COVID-19: Winter Is Coming – Part 3

By Ali Shaw, Gwilym Still & Bill Watts

23 October 2020

Winter is coming, but Coronavirus hasn’t left. This third part of our blog post series is concerned with the coexistence of cooler temperatures and viral particles within buildings.

Current guidance on managing the risk of transmitting Coronavirus identifies transmission through direct contact, large droplets, and microscopic aerosolised particles. People can address the first three mainly through social distancing and cleaning regimes. The principal methods for limiting the aerosolised particle risk are to remove particles through dilution (by ample ventilation), or removal (filtering the air).

There are epidemiological models for the transmission of aerosolised diseases, and scientists have applied these to Coronavirus. The models suggest ventilation rates significantly above standard practice, to reduce the risks of this transmission to a low level. For rooms with openable windows, these rates can often be achieved using natural ventilation.

During clement weather, opening windows and doors provides a robust and straightforward way of providing ventilation. As the weather gets colder, this approach will lead to the following issues:

  • high energy consumption for space heating
  • low room temperatures, cold draughts and discomfort, causing people to simply close the windows
  • low relative humidity inside, leading to discomfort and increased susceptibility to disease

For mechanically ventilated buildings without opening windows, the maximum fresh air ventilation rates are fixed and cannot be improved without major upgrade works. 

An alternative to increased dilution through increased freh air volumes is to filter the air and recirculate it within individual spaces. The fan energy used with this approach is significantly lower than the space heating and humidification energy needed to maintain comfortable conditions. These changes are also easily reversible when the virus is no longer a threat.

We recommend guidance is updated to reflect this, and building operators investigate using filtration effective for aerosolised COVID-19 particle sizes and carefully considered recirculation to allow lower energy, lower risk building use in cold weather. 

As we move into winter, building owners and operators must now consider new strategies for adequately addressing aerosol transmission risks so they can also conserve energy and maintain comfort conditions for their students, staff and visitors.


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