Single-zone simulations using FaTIMA for reducing aerosol exposure in educational spaces

Abstract

On March 11, 2020, the World Health Organization declared the novel coronavirus (COVID-19) outbreak a global pandemic. Following this announcement, school closures around the United States began, and starting in June 2020, many schools decided to reopen for the next academic year or were planning to reopen. Guidance from ASHRAE and other organizations for reoccupying school buildings include wearing masks, increasing ventilation, increasing filtration, and using portable air cleaners (PAC).This report describes a simulation study that: (1) compares the relative reduction in aerosol exposure in education spaces as a result of changes to the operation of heating, ventilating, and air conditioning (HVAC) systems and inclusion of non-HVAC controls (e.g., wearing of masks and operating PAC) and (2) estimates the uncertainties in effectiveness associated with these controls and combinations of controls. This analysis used Fate and Transport of Indoor Microbial Aerosols (FaTIMA), a tool developed by the National Institute of Standards and Technology (NIST), to evaluate aerosol exposure in ventilated spaces. FaTIMA was used to model the release of biological aerosols in two classrooms and an assembly room with four different types of HVAC systems, including central air handling systems (CAHS), dedicated outdoor air systems (DOAS), terminal unit systems (TUS), and wall unit systems (WUS). Exposure was evaluated for 1 ?m particles (or aerosols) by which viruses are transported. In the classrooms, exposure was evaluated for 6 h of continuous exposure to a continuous source. In the assembly room, exposure was evaluated for 1 h of continuous exposure, where the source was present for the 5 h prior to the exposure period. For all the spaces and HVAC systems, the wearing of masks alone (assuming a protection efficiency of 25 % for both intake and exhalation) reduced exposure by 44 % relative to the base cases without masks. Masks with higher protection efficiencies (85 % to 90 %) reduced exposure 99 %, and masks with lower protection efficiencies reduced exposure 15 %. Masks with inhalation and exhalation protection efficiencies of 25 % combined with MERV 13 filtration in the HVAC systems reduced exposure by an average 71 % for the CAHS, DOAS, and TUS. For the CAHS, these masks combined with 100 % outdoor air (OA) intake reduced exposure by an average of 70 %. Using either a 297 clean air delivery rate (CADR) PAC or a 569 L?s-1 (1 200 cfm) exhaust fan with these masks, exposure was reduced by an average of 68 %. Combining all these controls, exposure was reduced by an average of 85 % assuming all controls were performing as intended. The results of this analysis may be useful in the selection of control options in educational facilities.