In the air you breathe, you can be exposed and get sick from an infectious disease when disease-causing microorganisms are released into the air from the disturbance of environmental reservoirs (i.e., soil, water, dust, living things). The movement of things (e.g., people, air currents, water, equipment, stuff) can bring infectious disease agents indoors, where they can proliferate on various indoor surfaces and, if subsequently disbursed into the air, serve as a source for airborne infections.

Can infectious diseases spread in the air?

Infections can occur from exposure to germs contained in droplets or droplet nuclei. Exposure to germs in droplets (e.g., when an infected person talks, sings, coughs, sneezes, and breathes) constitutes a form of direct contact transmission. When droplets are produced during a sneeze or cough, a cloud of infectious particles >5 µm in size is expelled, and people nearby (usually within 3 feet or 1 meter) are potentially exposed. Examples of germs spread in this manner are the influenza virus, rhinoviruses, adenoviruses, and respiratory syncytial virus (RSV). These droplets tend to fall out of the air quickly and can sometimes remain infectious on surfaces, but measures to control airflow in a facility (e.g., use of negative pressure) generally are not indicated for preventing the spread of diseases caused by these germs.

(Note—µm is a micrometer or micron equal to 0.001 mm or 0.000039 inches and used to measure particles in the air. The smallest particles we can see with our eyes are those that are larger than 50 µm.)

The spread of infectious diseases in the air via droplet nuclei is a form of indirect transmission. Droplet nuclei are the residuals of droplets that, when suspended in air, subsequently dry and produce particles ranging in size from 1–5 µm. These particles can:

• Contain potentially viable infectious germs.
• Be protected by a coat of dry secretions.
• Remain suspended indefinitely in air.
• Be transported over long distances.

Germs in droplet nuclei persist in favorable conditions (e.g., dry, cool indoor spaces with little or no direct exposure to sunlight). Germs are so small and light that they may remain suspended in the air for hours. Airborne droplet nuclei can be widely dispersed by air currents. Anyone who enters a room with airborne droplet nuclei can be infected. Infectious diseases caused by germs that can be spread via droplet nuclei include tuberculosis, COVID-19, measles, and influenza.

Infectious diseases transmitted by air

Fungal infections

The air is full of fungi. On average, there are between 1,000 and 10,000 fungal spores in every cubic meter of air. A person breathes between 10,000 and 20,000 liters of air every day, and every breath contains between 1 and 10 spores. The Centers for Disease Control and Prevention (CDC) reported that in 2017, over 75,000 people in the United States were hospitalized and 8.9 million were seen as outpatients for fungal diseases at $7.2 billion in direct medical costs.

Aspergillosis is caused by a common mold (fungus) belonging to the genus Aspergillus. Aspergillus spp. infections are associated with dusty or moist environmental conditions. People breathe in numerous Aspergillus spores every day without becoming sick. People with weakened immune systems or lung diseases are at risk of developing health problems caused by Aspergillus. There are many types of aspergillosis, ranging from mild to serious disease. CDC reports total direct medical costs were $1.25 billion in 2017.

The spores of Aspergillus fumigatus have a diameter of 2–3.5 µm, with a settling velocity estimated at 0.03 cm/second (or about 1 meter/hour) in still air. With this enhanced buoyancy, the spores, which resist desiccation, can remain airborne indefinitely in air currents and travel far from their source. As a result, you get infected by breathing in the fungal spores.

Infections due to Cryptococcus neoformans, Histoplasma capsulatum, or Coccidioides immitis can occur indoors if soil or bird droppings are disturbed, and the facility’s air-intake components allow these pathogens to enter the ventilation system.

Cryptococcus neoformans is a yeast usually 4–8 µm in size. However, viable particles of <2 µm diameter have been found in soil contaminated with bird droppings, particularly from pigeons. CDC reports total direct medical costs were $258 million in 2017.

Histoplasma capsulatum range in size from 2–5 µm and is endemic in the soil along the Ohio and Mississippi River valleys of the United States. Substantial numbers of these infectious particles have been associated with chicken coops and the roosts of blackbirds. People develop an infection after breathing in fungal spores, particularly during activities that disturb contaminated soil. In healthy people, the disease is usually self-limiting and is characterized by mild flu-like symptoms. In people with weakened immune systems, histoplasmosis can be severe and require long-term antifungal treatment to resolve the disease. CDC reports total direct medical costs were $216 million in 2017.

