An airborne disease is any illness caused by a pathogen that travels through the air and infects someone who breathes it in. Unlike infections that require direct contact or contaminated food, airborne diseases can spread across a room, through ventilation systems, and sometimes linger in the air long after an infected person has left. Tuberculosis, measles, chickenpox, influenza, and several fungal infections all spread this way.
How Pathogens Travel Through the Air
When you breathe, talk, sing, cough, or sneeze, you release a spray of tiny particles from your mouth and nose. These particles exist on a spectrum of sizes. The larger ones are heavy enough that gravity pulls them down within seconds. A 50-micrometer particle, for example, falls about five feet in roughly 20 seconds. But smaller particles, those under 5 micrometers, behave very differently. A 5-micrometer particle can stay suspended in still air for around 32 minutes. These tiny particles are small enough to be inhaled deep into the lungs, reaching the smallest air sacs where gas exchange happens.
For decades, scientists drew a hard line at 5 micrometers: anything larger was a “droplet,” anything smaller was an “aerosol.” In 2024, the World Health Organization moved away from this binary. The updated terminology uses “infectious respiratory particles” to describe the full range of sizes and recognizes that transmission through the air happens on a continuum. Under this framework, airborne transmission specifically refers to particles that are expelled, travel through the air, and are then inhaled by another person, whether they’re standing nearby or across the room.
Diseases That Spread This Way
The list of confirmed airborne pathogens is broader than most people realize. It includes viruses, bacteria, and fungi.
- Measles is one of the most contagious airborne diseases known. The virus can remain infectious in the air for up to two hours after an infected person leaves a room. The CDC recommends keeping a room vacant for that full period to allow for removal of 99.9% of airborne contamination.
- Tuberculosis is caused by a slow-growing bacterium with an extraordinarily low infectious dose. It takes fewer than 10 inhaled bacteria, on average, to establish an infection. This is why TB has historically been so difficult to control in crowded, poorly ventilated settings like prisons, shelters, and hospitals.
- Chickenpox and shingles are caused by the varicella-zoster virus, which spreads through the air and has no animal reservoir. Humans are the only host.
- Influenza spreads through both large and small respiratory particles. The virus itself is tiny, only 80 to 120 nanometers, and easily becomes airborne during coughing and sneezing.
- Fungal infections like aspergillosis, Valley fever, and histoplasmosis come from inhaling spores found in soil, compost, bird droppings, and building materials. These aren’t passed person to person but are truly airborne in that the infectious particles travel through air into the lungs.
Less commonly recognized airborne pathogens include Legionella bacteria (spread through contaminated water systems that aerosolize the organism), hantavirus (shed in rodent urine and droppings that become airborne when disturbed), and norovirus, which can become aerosolized during projectile vomiting.
Why Indoor Spaces Are the Primary Risk
Airborne transmission is overwhelmingly an indoor problem. Quantitative modeling published in Environmental Research showed that outdoor transmission risk is typically orders of magnitude lower than indoor risk. Outdoors, wind disperses infectious particles rapidly, and UV light from the sun degrades many pathogens. Indoors, particles accumulate in the shared air, concentrations build, and exposure time is longer.
The exception is very specific weather conditions: extremely calm winds combined with a strong temperature inversion that traps air near the ground. In those rare scenarios, outdoor risk can approach indoor levels, particularly in dense crowds. But for day-to-day life, moving activities outdoors dramatically reduces transmission of airborne pathogens.
Scientists use a tool called the Wells-Riley equation to estimate infection probability in enclosed spaces. The equation accounts for how many infected people are in a room, how much fresh air is being supplied, how quickly the pathogen is being released, and how long susceptible people are breathing that air. The key insight is that ventilation rate is the variable you can most easily change. Doubling the fresh air flowing through a room roughly halves the concentration of infectious particles.
What Affects How Long Pathogens Survive
Temperature and humidity play a significant role in how long airborne pathogens remain viable, but the relationship isn’t simple. Research at 23°C (about 73°F) found that some viruses survived best in very dry air at 10% relative humidity, while others lasted longest in humid air at 90% relative humidity. There’s no single “safe” humidity level that kills all airborne pathogens.
What is consistent is that very dry indoor air, common in heated buildings during winter, favors the survival and transmission of several important respiratory viruses, including influenza. This partly explains the seasonality of flu and other winter respiratory illnesses. Dry air also impairs the mucous membranes in your nose and throat, reducing your body’s first line of defense.
How Filtration and Masks Reduce Exposure
HEPA filters, used in hospital isolation rooms and portable air purifiers, capture particles down to 0.3 micrometers with at least 99.97% efficiency. Since most airborne bacteria are larger than 0.3 micrometers and virus-carrying respiratory particles typically are as well, HEPA filtration is highly effective at clearing contaminated air in enclosed spaces.
For personal protection, the difference between mask types is significant. N95 respirators filter 98 to 99.7% of airborne particles when tested with the most challenging particle size (around 0.075 micrometers, which is close to the size that’s hardest for any filter to catch). Standard surgical masks, tested the same way, filter only 55 to 88% of those particles. That gap matters most in high-risk environments like hospitals or during outbreaks of highly contagious airborne diseases. The key factor beyond filtration is fit: an N95 that doesn’t seal against your face performs more like a surgical mask.
Ventilation as the First Line of Defense
The single most effective environmental control against airborne disease is moving contaminated air out and clean air in. This is why tuberculosis control programs focus heavily on ventilation in clinics, why operating rooms use positive-pressure air systems, and why opening windows during respiratory illness season provides measurable protection. Even without specialized equipment, increasing airflow through a room by opening windows on opposite sides creates cross-ventilation that dilutes infectious particles rapidly.
Ultraviolet germicidal irradiation, installed in the upper portion of a room or inside air ducts, adds another layer by inactivating pathogens as air circulates past the UV light. Combined with adequate ventilation and filtration, these measures form the environmental triad that hospitals and public health agencies rely on to control airborne transmission in high-risk settings.