By Lingying Zhao
We have been in the COVID-19 pandemic for almost two years. Because of the pandemic, we started a new mode of working from home and/or staying at home as a measure to control the virus’ spread and protect our health. Recently, as the new Omicron variant of Coronavirus spread in the U.S. and all over the world, many of our friends and family members became infected with the virus. When we are sick, the instructions issued from the workplaces, doctors, and/or schools are to stay at home. Consequently, in this winter during the pandemic, we are spending even more time in our homes. However, most of our homes are not designed and equipped to deal with the air-borne transmission of infectious diseases. One sick family member at home may affect the safety of the entire household. Staying healthy at home in the pandemic, especially when we have sick family members living at home, becomes a significant challenge. The Center for Disease Control (CDC) has provided many general measures, including physical distancing, face coverings or masks, surface cleaning, and handwashing, to help protect us from the virus infection. This article summarizes engineering control strategies and systems that can help us stay healthy at home in the pandemic.
To understand the effective engineering controls of Coronavirus exposures, let’s review the engineering facts related to the virus. First, it is a type of airborne virus that transmits through droplets and particles generated from exhaling, coughing, sneezing, and speaking of infected people. The virus attached to the droplets can be transported over long distances and for long periods of time depending on the initial droplet velocities, droplet sizes, and the ambient environmental conditions (airflow speed, air temperature, and relative humidity) around the droplets. Sneezing can generate droplets that travel more than 24 feet and coughing can make droplets travel at least 6 to 8 feet. As droplets travel in the air, evaporation will reduce their sizes and enable them to transport far beyond 6 feet. They can disperse throughout an entire room and stay in the air for hours. Fresh airflow can dilute the concentrations of air pollutants including the virus carried by droplets and reduce the virus load in the air for people to breathe.
Based on the above facts, the following engineering control strategies are suggested by ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers) and USEPA (United State Environmental Protection Agency) to ensure the safety of people at home in this pandemic.
Increase ventilation rate
Ventilation is an organized air movement process that brings fresh air in and exhausts pale air out of an indoor space. This can remove air pollutants including Coronavirus out of the space to keep the occupants safe.
USEPA recommends increasing ventilation with outdoor air. This is an important strategy to keep homes safe in the pandemic. The amount of fresh outdoor air needed for homes, called the air exchange rate, is suggested by ASHRAE Standard 62.2-2019 “Ventilation for Acceptable Indoor Air Quality in Residential Buildings” as either 0.35 ACH (air changes per hour), or 15 cfm (cubic feet of air per minute) per person, whichever is higher. This ventilation rate was determined in consideration of normal home materials, structures, and activities. Special zones such as kitchens and bathrooms are also equipped with range hoods or exhaust fans in order to increase air exchange rate to mitigate high level of either air pollutants due to cooking or moisture due to showers. These exhaust fans can also be used to increase the air exchange rate, reduce virus concentrations, and improve safety at home. If the weather is mild, opening windows or doors are also options to increase the air exchange rate of homes to reduce possible high virus concentrations. In winter, exhaust fans can be used to increase ventilation and avoid cold drafts.
Improve furnace filtration
The Heating Ventilation and Air-Conditioning (HVAC) systems for residential houses are typically equipped with filters to remove dust particles in order to protect the furnace and improve indoor air quality. Viruses are very small in size and normally attach to fine droplets or particles. Normal filters cannot clean fine particles and viruses in the recirculation air. The filter in the home HVAC system needs to be upgraded with a high-efficiency particulate air filter with a minimum efficiency reporting value of 13 (MERV13) to capture all virus, bacteria, and droplets. Other type of filters with ratings such as FPR 10 (filter performance rating) and MPR 2200 (micro-particle performance) are also equivalent to MERV13 in terms of filtration efficiency for small particles. Air filters with a rating of FPR 9 or MPR 1900 are less efficient but are still capable of capturing virus and bacteria carriers.
Add in-duct air cleaning and disinfection units to the home HVAC system. Indoor air is circulated through the home HVAC system to be conditioned to make our rooms thermally comfortable. The air can also be cleaned and disinfected using in-duct air cleaning and disinfection equipment. Two types of these devices are available: Electrostatic Precipitator (ESP) and Ultraviolet Germicidal Irradiation (UVGI) air cleaners.
ESP is a device using high-voltage electrodes to generate electrons and ions, which in turn charge particles or droplets in the air stream passing through the electrodes. The charged particles are then collected by the oppositely charged collection plates near the electrodes. The electrical fields of an ESP also can directly deactivate the bacteria and viruses in the air stream.
UVGI air cleaners use UV lamps to deactivate airborne microorganisms such as viruses, bacteria, and fungal spores. With sufficient exposure time and lamp power, UV light can damage the outer layer of a microorganism’s cell(s) to cause cell death. For details on the air cleaners, please see the USEPA publications on Air cleaners and Air filters in the Home (https://www.epa.gov/indoor-air-quality-iaq/air-cleaners-and-air-filters-home)
Portable air cleaners
For easy installation and flexible use, portable air cleaners, which are typically small in size, can be used to clean the air in a room. A portable air cleaner consists of a cleaning technology, a filter, and a fan propelling room air through the air cleaner. The air cleaning technologies used in a portable air cleaners can be the same as the ones used in the home HVAC system: filtration, ESP, and UVGI. See the above mentioned USEPA publication for details. The disadvantage of the portable air cleaners is that they are not as effective as the ones installed in the home HVAC system because of the challenges in organizing air recirculation through the portable air cleaners. A clean air delivery rate (CADR) with a unit of cubic feet per minutes is used to measure the capacity of a portable air cleaner in generating clean air. If a room is big, a couple of air cleaners may be needed to achieve the ASHRAE’s recommendation of 0.35 ACH.
Set an isolation room for a sick family member
If a family member is infected with COVID-19, ASHRAE strongly suggest to create an isolation room for the family member to prevent other family members from being infected. The isolation room should be on a high floor in the home in winter and at a corner of the house. The air supply diffusers and return grills in the isolation room should be closed. A portable heater or air conditioner can be used to condition the air in the isolation room. A small window opening or a window fan and bathroom exhaust fans can be used to ventilate the isolation room separately. Air pressure in the isolation room should be kept low to prevent air leakage to other rooms through the door. Obviously, the door should be closed. In addition, air barriers for the seams of the doors should be used to prevent air exchange with other rooms in the house, and the toilet should be capped when flushing.
In summary, the COVID-19 pandemic reminds us that control of pathogens in indoor environment is a necessary but challenging task that cannot be accomplished by the existing ventilation systems of homes. In additional to the CDC general guidance, engineering control strategies are needed at our homes to ensure we can stay heathy at home even with a sick home member. Upgrading furnace filters or installing air cleaning and disinfecting equipment in the ducts of home HVAC systems will ensure a clean and safe environment of the whole house, but the costs may be high and installation requires more time. The portable air cleaner is an easy and fast solution. Attentions need to be paid in order to size and position the portable air cleaners effectively. In the case of a sick family member, which is very likely in the current pandemic situation, a separation room is highly suggested for each home to prevent or delay other family member’s infection.
Lingying Zhao, Professor and Extension Agricultural Engineer, can be reached at email@example.com. This column is provided by the OSU Department of Food, Agricultural and Biological Engineering.