Coccidioidomycosis, also called Valley Fever, is an infection caused by the fungi Coccidioides immitis and Coccidioides posadasii. These soil-dwelling fungi with a diameter of 3–5 µm are found in arid, desert-like conditions throughout the southwestern United States (Arizona, California, Nevada, New Mexico, Texas, and Utah), Mexico, Central and South America. Infection occurs breathing Coccidioides spores into the lungs, and unlike most serious fungal diseases, healthy people are at risk for Coccidioides infection. In the United States, the highest incidence of infection occurs in California and Arizona. These states reported 18,407 cases of Valley Fever in 2019. It is estimated that Valley Fever causes up to 30% of community-acquired pneumonia. However, due to low testing rates, Valley Fever may be under-reported. Valley Fever can range from a self-limiting, mild, flu-like disease to severe disseminated infection that can require life-long therapy. CDC reports total direct medical costs were $198 million in 2017.

Bacterial diseases

Mycobacterium tuberculosis is carried by droplet nuclei generated when persons who have pulmonary or laryngeal TB sneeze, cough, speak, or sing. Normal air currents can keep these particles airborne for prolonged periods and spread them throughout a room or building.

Gram-positive bacteria, such as Staphylococcus aureus, are resistant to inactivation by drying and can persist in the environment and on environmental surfaces for extended periods. Airborne dispersal of Staphylococcus aureus is directly associated with the concentration of the bacterium in the nose. Approximately 10% of healthy carriers will disseminate Staphylococcus aureus into the air. The dispersal of Staphylococcus aureus into the air can be exacerbated by concurrent viral upper respiratory infection, thereby turning an infected person into a “cloud shedder.”

Gram-negative bacteria rarely are associated with episodes of airborne transmission because they generally require moist environments for persistence and growth. The main exception is Acinetobacter spp., which can withstand the inactivating effects of drying. In a study of bloodstream infections among children, identical Acinetobacter spp. were cultured from the patients, air, and room air conditioners in a nursery.

Aerosols generated from showers and faucets may potentially contain Legionella pneumophila, which causes Legionnaires’ disease, a serious type of pneumonia. Exposure occurs through direct inhalation.

Virus diseases

Some human viruses are transmitted from person to person via droplet aerosols, but very few viruses are consistently airborne in transmission (i.e., are routinely suspended in an infective state in air and capable of spreading great distances). If an infectious virus spreads predominantly through large respiratory droplets that fall quickly, the key control measures are reducing direct contact, cleaning surfaces, physical barriers, physical distancing, use of masks within droplet distance, and respiratory hygiene.

But if an infectious virus is mainly airborne, an individual could potentially be infected when they inhale aerosols produced when an infected person exhales, speaks, shouts, sings, sneezes, or coughs. Reducing airborne transmission of viruses requires measures to avoid inhalation of infectious aerosols, including ventilation, air filtration, reducing crowding and time spent indoors, using masks whenever indoors, and attention to mask quality and fit.

Airborne transmission of respiratory viruses is difficult to demonstrate directly. Mixed findings from studies that seek to detect viable viruses in air are therefore insufficient grounds for concluding that a virus is not airborne if the totality of scientific evidence indicates otherwise. Decades of research, which did not include capturing live pathogens in the air, indicate that diseases once considered to be spread by droplets are airborne.

The COVID-19 pandemic revealed critical knowledge gaps in our understanding of and a need to update the traditional view of transmission pathways for respiratory viruses. The long-standing definitions of a droplet and airborne transmission do not account for the mechanisms by which virus-laden respiratory droplets and aerosols travel through the air and lead to infection. The evidence that collectively supports that the SARS-CoV-2 virus that causes COVID-19 disease is transmitted primarily by the airborne route includes:

Superspreader events account for substantial SARS-CoV-2 transmission. Studies of people’s behaviors and interactions, room sizes, ventilation in choir concerts, cruise ships, slaughterhouses, nursing homes, and correctional facilities, have shown patterns of long-range transmission consistent with airborne spread and cannot be adequately explained by droplets or fomites.

Long-range transmission of SARS-CoV-2 between people in adjacent rooms, but never in each other’s presence, has been documented in quarantine hotels.

Asymptomatic transmission of SARS-CoV-2 from people who are not coughing or sneezing, and direct measurements show that speaking produces thousands of aerosol particles and few large droplets, which supports airborne spread.

Transmission of SARS-CoV-2 is higher indoors than outdoors and is substantially reduced by indoor ventilation.

Viable SARS-CoV-2 has been detected in the air. In laboratory experiments, SARS-CoV-2 stayed infectious in the air for up to three hours.

Viable SARS-CoV-2 was identified in air samples from rooms occupied by COVID-19 patients in the absence of aerosol-generating health care procedures, and in air samples from an infected person’s car.

Some people have avoided SARS-CoV-2 infection when they have shared air with infected people, but this situation could be explained by a combination of factors, including variation in the amount of viral shedding between infectious individuals by several orders of magnitude and different environmental (especially ventilation) conditions.

High-quality contact tracing data from several countries support a small number of primary cases (notably, individuals shedding high levels of virus in indoor, crowded settings with poor ventilation), accounting for many secondary infections.

