However, the need for appropriate ventilation has been a long-standing issue, and the justification goes far beyond diluting the potential presence of SARS-CoV-2 in the classroom. This article will focus specifically on the drivers for good-quality ventilation in schools – although the messages are equally appropriate for all places of learning.
Indoor environmental quality (IEQ) is dependent on an amalgam of factors, and when assessing strategies and options for ventilation it cannot be considered in isolation. CIBSE TM571 Integrated school design describes the four interrelated factors for successful environmental design of school buildings as daylight, thermal comfort, low and zero carbon energy, and ventilation. TM57 covers all these areas in some detail, and emphasises that building form and thermal mass underpin successful strategies for daylight, ventilation and energy. While recognising the interdependence of all the factors for a successful internal environment, this article will specifically consider ventilation.
The cocktail of contaminants that make up the internal environment has been discussed frequently (including in the first section of CPD 180 in CIBSE Journal June 2021) with the key chemical contaminants (in Europe) being2 carbon monoxide (CO) and nitrogen dioxide (NO2), and the volatile organic compounds (VOCs) formaldehyde, benzene and naphthalene. With potentially lesser significance, but highly dependent on exposure levels, are CO2, bacteria and viruses, particulate matter (PM2.5, PM1.0 and ultrafine particles) and, in some locations, gases such as radon. It is important to note that CO2, which has a concentration of just over 410ppm in outdoor air, is used both as a marker – or proxy – for estimating the actual ventilation rates in buildings, as well as being a potential contaminant that is being exhaled by occupants. This article relates to CO2 as a contaminant.
CIBSE Technical Memorandum TM40 Health and wellbeing in building services notes that, in general, exposure to air pollutants can have both acute and chronic health effects, from mild to severe – and occupants are unlikely to even be aware of the contaminants in the air that they are breathing. The likelihood and severity of the impact of contaminants on humans depends on age, any pre-existing medical conditions and individual sensitivity. In schools, there are often pollutants generated from the building and various materials used in the teaching spaces, together with local complications such as mould, asbestos and radon. A recent review paper produced by Swegon, Air quality and ventilation in schools,3 reiterates the much-voiced opinion that classroom ventilation rates are directly associated with students’ academic achievements,4 with a poor-quality indoor environment thought to cause discomfort, distract attention and reduce motivation.
Much of the potential contamination in the teaching rooms can emanate from the space itself, but will be dependent on the situation. For example, the presence of materials that emit VOCs – which can include adhesives, insulation, wall boards, furniture, carpets and fabrics – have been widely implicated in adversely affecting occupants. Levels of CO2 will primarily depend on the occupants. The Swegon review paper notes that a child aged seven to nine years old will produce half the amount of CO2 compared with a teenager – this will make a significant difference when assessing the accumulating CO2 levels in the internal environment. A recent research review by Fisk,5 which considered research across the world, indicates time average CO2 values in surveyed classrooms ranged from 1,400ppm to 5,200ppm (in studies where 20 or more classrooms where investigated). This is illustrated, categorised by type of ventilation system, in Figure 2. Fisk notes that ‘concentrations of CO2 do not appear to be systematically higher or lower in naturally ventilated classrooms relative to mechanically ventilated classrooms’. His review reports that studies found average or median ventilation rates to be in the range of 3L·s-1 to 5L·s-1 per person, with one average as low as 1L·s-1 per person, and he concludes that there is ‘a widespread failure to provide the minimum amount of ventilation specified in standards for classrooms’.
Adverse effects have been reported for elevated CO2 levels in classrooms, including decreased satisfaction with indoor air quality (IAQ),6 students experiencing greater fatigue and impaired attention span, and lower levels of focus among university students during lectures.
The link between ventilation and achievement was observed in a study by Toyinbo et al,7 where preliminary analyses indicated statistically significant poorer results in mathematics tests in schools where the ventilation rate was lower than 6L·s-1 per person. Toyinbo also links lower ventilation rates to more missed school days caused by respiratory infections. Lower ventilation rates may lead8 to increased asthmatic symptoms, and the risk of viral infections through the concentration of bioaerosols emitted from the occupants. Carrion-Matta9 noted research that indicated that high indoor concentrations of PM2.5 have been associated with asthma and cognitive impairment. The levels of particulates in the indoor air were determined as being strongly related to the outdoor air quality (rather than resulting from activities in the space itself) and were typically worse when ventilation rates were higher. This can pose a difficult compromise in locations with poor outdoor air quality and will increase the dependence on air-cleaning devices, such as particulate filters.
In the US, Haverinen-Shaughnessy observed10 a link between ventilation rates and performance on standardised tests in maths and reading for nine to 10-year-olds, estimating that each 1L·s-1 per person increase in ventilation rate was associated with a mean increase of 0.5% in maths scores. A UK study11 by Clements-Croome et al indicates that pupils’ performance is increased by a very significant 15% in various tasks when ventilation rates in teaching spaces are increased from 0.7–1L·s-1 per person to 6-8L·s-1. Fisk’s review5 indicates similar improvements in performance with increased ventilation rates ranging from a few per cent up to as high as 15%.
