Evaluation of airborne transmission risk in university towns based on IEQ surveys

This study aims to evaluate airborne transmission risk in university towns during the COVID-19 pandemic based on surveys of indoor environmental quality(IEQ).Both on-site measurements and questionnaire surveys were carried out in public buildings in university towns in Changsha,China.Air temperature,relative humidity,and CO 2 concentration in one library,ten classrooms,eight canteens,seven restaurants,and sixteen malls were mea-sured.2220 valid questionnaires concerning occupants’sensation on thermal environment,air movement,and indoor air quality were collected.A 3-level evaluation method of airborne transmission risk that is dependent on building type and indoor CO 2 concentration was developed.Excessive CO 2 concentration is found in library(1045 ppm),classrooms(1151 ppm),restaurants(1242 ppm),and malls(1057 ppm).The percentage time of“high risk”accounts for 18-100%in these buildings.The results reveal a serious problem:numerous public buildings in China and probably other resource limited countries are not basically prepared and equipped to cope with airborne transmission.This fact should be taken into account when developing guidelines and formu-lating mitigation measures.Real-time monitoring and displaying IEQ and thus the transmission risk level should be an important way to be widely implemented in public buildings.

Airborne transmission during short-term events: Direct route over indirect route

Numerous short-term exposure events in public spaces were reported during the COVID-19 pandemic,especially during the spread of Delta and Omicron.However,the currently used exposure risk assessment models and mitigation measures are mostly based on the assumption of steady-state and complete-mixing conditions.The present study investigates the dynamics of airborne transmission in short-term events when a steady state is not reached before the end of the events.Large-eddy simulation(LES)is performed to predict the airborne transmission in short-term events,and three representative physical distances between two occupants are examined.Both time-averaged and phase-averaged exposure indices are used to evaluate the exposure risk.The results present that the exposure index in the short-term events constantly varies over time,especially within the first 1/ACH(air changes per hour)hour of exposure between occupants in close proximity,posing high uncertainty to the spatial and temporal evolutions of the risk of cross-infection.The decoupling analysis of the direct and indirect airborne transmission routes indicates that the direct airborne transmission is the predominated route in short-term events.It suggests also that the general dilution ventilation has a relatively limited efficiency in mitigating the risk of direct airborne transmission,but determines largely the occurrence time of the indirect one.Given the randomness,discreteness,localization,and high-risk characteristics of direct airborne transmission,a localized method that has a direct interference on the respiratory flows would be better than dilution ventilation for short-term events,in terms of both efficiency and cost.

A cost-effectiveness assessment of the operational parameters of central HVAC systems during pandemics

The present study develops a cost-effectiveness assessment model to analyze the performance of major operational parameters of central HVAC systems in terms of airborne transmission risk,energy consumption,and medical and social cost.A typical multi-zone building model with a central HVAC system is built numerically,and the effect of outdoor air(OA)ratio(from 30%to 100%)and filtration level(MERV 13,MERV 16,and HEPA)are assessed under the conditions of five climate zones in China.Compared with the baseline case with 30%OA and MERV 13 filtration,the airborne transmission risk in zones without infector is negligibly reduced with the increase in OA ratio and the upgrade of filtration level,owing to their slight modification on the equivalent ventilation rate of virus-free air.However,depending on climate zone,a 10%increase in OA ratio results in 12.5%-78.6%and 0.1%-8.6%increase in heating and cooling energy consumption,respectively,while an upgrade of filtration level to MERV 16 and HEPA results in an increase of 0.08%-0.2%and 1.4%-2.6%,respectively.Overall,when compared to the use of 100%OA ratio and HEPA filtration,the application of 30%or 40%OA ratio and MERV 13 filtration would save annually an energy and facility related cost of$29.4 billion in China,though giving an increase of approximately$0.1 billion on medical and social cost from the increased number of confirmed cases.This study provides basic method and information for the formulation of cost-effective operational strategies of HVAC systems coping with the airborne transmission,especially in resource-limited regions.