  The quality of the classroom environment, including ventilation, air quality
and thermal conditions, has an important impact on children's health and
academic achievements. The use of portable HEPA filter air cleaners is widely
suggested as a strategy to mitigate exposure to particulate matter and airborne
viruses. However, there is a need to quantify the relative benefits of such
devices including the impacts on energy use. We present a simple coupled
dynamic thermal and air quality model and apply it to naturally ventilated
classrooms, representative of modern and Victorian era construction. We
consider the addition of HEPA filters with, and without, reduced opening of
windows, and explore concentrations of carbon dioxide (\co), \PM, airborne
viral RNA, classroom temperature and energy use. Results indicate the addition
of HEPA filters was predicted to reduce \PM~ by 40--60\% and viral RNA by
30--50\% depending on the classroom design and window opening behaviour. The
energy cost of running HEPA filters is likely to be only 1\%--2\% of the
classroom heating costs. In scenarios when HEPA filters were on and window
opening was reduced (to account for the additional clean air delivery rate of
the filters), the heating cost was predicted to be reduced by as much as -13\%,
and these maximum reductions grew to -46\% in wintertime simulations. In these
scenarios the HEPA filters result in a notable reduction in \PM~and viral RNA,
but the \co\ concentration is significantly higher. The model provides a
mechanism for exploring the relative impact of ventilation and air cleaning
strategies on both exposures and energy costs, enabling an understanding of
where trade-offs lie.
