Airborne pathogens: How aerosolized infectious agents spread

Many infectious diseases, from influenza to measles, can travel through the air in microscopic particles known as aerosols. These particles can linger, move through indoor spaces, and be inhaled by others—making them a critical factor in how infections spread.

The risks from airborne pathogens are ever-present, and effective strategies are essential to mitigate their impact.

How infectious agents become airborne

Aerosols are microscopic solid or liquid particles in air or other gases (1). Aerosols range in size from extremely small particles to larger droplets, and many are small enough to remain suspended in the air for extended periods; research has revealed that the smaller sized aerosols can remain suspended in the air for hours, making them a highly efficient vehicle for infectious agents (2). 

Some of the smallest airborne particles, such as PM2.5, or particulate matter measuring 2.5 micrometers or smaller, can carry viruses, bacteria, or fungi, allowing them to spread beyond close contact and infect individuals who inhale them (3)(4).

Pathogens – including viruses, bacteria, and fungi – become airborne through coughing, sneezing, and talking; even normal breathing can release infectious aerosols into the environment. When an infected person exhales, tiny droplets containing the pathogen evaporate, leaving behind even smaller particles that remain airborne. This infection process is particularly effective in indoor settings, where poor ventilation can trap and concentrate these particles, increasing the risk of transmission (5).

Several factors influence how effectively airborne pathogens are spread:

• Particle size determines how long they stay aloft—smaller particles travel farther and remain suspended longer.
• Humidity affects their stability; some viruses thrive in dry air, while others persist better in humid conditions (6).
• Ventilation plays a crucial role. Stagnant air allows aerosols to accumulate, while proper airflow dilutes and removes them. As a group of Montreal-based teachers, scientists, and doctors demonstrated in an unofficial study conducted in 2020, increased carbon dioxide (CO2) accumulated beyond acceptable levels in classrooms through poor ventilation, which may have in turn exposed students and staff to increased risk of exposure to SARS-CoV-2 (7).
• The duration of exposure also matters; more time spent in contaminated air increases the likelihood of infection. 
  

All these factors combined shape the dynamics of airborne transmission. Although airborne transmission is a primary pathway, pathogens can also settle onto surfaces, where they may be transferred through contact—making both air and surface hygiene important in limiting spread.

Common airborne pathogens and their risks

Among the most well-known airborne threats are viruses that exploit aerosol transmission to infect new individuals. These can include: 

• Influenza: Influenza (or flu) is a contagious respiratory illness. It relies on airborne particles to move between people, often leading to outbreaks in schools, workplaces, and public gatherings (8).
• Measles: Measles, typically presented by a rash, fever, cough, runny nose, and watery eyes, are extremely contagious. Measles can linger in the air for up to two hours after an infected person leaves a room, infecting those who enter later (9).
• SARS-CoV-2: This respiratory virus spread COVID-19 globally, with aerosols playing a key role in its transmission (10).
• Chickenpox: Also known as varicella-zoster, chickenpox spreads through contact, through fluids, and is airborne. (11)

Bacteria also pose a significant airborne risk. Mycobacterium tuberculosis, the bacterium responsible for tuberculosis (TB), spreads when infected individuals cough or sneeze, releasing aerosols that can remain infectious for hours (12). Legionella, another bacterial pathogen, thrives in water systems but becomes airborne through contaminated mist or droplets, causing severe pneumonia when inhaled (13).

Where and how aerosol transmission happens

Aerosol transmission can flourish in spaces with limited airflow. 

Hospitals, schools, and public transit systems are notable high-risk settings, where infected individuals can unknowingly expose many others to airborne pathogens. 

• In hospitals, medical procedures such as intubation or even routine patient care can generate infectious aerosols, affecting both patients and healthcare providers.
• Schools, with their densely populated classrooms and shared facilities, can become epicenters during outbreaks of diseases like measles or influenza.
• Public transit, where ventilation is frequently inadequate, increases the risk of infection as passengers inhale recirculated air over prolonged periods. In these environments, people may be exposed to airborne pathogens even without direct contact with an infected individual.
  

Research has shown that certain public spaces provide ideal conditions for rapid transmission. For example, a study on measles transmission in a pediatric practice demonstrated that airborne transmission occurred in an office setting, with unvaccinated infants facing an attack rate of 80% (4/5), compared to 7% (2/27) for vaccinated children (14). A 2024 study conducted in Finland found that all but one participant in a senior choir rehearsal were infected with SARS-CoV-2 (15). Computational modeling confirmed aerosol transmission as the likely cause.

Environmental factors may further increase risk infection. Poor ventilation traps aerosols, allowing them to accumulate and linger. High occupancy increases the likelihood of exposure, while activities that involve heavy breathing—singing, shouting, or exercise—produce more aerosols and project them farther. 

Together, these factors determine how far airborne pathogens travel and how long they remain a risk.

Protecting yourself and your community

Reducing the risk of airborne transmission typically involves multiple layers of protection, combining environmental controls with personal precautions.

• Source control: Handwashing helps reduce transmission after particles settle on surfaces, complementing strategies that address airborne exposure.
• Ventilation: Opening windows, using exhaust fans, or upgrading to high-efficiency HVAC systems can dilute and remove infectious particles from indoor air.
• Carbon dioxide monitoring: Monitoring carbon dioxide serves as a practical indicator of ventilation quality. By tracking CO2, building managers and individuals can identify when ventilation needs improvement, reducing the risk of aerosol buildup and transmission.
  
• Filtration: Using a high-efficiency air purifier with advanced filtration can help reduce exposure in homes, schools, and workplaces.
• Masks: While loose-fitting cloth masks provide minimal defense against aerosols, wearing well-fitted KN95/FFP2 mask can help filter out 95% of airborne particles measuring down to .03 microns when worn correctly.

All of these strategies combined create a robust defense against airborne threats. 

The takeaway

By improving ventilation, monitoring indoor conditions, and using layered protection strategies, individuals and communities can better manage exposure and create safer indoor environments.

This layered approach reflects a broader principle in infection prevention: reducing risk depends on combining multiple strategies rather than relying on a single intervention.

In that context, World Hand Hygiene Day, observed each year on May 5, highlights how these measures work together to reduce transmission risk. While hand hygiene remains essential, growing recognition of airborne transmission points to the role of air quality and ventilation as part of a more complete approach.