Airborne vs. Aerosol vs. Droplet: What's the Difference?

This article was updated on July 13 with new information from the World Health Organization.

COVID-19 has introduced several new terms to the national lexicon. In addition to phrases like social distancing, self-quarantine, and N95 respirator, you may have seen the word airborne popping up more in recent weeks. It usually appears in discussions concerning how the novel coronavirus that causes COVID-19 is transmitted—a question that's still debated by health experts. So what exactly does it mean when a pathogen is airborne, what do droplets and aerosol have to do with it, and which of these terms apply to the new coronavirus?

What are droplets?

We know that there are at least two ways to catch the new coronavirus: by coming in close contact with infected individuals and touching contaminated objects and surfaces. In both cases, the root of the transmission is often a cough or sneeze. When someone with COVID-19 coughs—which is a common symptom of the disease—they send a spray of mucus and saliva droplets flying from their mouths. These tiny, sometimes invisible droplets, measuring between five and 10 micrometers in diameter, contain particles of the virus. Coughing into a shirt sleeve or mask can catch a lot of these droplets, but with nothing to block them, many will land on objects and people in the immediate vicinity. This is why a sick person is more likely to infect more people standing elbow-to-elbow with them in crowded subway car than they are keeping a 6-foot distance from others in a spacious park.

You don't need to be touching someone to contract coronavirus from them. If someone is standing directly behind you in line at the grocery store, they can infect you through the droplets in a sneeze or cough. But while these droplets technically travel through the air, that doesn't automatically make COVID-19 an airborne disease—at least not according to the definition of the word used by health officials. In order to understand what airborne really means, you need to know about aerosols.

What are aerosols?

Saliva and mucous droplets are heavier than air, which means gravity starts pulling them—and whatever viral particles they contain—towards the ground as soon as they leave someone’s body. By the time someone walks out of a room, any droplets they may have emitted have likely already settled on the floor or nearby surfaces—so usually, you don’t need to worry about breathing in those droplets if you’re social distancing correctly.

Aerosols are a different story. They form when smaller droplets evaporate faster than they fall to the ground, leaving nuclei measuring less than five micrometers in diameter. Without heavy liquids dragging them down, virus particles from these evaporated droplets are able to float through the air for up to half an hour. When a virus travels via aerosols, it’s possible to contract it by entering an empty room that a sick person was in several minutes earlier. This transmission via free-drifting aerosols is how the World Health Organization defines an airborne disease.

Is the new coronavirus airborne?

The WHO updated its scientific brief in July to say that airborne transmission of the new coronavirus appears to be possible. Citing three studies of infections in a gym, a choir practice, and a restaurant, WHO stated, "short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons, cannot be ruled out."

Though there may be a chance of the novel coronavirus wafting in ambient air, it's not unsafe to open a window or go outside. Airborne viruses are more likely to spread in rooms with poor ventilation than they are outdoors, so allowing air to circulate in your home can actually help prevent the spread of disease. Plus, getting fresh air and exercise is important for staying mentally and physically well in stressful situations.

When leaving the house, just keep some safety precautions in mind. Maintain at least a 6-foot distance from others to protect yourself from droplet transmission, and wash your hands thoroughly after touching surfaces that may be contaminated. Wearing an N95 respirator is the best way to prevent airborne infection, but because supplies are limited, new masks should be reserved for especially vulnerable people like healthcare workers. Homemade cloth masks and surgical masks aren’t built to filter out smaller airborne particles, but they can stop larger droplets from splashing onto your face, and just as importantly, stop infected wearers from spreading droplets to others.