Whether SARS-CoV-2, the virus that causes COVID-19, can travel through a building’s air vents is a major concern for public health in indoor environments. The focus on Heating, Ventilation, and Air Conditioning (HVAC) systems stems from the understanding that air movement plays a significant role in viral transmission. While the risk varies greatly depending on a system’s design and maintenance, improving ventilation is now widely recognized as a defense against airborne pathogens and a subject of inquiry for mitigating the spread of respiratory illness indoors.
Understanding Airborne Transmission
The ability of the virus to travel through ventilation systems depends on the physical characteristics of the particles carrying it. When an infected person breathes, talks, or coughs, they release respiratory fluid into the air in a range of sizes. Larger respiratory droplets, typically greater than 100 micrometers, are relatively heavy and fall quickly to surfaces within one to two meters.
The concern for long-distance travel, including movement through air ducts, is the smaller, lighter particles known as aerosols. These tiny particles, often less than 5 micrometers, can remain suspended in the air for minutes to hours, drifting on air currents like smoke. Since a cough or loud conversation can produce thousands of these particles, they can be pulled into a building’s ventilation system and distributed over a wider area. The risk of transmission is higher in poorly ventilated indoor spaces because viral aerosols are not diluted or removed quickly, allowing them to accumulate to infectious concentrations.
How HVAC Systems Influence Viral Spread
HVAC systems can act as a pathway for viral aerosols, most notably through air recirculation. In many commercial buildings, the system draws air from occupied spaces, filters only a portion of it, and then mixes it with fresh outdoor air before delivering it back to different rooms. If the filtration is insufficient, the system can inadvertently distribute virus-containing aerosols from an infected zone to other areas of the building via shared ductwork.
The movement of air is also governed by pressure differentials, which can move contaminated air. For instance, creating a negative pressure environment, typically used in hospital isolation rooms, is designed to keep air in the room. A failure in this system could cause air to leak into surrounding hallways and spaces. Conversely, some systems may “short-circuit” the ventilation flow, meaning incoming clean air bypasses occupants and is immediately pulled toward the return vent. This creates pockets of stagnant air where viral aerosols can concentrate, even if the system is moving the correct volume of air.
Assessing Building Risk Factors
The risk of viral spread through a building is quantified by measuring the Air Changes per Hour (ACH), which is the number of times the total volume of air in a space is replaced with clean air every sixty minutes. A higher ACH dilutes the concentration of airborne viral particles faster, reducing the likelihood of infection. While a minimum of three ACH is often suggested, public health guidance recommends aiming for five to six ACH in typical indoor settings like classrooms or offices.
The size and use of a room also influence the risk, even with a functioning ventilation system. High occupant density, such as in a crowded meeting room, increases the rate at which viral aerosols are generated, requiring a proportionally higher ACH to maintain low risk. A centralized HVAC system that serves an entire building may perform differently than decentralized units, like window air conditioners. Decentralized units only recirculate air within a single room without bringing in fresh outdoor air. Evaluating the total volume of the space, including ceiling height, is necessary to accurately calculate the required airflow for maintaining adequate air change rates.
Strategies for Reducing Airborne Risk
The most direct way to mitigate the risk of aerosol travel is through engineering controls that clean the air before it is recirculated. This strategy relies on the use of high-efficiency filters, rated by the Minimum Efficiency Reporting Value (MERV) system. A MERV 13 filter is recommended as a minimum standard for building HVAC systems because it can capture at least 85% of particles between 1.0 and 3.0 micrometers, which includes the size range of infectious aerosols.
For systems that cannot accommodate a MERV 13 due to fan capacity or air pressure drop, supplemental air purification is an effective solution. Portable air cleaners equipped with High-Efficiency Particulate Air (HEPA) filters are useful because they capture 99.97% of particles at the most penetrating particle size of 0.3 micrometers, providing the equivalent of additional ACH to a space. Beyond filtration, maximizing the intake of fresh outdoor air is a simple operational change that increases dilution, reducing the concentration of aerosols inside a building.
Another mitigation technique is the installation of Ultraviolet Germicidal Irradiation (UVGI) technology within the ductwork or air handling units. UVGI uses short-wavelength UVC light, typically in the 200–280 nanometer range, to inactivate the virus by scrambling its DNA and RNA, rendering it unable to reproduce. This non-chemical method provides an additional layer of disinfection as the air moves through the system, supplementing the particle capture provided by high-efficiency filters.