Airborne Contaminant Removal
A Practical Approach to Infection Risk Reduction
Last month we explored science-based guidelines and recommendations for health and safety as we prepare to return to work, school and recreation. These recommendations come from OSHA, CDC, WHO, ASHRAE and other sources that outline best practices to reduce pathogen transmission and infection risks in buildings from SARS-CoV-2 (virus causing COVID-19 disease) and other pathogens. Since writing that column, I have learned a great deal on the subject and engaged in efforts with collaborators to create a model for customized solutions in buildings. Specifically, while many administrative controls like planning, education and enhanced cleaning can be broadly applied, engineering controls focused on indoor environmental quality and airborne contaminant removal require customized solutions.
We know that respiratory droplets are a primary means of direct and indirect transmission for SARS-CoV-2. These droplets, expelled by coughs, sneezes and even loud talking and singing, are heavier than air and tend to precipitate quickly on adjacent surfaces. We also know that an infected person can shed this and other viruses as smaller aerosolized particles (generally in the 0.1 to 1-micron range) that remain suspended in the air for extended periods and are transported by air movement. An effective transmission risk reduction strategy needs to address both.
The ASHRAE Position Document on Airborne Infectious Disease and other sources describe the benefits of dilution ventilation, higher efficiency filtration and UV sanitization to remove contaminants of concern from air in the breathing zone (generally 3 to 6 feet above the floor). Dilution ventilation means increasing fresh air ventilation above minimum standards (ASHRAE 62.1) moving more airborne contaminants out of the space before occupants breathe them in. Enhanced filtration of recirculated air removes contaminants from the air that occupants are breathing, and the higher the MERV rating (Material Efficiency Reporting Value) of the filter, the smaller the particles it will remove. Finally, evidence shows UV-C light (UVGI lamps) has a germicidal effect and can deactivate viruses in the air and on surfaces. What the resources noted above don’t necessarily tell us is how best to apply these measures in specific circumstances and how effective each would be. Some of the application limitations in existing facilities include:
- A partial-building tenant cannot implement the same strategies as a whole building manager.
- Some types of HVAC systems cannot accommodate significant ventilation rate increases or filter efficiency upgrades.
- Some extreme climates limit the potential to increase ventilation while also maintaining thermal comfort.
- Filters in an HVAC system are only effective at removing contaminants from interior sources if indoor air is circulated at a sufficient rate.
- UVGI lamps in HVAC systems must be of adequate intensity to disinfect air passing by at a rapid rate, and deployments in occupied spaces must avoid human exposure and potential degradation of finish materials.
To address these limitations and guide the selection of the optimal combination of strategies, Engineering Economics Inc. (EEI) has been combing through the available data on the efficacy of these strategies and developing a model to calculate the cumulative benefits. Rather than focus on prescribed requirements like percent increases above ASHRAE 62.1-2016 minimum ventilation rates or filters of a specific MERV rating, their model sets a contaminant removal target (e.g., 99%) and calculates the time required to reach that target after a presumed contaminant is released in the space, based on selected strategies. This approach provides flexibility to achieve the results that matter most, contaminant removal and reduced virus spread, in a wide array of space types with various HVAC system configurations. The desired time-to-target could vary based on risk levels and owner expectations, where a longer time may be acceptable in a private office space, but a dental clinic or retirement 德扑圈官网版下载home would benefit from more rapid removal. Let’s look at an example to illustrate this.
A typical office space would have a ventilation rate of 0.5 air changes per hour (ACH) and MERV 8 filters (that primarily remove only larger dust particles) in the air handling equipment. In this scenario, it would take approximately 299 minutes to reach a 99% contaminant removal target after an interior release (like an occupant coughing). If feasible, increasing the ventilation rate to 2.4 ACH with the same MERV 8 filters cuts that time by almost two-thirds (104 min.). Alternatively, retaining the lower ventilation rate but upgrading to MERV 13 filters gets to the target in 84 minutes. If the HVAC system supports both an increased ventilation rate and filter upgrade, the target can be reached in just over an hour (65 min.). Encouragingly, if no HVAC upgrades are possible (e.g., in a tenant space), deployment of portable HEPA filter air cleaners (with sufficient air flow for the volume of space) could achieve 99% contaminant removal in about 40 minutes. Finally, if all three approaches were implemented in combination, the target could be reached in less than 30 minutes.
This approach also allows consideration of the energy use impacts of various strategies, supporting a balanced approach. A similar assessment could be conducted for other space types and with other strategies, such as upper room UVGI. Importantly, these measures not only limit transmission risks of SARS-CoV-2, they will also reduce absenteeism due to seasonal flu and the common cold, and other respiratory health impacts. Improved ventilation also improves productivity by increasing cognitive functioning in occupants.
While frequent cleaning, hand washing and mask wearing are effective measures to protect occupant health, they are also subject to inconsistent human execution. Engineering controls provide a consistent baseline infection risk reduction to backstop for these variable administrative control measures. Employers and facility managers want to protect the health and wellbeing of occupants, and employees, customers, students and others who occupy or visit buildings want assurance that they are safe to enter and occupy. While there is no way to guarantee that occupants won’t get sick if an infected person enters a facility, implementing best practices to reduce risks can be reassuring for all stakeholders, and enhance the on-going health and wellness of occupants.
Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is a senior consultant with Intertek Building Science Solutions in Portland, Ore. Jarrell Wenger, PE, LEED AP, is a mechanical engineer with over 30 years of experience, and principal with Engineering Economics Inc. in Denver. To learn more, follow Scott on Twitter, @alanscott_faia.