EDUCATION | CLASSROOM VENTILATION To optimise classroom indoor air quality, schools need to take a holistic approach to natural ventilation and air cleaning technology, argues CIBSEs Chris Iddon Optimising ventilation in the post-Covid classroom T here has been much clamour in recent months about the poor state of school ventilation and the need for investment in air cleaners for classrooms1 but is ventilation in schools poor? Since the start of the pandemic, there has been growing recognition of the importance of virus carried in exhaled breath in the transmission of SARS-CoV-2.2 Virus shed in the respiratory system of an infected individual become encapsulated in droplets of respiratory fluid. These droplets have a continuum of sizes (and, therefore, volume of fluid) ranging from <1m to 100m-plus. Once exhaled, they can decrease in size because of evaporation, and the smallest droplets (sub 5m) often termed aerosols can remain airborne for hours, building up in poorly ventilated indoor spaces. Over time, the concentration of viable virus in the air of a room containing an infector will reach a steady state. The poorer the ventilation, the greater the steady-state concentration of virus in the air. Are schools poorly ventilated? The importance of ventilation in providing suitable air quality for comfort, concentration and health has long been understood. Before the early 20th century,3 the only way to provide outside air was by natural means exploiting the natural forces to encourage outside air in and exhausting contaminated air out. This principle has remained popular for UK classrooms and most schools have a natural ventilation strategy, although mechanical and mixed-mode ventilation are becoming more popular especially in newer schools. Most natural ventilation designs require the opening of a vent, usually a window, to provide a means of incoming and exhaust air. This can create issues with cold draughts in the winter and often because of a lack of occupant understanding of the ventilation strategy results in vents being kept shut. The resulting lack of ventilation leads to a build up of exhaled breath, bio-effluents, off-gassing pollutants from furnishings, and pathogens, leading to a less healthy environment, which has been shown to affect pupil concentration and cognitive ability.4 Perhaps this has led to the notion that classrooms are poorly ventilated? Before Covid-19, much research into school indoor air quality (IAQ) concluded that, in the main, poorly ventilated classrooms are the result of a lack of occupant interaction with the ventilation design strategy rather than a sub-optimal strategy.5 Often, classes noted as being poorly ventilated in winter are assessed as being well ventilated in summer, despite the design strategy remaining the same.6 Indeed, if we are to say that most school classrooms are poorly ventilated, it would suggest that the many architects, engineers, contractors and building control personnel involved in the design and build of schools for more than a century have failed to consider this important aspect of building design. As well as providing adequate IAQ, ventilation is often used to cool classrooms. Typically, the flowrates required to provide ventilative cooling are larger than the flowrates required for IAQ. As such, there should be the capacity within the ventilation design to provide adequate IAQ during the heating season. How can we balance ventilation provision with occupant comfort? This is not Classes noted as poorly ventilated in winter are assessed as well ventilated in summer, despite the design strategy being the same 40 April 2022 www.cibsejournal.com CIBSE Apr 22 pp40-41 Classroom ventilation Supp.indd 40 25/03/2022 14:29