by country

• England: Building Bulletin 101 (BB101) is Department for Education guidance on ventilation in schools, last updated 23 August 2018, and the gov.uk page says it is used to satisfy Section 1 of Approved Document F of the Building Regulations 2010. The DfE's August 2018 consultation response, which calls these DfE performance standards set in addition to the regulations, says naturally ventilated teaching spaces are allowed a daily average CO2 of 1,500 ppm, higher than the 1,000 ppm for mechanically ventilated ones. It adds a further criterion of under 1,200 ppm for the majority of the occupied period in new natural or hybrid buildings and under 1,750 ppm in refurbishments. Neither the response nor the gov.uk page says whether these figures are legally binding, and the full bulletin was not read.
• Germany: on an undated page, the Federal Environment Agency (UBA) says classroom air with up to 1,000 ppm CO2 counts as good, a CO2 traffic light turns yellow above that and usually red above 2,000 ppm, and a room is adequately ventilated if the average stays at or below 1,000 ppm during a lesson. The underlying guide values from 2008 list three ranges: below 1,000, 1,000 to 2,000 and above 2,000 ppm. UBA calls its school advice recommendations (Empfehlungen) and the text does not present the numbers as legal limits. It advises airing every 20 minutes or more with windows wide open.
• Netherlands: the Programma van Eisen Frisse Scholen 2025, published by RVO, sets a maximum CO2 level in the breathing zone during use of 800 ppm for class A, 950 ppm for class B and 1,200 ppm for class C, with fresh air of at least 12, 8.5 and 6 dm³/s per person. It assumes 400 ppm outdoors and a class of 30 pupils and one teacher. The document carries no publication date and describes itself as a guide ("een handreiking"), explicitly no label, and the client chooses the level. It says class C is below the legal requirement for new buildings (Bbl article 4.122). The quality assurance table lists a CO2 sensor that meets Bbl specifications (articles 3.143 and 4.243) in the class C column, and class A adds CO2 logged every 10 minutes with data kept for at least 12 months.
• Ireland: a May 2021 government document about the CO2 monitors that the Department of Education funded for schools (the text does not name its publisher) says CO2 concentrations above 1,400 to 1,500 ppm, citing CIBSE and AIVC, are likely to indicate poor ventilation, and advises opening windows and doors. A Dáil answer of 2 December 2021 says more than 35,400 monitors were delivered at a cost of about €4 million. The document presents the figure as an indicator of poor ventilation, not as a legal limit.

Thresholds and rates from WHO, REHVA and ASHRAE

• WHO: its 2021 roadmap for good indoor ventilation in the context of COVID-19 gives no CO2 limit. For non-residential settings it proposes a minimum of 10 L/s per person, citing EN 16798-1, and names schools only as an example, with no classroom-specific rate.
• REHVA: version 4.1 of its COVID-19 guidance (15 April 2021) suggests setting traffic-light CO2 monitors so the warning comes at 800 ppm and the alarm at 1,000 ppm, as a temporary change during the epidemic, and repeats this for schools in Appendix 4. It is interim guidance. It also gives an office example of how rates and CO2 relate: 2 L/(s·m²) can keep CO2 below 800 ppm if there are at least 7 m² of floor area per occupant, and 1 L/(s·m²) below 1,000 ppm at 10 m² per person.
• ASHRAE: Standard 62.1-2022, Table 6-1, as shown in addendum w (approved 30 September 2025), gives classrooms for ages 5 to 8 and for age 9 and older 5 L/s per person plus 0.6 L/s per m² of floor area (10 cfm per person plus 0.12 cfm per ft²). ASHRAE states that Standards 62.1 and 62.2 do not include CO2 limits and that claims of a 1,000 ppm requirement in 62.1 are incorrect. The standard itself is paywalled, so these values come from ASHRAE's published addendum.

How classroom CO2 monitors are used

For monitoring ventilation in schools, the Welsh Government's guidance on CO2 monitors in education settings (15 October 2021) sets out three bands and an action for each. The page is labelled guidance.

The same guidance says to wait five minutes and retake a high reading, to take several readings across the day and average them for the occupied period, and to keep the monitor at least 0.5 m from people. The Irish guidance says to measure for at least an hour because spot readings mislead. UBA suggests breathing height (about 1.5 m for seated people) in the middle of the room, away from windows, walls and the corridor, and REHVA says away from fresh-air inlets. ASHRAE adds that sensor accuracy, location and calibration are all critical for meaningful readings.

The Welsh guidance states that CO2 monitors are "an indicator of ventilation status and not infection risk", and ASHRAE notes that CO2 is unaffected by filtration and most other air-cleaning methods.

What a CO2 reading leaves out

CO2 tracks ventilation per person. It does not show particles, moisture or gases from materials, which Indoor air quality in schools: what to monitor beyond CO2 covers. Ventilation rates in offices and classrooms explains how rates and CO2 relate. ASHRAE also notes that the evidence on CO2 and health, learning and work performance is inconsistent.

Nomestic measures indoor climate room by room (CO2, temperature, humidity and more), so a school can see which classrooms run high and when. The devices are wireless and battery-powered, so a room takes minutes to equip, and the system works with or without a building management system. The Airsight CO2 Monitor uses an NDIR CO2 sensor, and the schools page has more.