Thread by @CathNoakes

Our paper led by @HenryBurridge on modelling the combined IAQ, infection risk and thermal aspects of nat vent school classrooms with and without HEPA filters is published
A short 🧵 on what we did 1/sciencedirect.com/science/articl…

We assume a well mixed space and use a box model together with indoor and outdoor temperatures and assumptions about the building fabric and heating to model the thermal environment and air quality over time. We also model virus 2/

We assume the classroom is naturally ventilated and that ventilation is controlled by window openings to different extents depending on weather. We consider HEPA filters as a simple addition or used in conjunction with reduced window opening in response to colder weather. 3/

We consider two schools - a modern construction and a Victorian era building. In each scenario we consider the comfort, air quality, infection risk and energy consumption, and do this for weather conditions in Leeds and London 4/

We find that the addition of HEPA filters reduces exposure to PM by 40-60% and viral RNA by 30-50% depending on the classroom design. Keeping window behaviours as normal means this only adds an energy cost around 1-2% of the heating costs - heating bills dominate 5/

The most interesting case is when the windows are reduced because it’s cold and HEPA is added as a supplement. This still results in reduced PM and viral RNA (though CO2 is higher) but the heating cost is actually reduced by 13-46%. 6/

This means that, with the right choice of device, HEPA units can be an effective low energy supplement to enable better IAQ at times of year when windows get closed to save energy. 7/

Of course this is no silver bullet and there’s always compromise - the CO2 will not be reduced and hence the environment is still sub optimal for learning, as well as the potential risks that higher CO2 is better for virus survival 8/

Ideally we need good ventilation with appropriate filtration for outdoor PM in all classrooms. But in the absence of funding for this, we show that balancing HEPA units and window opening can create better environments that are lower risk and without increasing energy costs 9/

And for all of the “ah but what about” folks out there (as I know you’re waiting). Yes this is a model so it comes with all the many caveats, yes we’ve only looked at certain scenarios so there could be other factors, yes you have to consider maintenance, waste, behaviours….10/

Yes you have to consider spend o indoor air against spend on many other things in schools and the priorities may vary in different settings. But despite all this, the model allows quantitative consideration of trade offs and adds to the evidence around improving school IAQ 11/

A big thanks to @HenryBurridge, Sen and Sam for lots of hard work in creating the model, and Sara for schools thermal data - I can’t claim much credit at all! And a thank you to @SAMHEProject @BreathingCity and the TAPAS network