The CIBSE Air-cleaning Calculator has been developed to assess the Relative Exposure Index (REI) of far field (> 2 m) indoor transmission of SARS-CoV-2 encapsulated in respiratory droplets that are small enough to become airborne, as described in Modelling uncertainty in the relative risk of exposure to the SARS-CoV-2 virus by airborne aerosol transmission in well mixed air in buildings (Jones et al., 2021).

It does not consider the risk of transmission via other pathways, and should be used in conjunction with government guidance. It models the relative exposure risk in a space, not absolute infection risk; this will be dependent upon community infection rates, virus variant and individual susceptibility.

REI values in excess of 1.0 demonstrate that the risk of a person in a space being exposed to SARS-CoV-2 is higher than the base case scenario. An REI of 1.0 does not indicate safety; it is a method used to compare the impact of various mitigations, such as reduced exposure time, increased ventilation and the addition of an air cleaning device.

A simple calculation can be achieved using the 'Input' tab. This allows users to input the elements a space: dimensions, number of occupants, breathing rate, respiratory activity (the amount of time spent breathing, talking or vocalising, respectively), occupation time and ventilation provision. The ventilation equivalent flow of the proposed air cleaning system can then be entered, and the median improvement on exposure risk is calculated. The REI is a measure of relative exposure in relation to the reference basecase scenario of a UK junior school classroom occupied for seven hours and ventilated at five litres per second per person. The breathing rate is assumed to be that of a seated child whose respiritory activity is 75% breathing and 25% talking.

A more complex calculation can be achieved using the two 'Scenario' tabs, which can be compared to assess the impact of mitigation measures in reducing relative risk of secondary transmissions. This allows an assessment of the exposure risk to susceptible persons sharing the space with a single infected person (although this assumption can be edited for each scenario), and therefore assumes no risk to an indoor space occupied by a single person.

These calculations should  be used with caution for large spaces (> 1000m3), where it should not be assumed that the air is fully mixed.

Further information is available in COVID-19: Air cleaning technologies (CIBSE, 2021) and Modelling uncertainty in the relative risk of exposure to the SARS-CoV-2 virus by airborne aerosol transmission in buildings (Jones et al., 2021).