When we breathe, talk, laugh, cough, or sneeze, we release droplets into the air. Some of these are microscopic and can remain floating in the air for an hour or more. Others are large, fly through the air briefly, and fall to the ground or other surfaces within seconds. If a person is infected with a virus, some of the droplets may contain viruses.
The droplets also contain large amounts of salts and proteins and other chemicals that are in respiratory fluid. If the air is not very humid, some or all of the water in the droplet will evaporate, leaving behind a smaller droplet that is about half the original size.
From research on flu patients, we know that at least half of the viruses that are expelled into airborne droplets are found in those smaller than 5 microns, which can remain floating in air for half an hour or more. During this period, they may be carried by air currents many meters from the person who released them, but this process also dilutes them in the air, reducing the chance that you could breathe in a lot of them.
Viruses in droplets that land on surfaces and in the microscopic ones that float in air break down over time and lose their ability to cause infections. How fast this happens depends on the type of virus and conditions such as temperature, humidity, and the type of surface material, whether it’s cloth, paper, plastic, or metal.
There are a lot of things we still don’t know about viruses in the air. We don’t know how much transmission occurs through inhalation of such viruses vs. them landing in our eyes, nostrils, or mouth vs. touching them on objects. For most types of viruses, we don’t know how many are found in different size droplets and how well the viruses survive under different conditions in the real world. What is clear is that for some types of respiratory viruses, transmission at close range can occur through multiple routes, including direct spray of large droplets onto the body and inhalation of microscopic droplets.