In his paper Particle sizes of infectious aerosols: implications for infection control, Kevin Fennelly stated that historically a flawed assumption was that transmission through close proximity implied large respiratory droplets or fomites, was used for decades to deny the airborne transmission of tuberculosis and measles. It was argued that since respiratory droplets are larger than aerosols, they must contain more viruses. However, in diseases where pathogen concentrations have been quantified by particle size, smaller aerosols showed higher pathogen concentrations than droplets when both were measured.

The COVID-19 pandemic has exposed significant gaps in our understanding of the transmission of viruses through the air. The above statements, as well as other studies not mentioned in this article, provide strong evidence that the SARS-CoV-2 virus spreads by airborne transmission, although other routes can contribute, such as contact with contaminated surfaces.

Though the risk of infection by breathing in particles carrying the virus generally decreases with distance from infected people and with time, some circumstances increase the risk of infection:

• Being indoors rather than outdoors, particularly in indoor environments where ventilation with outside air is inadequate.
• Activities that increase the emission of respiratory fluids, such as speaking loudly, singing, or exercising.
• Prolonged time of exposure (e.g., longer than a few minutes).
• Crowded spaces, particularly if face coverings are inconsistently or improperly worn.

Institutional outbreaks of influenza virus infections have occurred predominantly in nursing homes, schools, and workplaces. Evidence supports airborne transmission of influenza viruses by droplet nuclei, and case clusters in pediatric wards suggest that droplet nuclei may play a role in transmitting certain respiratory pathogens (e.g., adenoviruses and RSV).

Evidence also supports airborne transmission of enteric viruses, which invade and replicate in the intestinal tract. An outbreak of a Norovirus infection involving more than 600 staff over a three-week period was investigated in a Canadian hospital in Toronto, Ontario, in 1985. Common sources (e.g., food and water) were ruled out during the investigation, leaving airborne spread as the most likely mode of transmission.

Infectious disease-causing germs proliferate in environments wherever air, dust, and water are present. Accumulation of dust and moisture increases the risk for the spread of fungi and bacteria. Clusters of infections caused by Aspergillus spp., Pseudomonas aeruginosa, Staphylococcus aureus, and Acinetobacter spp. have been linked to poorly maintained and/or malfunctioning air conditioning systems.

Particle size is important

Particle size is the most important determinant of aerosol behavior. Infectious aerosols are particles with viruses, bacteria, and fungi suspended in the air, which are subject to the same physical laws as other airborne particulate matter. Particles that are 5 µm or smaller in size can remain airborne indefinitely under most indoor conditions unless there is removal due to air currents or dilution ventilation.

A re-analysis of the size of particles emitted by an average person that would fall to the ground within 2 meters, or 6 feet is 6-100 µm, and these can be carried more than 6 meters or 19 feet away by sneezing. The biology of the pathogens predicts their airborne survival, infectivity, and virulence. A detailed understanding of droplet size physics, the flow dynamics in space and time, and their measurement are critical to providing sound scientific interventions for infection prevention and control.

Throughout the COVID-19 pandemic, social and physical distancing rules have been an important public health intervention, but the recommended distance to avoid infection varied from 1 meter or 3 feet by the World Health Organization (WHO) and in parts of Europe, to 1.5 meters or 5 feet in Australia, to 2 meters or 6 feet in the United States, Canada, and the United Kingdom.

The scientific basis that links respiratory disease transmission mechanism to a prescribed distance requires urgent and careful revisiting in the context of specific pathogens, environments, and infectious doses.

The differentiation between droplets and aerosols or droplet nuclei by WHO and CDC is based on an arbitrary cut-off in droplet diameter, particles larger than 5 µm are considered droplets, and those smaller than 5 µm are considered aerosols or droplet nuclei.

There are other postulations that a diameter of 20 µm or 10 µm or less should be aerosols, based on their ability to linger in the air for a prolonged period. But studies of the SARS-CoV-2 virus showed that the framework of droplet versus aerosol routes of transmission is not a perfect dichotomy with a sharp boundary in particle size. We need to better understand where, how long, and in which environment pathogen-laden droplets can remain airborne and infectious.

We need to think more about airborne spread

Germs can become airborne when aerosol droplets are generated and released during speaking, coughing, sneezing, vomiting, or the atomization of feces when flushing the toilet. The fate of the droplets in the air is governed by the physical principles of transport, with droplet size being the most important factor affecting the distance traveled by droplets immediately after generation, their dispersion, and deposition on surfaces.

Droplet size is also the key factor determining the survival of microorganisms within the droplets. In addition to the droplets’ physical properties, physical characteristics of the indoor environment, such as temperature, humidity, and airflow characteristics, as well as the design and operation of building ventilation and filtration systems, are of critical importance in affecting indoor infection spread. There are many pathways to infection spread, and among the most significant is airborne transport.