Recently, Wargocki et al12 reviewed data from published studies to derive systematic relationships between learning outcomes and air quality in classrooms, which predict that reducing CO2 concentration in classrooms (from the typically higher values as illustrated in Figure 2) to 900-1,000ppm would significantly improve performance in school tasks, concentration and daily attendance. In terms of ventilation rates, these results suggest that increasing ventilation rates from 2L·s-1 to 7.5L·s-1 per person will improve pupils’ performance in national tests by 5%, and children’s daily attendance by 1.5%. Notably, Wargocki concludes that the results provide a strong incentive for improving classroom air quality, and this might be assessed as part of cost-benefit analyses in systems design and operation.
As classrooms are typically densely occupied spaces – with a common rule of thumb being between 2m2 and 4m2 floor area per student – TM57 notes that the ventilation rates required to maintain good IAQ are high in winter and even higher in summer. It is noted, in this pre-Covid publication, that the wider interest in IAQ of educational buildings is underpinned by the rising incidence of asthma and respiratory disease among children.
In urban applications, the outdoor air may not be necessarily ‘fresh’ (see panel, ‘Fresh’ air?) but, practically, to maintain a good indoor environment, the school building needs to have an air replacement system employing adventitious and/or controlled air movement. Outdoor air may be introduced by infiltration (through a leaky building envelope), airing (manual operation of openings such as windows and doors) and controlled ventilation (natural, mechanical or hybrid). TM57 highlights that the ventilation should always be available, and that this should include secure night ventilation. Typically, the ventilation solution must provide adequate IAQ that, while minimising heating requirements, should also be configured and integrated to reduce the potential risks of overheating in an energy efficient way and be able meet the need for ‘summer’ cooling. The authors of TM57 note that ‘common complaints in new schools are that they are stuffy, suffer from overheating, and are under-ventilated’.
Current regulatory frameworks focus on CO2 concentrations as an indication of air quality, as it is a useful metric that can be used to estimate ventilation rates, the dilution of pollutants from indoor sources, and the improvement of perceived IAQ. The recently introduced BS EN 16798-113 gives three methods (in section 6.3.2) of determining minimum ventilation rates for non-residential buildings – such as schools – to provide a satisfactory IAQ. Each method provides calculation constants (in Annex B3) based on four categories of indoor environmental quality – I, II, III and IV (high to low). Calculation in the three methods is based on simple, steady-state conditions.
Method one is established on perceived air quality and is provided by the sum of empirically derived, tabulated ventilation rates, based on the number of occupants plus an area-based rate (to allow for pollution from the building and the ‘systems’). Method two provides a simple formula for the dilution of a contaminant in the air through the introduction of outdoor air. Practically, this would be used to determine the ventilation required to dilute the level of CO2, based on steady-state ventilation (regardless of other contaminants). This requires careful consideration to ensure systems are designed appropriately, as the guesstimates of occupant CO2 emissions appear to be highly variable. The example data provided in the standard is based on CO2 emissions of 23.3L·h-1 per person for a ‘kindergarten’ – it is challenging to find peer-reviewed estimates. Method three is based on pre-defined ventilation airflow rates (based either on occupant numbers or room area), and is probably the least useful – although possibly the simplest – method when assessing teaching spaces. It is probably included in the standard as a catch-all method to allow application of the standard where local regulations have specific designated ventilation rates. For worked examples, a reliable reference – with extended and corrected data and explanations – is the interpretation document PD CEN/TR 16798-2:2019.23
The quality of outdoor air – which is often referred to in building services as ‘fresh’ air – will have a significant influence on the ability to ventilate. The World Health Organization (WHO) guidlines14 for outdoor air quality are becoming more stringent, as illustrated in Figure 1. Outdoor air quality has shown general improvement15 over recent years, particularly in the ‘western’ world,16 as a result of various regulatory and technological measures, with the exception of ground-level ozone,17 which – as well as increasing in warm weather – perversely increases as nitrogen dioxide (NO2) levels reduce.18 However, a changing climate has increased the prevalence of wildfires19 with their associated particulate matter, wind-carried dust,20 and uncertainties in ozone levels21 that can have potentially significant health impacts22 on building occupants if transferred into the internal space.
The recommendations for IAQ, published by WHO,24 CIBSE, ASHRAE and, in the UK, Building Bulletin 10125 typically set limiting values of average daily CO2 concentration to between around 1,000ppm and 1,500ppm, with various restrictions of acceptable peak values. For a category-II application, which might be considered appropriate for a teaching space, BS EN 16798-1 suggests a level of 800ppm.
