Far‐UVC lamps

Eight filtered KrCl excimer lamps (Biotile, Biocare UV) were installed in the ceiling of the classroom prior to the recruitment process. The location of the lamps within the room is shown in Figure 1, and the spectral emission of the lamps is given in Figure S1. The room has dimensions 12 m × 5.9 m × 3 m and was arranged as shown in Figure 1. As previously described, the room has mechanical ventilation with four air inlets and three open windows for outlets providing a ventilation rate of 6.8 air‐changes‐per‐hour (ACH). ²⁵

(Top) A photograph of the classroom used during the study. The KrCl excimer lamps can be seen secured to the ceiling between the normal room lighting. (Bottom) A schematic of the classroom with the location of the KrCl excimer lamps indicated by purple dots. The available seating positions for the participants are indicated with red circles, and the locations of the agar plates for microbiological sampling are indicated by the letters a–r.

Each study group experienced three different Far‐UVC exposures during the study. The exposures were

The three scenarios were randomly assigned to study day 1, 2 or 3. Participants were not informed of the Far‐UVC exposure scenario on each study day. However, there was also no attempt to disguise the Far‐UVC exposure scenario, and therefore, for the purposes of study design, the participants were not regarded as blinded. To provide an indication of the Far‐UVC dose received by participants each wore a brimless cloth cap with UVC Dosimeters (UVC 222 Dots, Intellego Technologies) fixed to the top. At the end of each study day, the dosimeters were visually assessed and compared to the included reference chart which provided a dose range, e.g. 0, 0–20, 20–50 mJ cm⁻², dependent on the resultant color of the dosimeter.