Aerosol transmission has emerged as a significant public health consideration, especially regarding the spread of respiratory illnesses. This mode of disease spread involves infectious agents carried by extremely small, lightweight particles that can remain suspended in the air. Unlike larger, heavier particles that fall quickly, these tiny remnants behave like smoke, moving with air currents throughout an enclosed space. Understanding the physics of these airborne particles is necessary for developing effective measures to limit their spread. Managing air quality is a foundational strategy for reducing disease transmission risk indoors.
Defining Aerosol Transmission and Particle Physics
Respiratory emissions are released across a wide spectrum of sizes, traditionally categorized to distinguish their transport behavior. Larger respiratory droplets, often defined as being 100 micrometers (µm) or greater in diameter, follow a ballistic trajectory. They are pulled quickly toward the ground by gravity, typically settling within a short distance of the person who expelled them. Conversely, aerosols are the smaller particles that remain suspended in the air for extended periods, behaving more like a gas than a falling object.
The size of these viral-laden aerosols often falls into the 1 to 10 µm range. A key physical process is the rapid evaporation of the water content from the initial, larger droplet after it is exhaled. This leaves behind a much smaller, stable particle composed of non-volatile materials and the infectious agent, known as a droplet nucleus.
The physics of suspension dictates that a particle’s settling time increases exponentially as its size decreases. This extended suspension time allows aerosols to travel far beyond the immediate proximity of the infected individual, circulating within a room’s entire air volume and increasing the potential for far-field transmission.
Environmental Factors Driving Airborne Spread
The concentration of infectious aerosols in an indoor environment is primarily determined by air movement and the density of people present. Ventilation serves to dilute and remove airborne contaminants, a process quantified by the Air Changes Per Hour (ACH) metric. ACH represents how many times the entire volume of air in a room is replaced with outside air or filtered air within one hour.
A low ACH rate allows infectious aerosols to accumulate over time, leading to a higher concentration and a greater risk of inhalation exposure. Public health recommendations often suggest aiming for five or more ACH in occupied spaces to effectively reduce airborne viral particles. Clinical settings, such as hospital isolation rooms, typically target much higher rates, sometimes reaching 6 to 12 ACH, for rapid contaminant removal.
Air recirculation in heating, ventilation, and air conditioning (HVAC) systems can compound the risk if the air is not adequately filtered. Systems that pull air from an occupied space and redistribute it without sufficient treatment can spread concentrated aerosols to other areas within a building. A strategy to mitigate this involves increasing the intake of fresh outdoor air, ideally operating systems on 100% outdoor air where feasible, to maximize dilution.
The combined factors of occupancy level and the duration of exposure are multipliers of risk. In crowded settings, the source of aerosol generation is greater, and the total viral load builds up faster in the shared air. The longer an individual spends in a poorly ventilated space with an infected person, the greater their cumulative inhaled dose of aerosols, which increases the probability of infection. This dynamic explains why super-spreader events are frequently linked to indoor environments characterized by high density and prolonged interaction.
Strategies for Reducing Airborne Risk
Reducing the risk posed by airborne aerosols involves a layered approach that combines personal protection with engineering and administrative controls. One effective engineered control is High-Efficiency Particulate Air (HEPA) filtration.
Engineering Controls
By definition, a HEPA filter must be at least 99.97% efficient at capturing particles that are 0.3 µm in size, which is considered the most penetrating particle size (MPPS) for filters. For central HVAC systems, upgrading to filters with a Minimum Efficiency Reporting Value (MERV) of 13 or higher is widely recommended. A MERV 13 filter is rated to remove over 50% of particles in the 0.3 to 1.0 µm range, which includes the size of many virus-carrying aerosols. Where central systems cannot accommodate high-efficiency filters due to airflow resistance, portable air cleaners with true HEPA filters can be deployed to provide equivalent air changes per hour (eACH).
Another engineered solution is Ultraviolet Germicidal Irradiation (UVGI). UVGI uses short-wavelength UV-C light to inactivate airborne pathogens. The peak germicidal effectiveness occurs near the 265 nanometer (nm) wavelength, damaging the DNA or RNA of viruses and bacteria and preventing them from replicating. UVGI is often installed in the upper part of a room or within ductwork to treat the air safely as it circulates.
Personal and Administrative Controls
Personal protection is maximized through the proper use of high-filtration masks, such as an N95 respirator. The “95” rating signifies that the device is at least 95% efficient at filtering airborne particles when the air passes through the filter material. Crucially, an N95 respirator must form a tight seal to the wearer’s face, verified by a user seal check, to ensure that air is not bypassing the filter media through gaps. Behavioral modifications, such as minimizing the time spent in crowded indoor settings and increasing distance from others, serve as fundamental administrative controls that further reduce the overall exposure to concentrated aerosols.