Natural ventilation would always be the preferred option, but constraints of location, occupant density and building configuration will often make this technically challenging to achieve in an energy- or cost-efficient way – or just simply impossible. For example, as reported26 by Jain et al, relating to a study of a London school campus, monitoring indicated increased levels of traffic-related pollutants during the heating season, and that ‘this suggests activated carbon filters or other measures are required’ (in addition to particle filters) that could be coupled with ventilation controls to balance the requirement for ‘fresh’ air to create a healthier environment while ensuring energy efficiency.
Ventilation is a key component of successful school design and operation. As with most subdisciplines within building services engineering, there is no single solution for how the ventilation should be best achieved. The key deliverable is the quality of the air that is being inhaled by the building occupant. Whether that is assessed by real-time monitoring – for example, by measuring levels of CO2 to provide demand-led ventilation – or by attempting to meet prescriptive minimum ventilation rates, current evidence indicates that, in many cases, ventilation is inadequate and, in all likelihood, may be adversely impacting the wellbeing and performance of the students.
© Tim Dwyer, 2021.
The freely downloadable BB101 Guidelines on ventilation, thermal comfort and indoor air quality in schools is based on UK requirements but provides comprehensive guidance that is widely applicable.
CIBSE TM57 Integrated school design provides accessible guidance for all members of the design team to bring their understanding of all aspects of building design and performance to influence a more informed design. It also provides a gateway to the myriad of other documents that will provide further knowledge and understanding of this topic.
A recent paper by Khovalyg et al provides a useful comparison of standards in a Critical review of standards for indoor thermal environment and air quality
1 CIBSE TM57 Integrated school design CIBSE 2015.
2 WHO global air quality guidelines, WHO 2021.
3 Emissions of air pollutants in the UK – accessed 1 November 2021.
4 Air Quality Historical Data Platform, The World Air Quality Project , – accessed 1 November 2021.
5 National statistics Concentrations of ozone, UK DEFRA –bit.ly/CJDec21CPD2– accessed 1 November 2021.
6 UK’s first lockdown reduced air pollution but caused surface ozone levels to rise, National Centre for Atmospheric Science, – accessed 1 November 2021.
7 Hirschlag, A, The long distance harm done by wildfires, BBC, 2020, – accessed 1 November 2021.
8 Milford, C, Impacts of Desert Dust Outbreaks on Air Quality in Urban Areas, Atmosphere, 2019, – accessed 1 November 2021.
9 Ozone threat from climate change, Science Daily, 2019, – accessed 1 November 2021.
10 Graham, A, et al, Impact on air quality and health due to the Saddleworth Moor fire in northern England, Environ, Res, Letters 2020, bit.ly/CJDec21CPD10 – accessed 1 November 2021.
11 The INDEX project – Critical Appraisal of the Setting and Implementation of Indoor Exposure Limits in the EU, – accessed 1 May 2021.
12 Air quality and ventilation in schools, Swegon, 2021.
13 Haverinen-Shaughnessy, U, et al, Association between substandard classroom ventilation rates and students‘ academic achievement, Indoor Air 2011, bit.ly/CJDec21CPD12
14 Fisk ,WJ, The ventilation problem in schools: literature review, Indoor air, 27(6) 2017.
15 Chatzidiakou, L, et al, A Victorian school and a low carbon designed school: Comparison of indoor air quality, energy performance, and student health, Indoor and Built Environment, 2014.
116 Toyinbo, O, et al, Building characteristics, indoor environmental quality, and mathematics achievement in Finnish elementary schools, Building and Environment, 2016,
17 Chatzidiakou, L, et al, What do we know about indoor air quality in school classrooms? A critical review of the literature, Intelligent Buildings International, 2012.
18 Carrion-Matta, A, et al, Classroom indoor PM 2.5 sources and exposures in inner-city schools, Environment International, 131, 2019.
19 Haverinen-Shaughnessy, U, et al, Effects of Classroom Ventilation Rate and Temperature on Students’ Test Scores, PLoS One, 2015,
20 Clements-Croome, DJ, et al, Ventilation rates in schools Building and Environment, Vol 43, Iss 3, 2008.
21 Wargocki, P, et al, The relationships between classroom air quality and children’s performance in school, Building and Environment,173, 2020.
22 BS EN 16798-1 Energy performance of buildings – Ventilation for buildings Part 1: Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics, British Standards Institution, 2019.
23 PD CEN/TR 16798-2:2019 Energy performance of buildings…, BSI 2019.
24 Selected Pollutants, WHO – accessed 1 November 2021.
25 Building Bulletin 101 Guidelines on ventilation, thermal comfort and indoor air quality in schools, UK Education and Skills Funding Agency, 2018,
26 Jain, N, et al, Measuring beyond energy in a school POE, CIBSE Journal, 2019,