An aerosol is any collection of tiny particles or liquid droplets suspended in a gas, usually air. The defining feature is not what the particles are made of, but that they are small and light enough to float rather than fall immediately to the ground. Aerosols range from natural phenomena like fog and wildfire smoke to manufactured products like spray paint and inhaler mist.
Size: The Key Factor
What separates an aerosol from a regular droplet or piece of debris is particle size. Aerosol particles typically measure from about 0.01 micrometers up to roughly 100 micrometers in diameter (a micrometer is one-thousandth of a millimeter). For context, a human hair is about 70 micrometers wide, so most aerosol particles are invisible to the naked eye.
Size determines how long particles hang in the air. CDC data on settling times in still air illustrates this dramatically: a 100-micrometer particle falls five feet in about 6 seconds, while a 10-micrometer particle takes over 8 minutes. Shrink that to 1 micrometer and it stays airborne for roughly 12 hours. At 0.5 micrometers, the particle can float for 41 hours. This is why smaller aerosols travel farther, penetrate deeper into buildings, and pose greater health risks.
Natural Aerosols
The atmosphere is full of aerosols from natural sources. Sea spray launches salt crystals and organic matter, including bacteria and microalgae, into the air above the ocean. Wind lifts mineral dust from deserts and dry soil. Wildfires produce massive plumes of smoke containing organic carbon and black carbon (soot). Volcanoes contribute two major types: ash, which is made of pulverized rock containing minerals like silica, and sulfate particles, which form when volcanic sulfur dioxide reacts with water vapor in the atmosphere.
NASA classifies these as “primary aerosols” because they enter the atmosphere directly as particles. “Secondary aerosols” start as gases and become particles only after undergoing chemical reactions in the air. Volcanic sulfate is one example of this process.
Human-Made Aerosols
Industrial activity, transportation, and agriculture all generate aerosols. Vehicle engines and power plants release nitrogen oxides, which react in the atmosphere to form nitrate aerosol particles. Burning fossil fuels produces sulfur dioxide that converts to sulfate aerosols, the same type volcanoes create but in far larger quantities. Agricultural land clearing and waste incineration add smoke and soot.
Consumer aerosol products, like spray deodorant, cooking spray, and spray paint, work by a different mechanism entirely. A pressurized container holds the product mixed with a propellant gas. When you press the nozzle, the pressure difference forces the liquid through a valve designed to break it into a fine mist of suspended particles. Modern aerosol cans use liquefied gas propellants, typically short-chain hydrocarbons, because they maintain constant pressure as the can empties. The older propellants, chlorofluorocarbons (CFCs), were banned under the 1987 Montreal Protocol after scientists discovered they were destroying the ozone layer.
How Your Body Creates Aerosols
Your respiratory system produces aerosols constantly. Breathing, talking, singing, coughing, and sneezing all expel tiny particles from the lining of your airways. A cough generates particles across a wide size range: research published in the Journal of Laryngology and Otology found that involuntary coughs produce particles both smaller than 1 micrometer and larger than 5 micrometers, with the smallest particles vastly outnumbering the larger ones.
This matters because respiratory aerosols can carry viruses and bacteria. In 2024, the World Health Organization updated its terminology to reflect current science, replacing the old “aerosols versus droplets” distinction with a single term: “infectious respiratory particles,” or IRPs. The WHO now recognizes that these particles exist on a continuous spectrum of sizes with no clean cutoff between what floats and what falls. Under this updated framework, “through the air transmission” is the umbrella term for any infection spread by particles that travel through or remain suspended in the air.
Why Particle Size Matters for Health
The smaller an aerosol particle, the deeper it can travel into your lungs. Particles larger than about 10 micrometers tend to get caught in your nose and throat. Particles under 10 micrometers (classified by the EPA as PM10) can reach the bronchial tubes. The most concerning category is PM2.5, particles smaller than 2.5 micrometers. These are small enough to pass through the nose’s natural filtration, travel all the way to the alveoli (the tiny air sacs where oxygen enters your blood), and irritate or corrode the tissue there. Their small size also gives them a large surface area relative to their volume, meaning they can carry toxic compounds deep into the respiratory system and, through gas exchange in the lungs, potentially affect other organs.
The EPA regulates PM2.5 levels in outdoor air. In February 2024, the agency tightened the annual standard to 9.0 micrograms per cubic meter, down from the previous limit, citing evidence linking fine particle exposure to heart attacks and premature death.
Aerosols in Medical Settings
Certain medical procedures generate unusually high concentrations of aerosol particles, earning them the designation “aerosol generating procedures,” or AGPs. These include intubation (placing a breathing tube), bronchoscopy (inserting a camera into the airways), suctioning of the respiratory tract, tracheostomy, and endoscopic sinus surgery. During the COVID-19 pandemic, AGPs became a central concern because healthcare workers performing them faced elevated risk of inhaling virus-laden particles. Hospitals responded by requiring enhanced protective equipment and developing specific safety checklists for these procedures.
The concern extends beyond hospitals. Any enclosed, poorly ventilated space where people are breathing, talking, or coughing can accumulate respiratory aerosols over time. This is why ventilation and air filtration became prominent public health topics: removing or diluting suspended particles reduces the chance of inhaling whatever they carry.