As people spend more time in offices, stores and other buildings, and colder weather forces many outdoor activities to be held indoors during the COVID-19 pandemic, construction contractors are increasingly being asked by building owners and operators to provide various mitigation strategies to improve indoor air quality to help occupants avoid being exposed to lingering airborne viral particles.
Lowering the concentration of SARS-CoV-2 and other pathogens in enclosed public spaces is of the utmost importance nowadays given that Americans, on average, spend nearly 90% of their time indoors, according to the EPA.
It’s fairly common knowledge that the best way to avoid infection is to follow guidelines issued by the Centers for Disease Control and Prevention that include:
But it’s also important to provide healthy air in buildings as a key strategy in limiting the virus’s transmission. LEED and WELL standards have been driving improved air quality and other building performance metrics for years. But the pandemic could accelerate those efforts, as building owners move to upgrade ventilation systems to ensure the safety and well-being of occupants or to make a property more marketable, or both.
Construction contractors should follow air-quality improvement strategies, recognized by the American Society of Heating, Refrigeration and Air Conditioning Engineers, to reduce the concentration of airborne viral particles and other contaminants.
It should be noted that these methods must be part of a multipronged approach, as improving air indoor quality needs to be included in a comprehensive mitigation strategy to effectively reduce viral transmission.
ASHRAE and the CDC have stated that increasing the flow rate of outside air ventilation can dilute indoor sources of contaminants including volatile organic compounds, pathogens and allergens. ASHRAE has recommended flushing the building with fresh air daily before and after typical occupancy periods. The strategy involves turning over the building with fresh air at least three times pre- and post-occupancy every day. This operation can be scheduled automatically using the building automation system.
Higher flow rates of outside air ventilation can increase HVAC heating and cooling loads, as well as energy consumption. It is critical that contractors use a registered mechanical engineer to determine if the ventilation rate can be increased and by how much. Excessive ventilation can have a negative impact on control of space temperature, relative humidity, building pressure and ultimately occupant comfort.
Ultraviolet Germicidal Irradiation (UVGI) is a technology that has been used for years in controlling airborne pathogens in HVAC systems and healthcare settings. Ultraviolet light is a form of electromagnetic radiation with a wavelength between 100 and 400 nanometers (about 100 times smaller than a human hair). A UV source at around 254 nanometers is considered the most effective at inactivating viruses like SARS-CoV-2.
Most air-handling unit installations of UV light focus on the disinfection of the system’s cooling coil and drain pan. Higher UV source power densities are sometimes considered for instantaneous inactivation of airborne pathogens, referred to as airstream sanitization.
The benefits of using UVGI as a mitigation strategy are that it doesn’t require a significant amount of energy consumption or adversely impact the performance of air-handling equipment. To avoid potential health problems associated with UV lamps, safety cutout switches and personal protective equipment should be used. Most portions of the UV spectrum can burn skin, corneas and cause cancer. Some UV lamps and devices produce ozone as a byproduct. Care should be taken in selecting UV-generating equipment.
It’s recommended that lower-rated Minimum Efficiency Reporting Value (MERV) mechanical filters are replaced with those with a higher MERV rating, such as a MERV-13 filter, to reduce the concentration of airborne pathogens.
However, since most high-efficiency mechanical filters create a larger pressure drop, it’s best to consult with a mechanical engineer to determine if the HVAC system has sufficient capacity for this type of an upgrade.
Electronic air filters might be the way to go if the HVAC system cannot handle the higher amount of pressure drop produced by a high-efficiency mechanical filter. While these filters can cost more to install than mechanical filters, their lower pressure drops can result in lower fan energy consumption. A mechanical engineer can determine the best electronic filters to use.
To reduce airborne transmission of viral pathogens indoors, spaces should maintain indoor relative humidity at between 40% and 60%, according a scientific study published on PLOS ONE. Studies have found that low levels of relative humidity can raise transmission rates.
Drier indoor air conditions can encourage evaporation of airborne droplets of water vapor containing viral particles, resulting in smaller particles which remain airborne for longer periods of time and travel longer distances.
Lower indoor relative humidity levels could also dessicate mucus membranes of the sinus and lungs, making a person more susceptible to respiratory virus illness. Humidifiers are commonly used to avoid this problem, but they can consume additional water and energy.
One of the technologies used to eradicate airborne pathogens is bipolar ionization, which creates charged ions in the air reacting with airborne contaminants like viruses, bacteria, mold and other allergens. This technology can be centrally located in the air-handling unit of an HVAC system, where it will purify air returned to the unit. Another application is to place it in the supply air duct that will release the ions into the space, resulting in a greater impact on indoor air quality.
Users should carefully review the manufacturer’s data before installing a bipolar ionization system and only use those products that have an Underwriter’s Laboratories listing (UL 2998) that certifies they don’t produce ozone, which can be harmful at excessive levels. Bipolar ionization is considered by the CDC to be an “emerging technology” and warrants additional research.
It’s highly recommended that contractors work closely with a registered mechanical engineer to ensure that the systems are properly used to help create healthy air and keep operating costs in check. A mechanical engineer also will advise how to avoid problems associated with improper installation and maintenance of air-quality upgrades in both new and older buildings.
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