The Influence of Overhead HVAC on Exposure to Airborne Contaminants from Simulated Speaking

The Challenge

Quantify the effect of room stratification from overhead heating on the mixing of respiratory aerosols emitted by people in meeting and classroom scenarios. The guidance to maintain a distance of at least six feet and to increase ventilation were core features of pandemic building operations. This distance was generally supported by studies of simulated emissions from one to two heated manikins in office-sized experimental chambers. Investigating the effectiveness of distancing in classroom and meeting room scenarios required a facility with advanced measurement tools and full control over the heat transfer processes that drive airflows.

The Solution

This research was supported by the U.S. Department of Energy Office of Science, through the National Virtual Biotechnology Laboratory, a consortium of DOE national laboratories focused on response to COVID-19, with funding provided by the Coronavirus CARES Act. Researchers at the U.S. Department of Energy’s FLEXLAB^® facility at Lawrence Berkeley National Laboratory experimentally investigated this challenge. The team conducted over 60 experiments to investigate the influence of supply air temperature on mixing of respiratory aerosols emitted by people speaking in a room that has forced air heating, ventilation, and air conditioning (HVAC) with supply and return registers mounted in the ceiling – which is common in schools and offices. The study used heated mixtures of carbon dioxide, air and generated aerosols to represent the emissions and heated manikins to represent occupants. The impact of portable air cleaners (PACs) was also studied.

The Bottom Line

Overhead heating creates air stratification, leading to higher concentrations and exposures in the breathing zone compared to a completely well-mixed space. In both simulated settings, the stratification from overhead heating created substantially higher exposures, compared to scenarios of no forced air, cool air, or air of a neutral temperature and reduced the benefits of outdoor air ventilation. Fortunately, there is a relatively simple solution. Fans or PACs can mix the air vertically and ventilation and filtration can be used to greatly reduce respiratory aerosol levels.

Download the full case study here: The Influence of Overhead HVAC on Exposure to Airborne Contaminants from Simulated Speaking