Frequently asked questions

Our atmosphere is made up primarily of gases — about 78% nitrogen and 21% oxygen, with the rest made up of other gases. In addition to these gases, our atmosphere also contains very small liquid droplets and solid particles, known as particulate matter (PM). These particles play an important role in human health and climate, but are so small that you usually can't see them with your bare eyes. In areas highly polluted with particulate matter, however, the difference is clear.

These very small liquid droplets and solid particles are lightweight, so they can float in the air for a long time without falling to the ground — they are suspended in the air. When particulate matter is suspended in a gas, it is called an aerosol. When you hear "aerosol," you might think of a can of hairspray or spray paint. But the particulate matter in our atmosphere can also be referred to as an aerosol because it is suspended within the gases in the air.

Adapted from: CAICE — Introduction to Aerosols

Aerosol particles contribute to a variety of health problems. According to estimates from the World Health Organization, particle pollution contributes to approximately 7 million premature deaths each year, making it one of the leading causes of worldwide mortality.

The health effects of particles are directly related to their size. Particles greater than 100 µm are generally too large to be inhaled. Particles between 10–100 µm can be inhaled but are usually stopped by our body's defenses — the mucus membranes in our respiratory system. Inhalable particles less than 10 µm can make it past these defenses and deep into the lungs, posing the most health risk.

Inhalable coarse particles (PM10, between 2.5 and 10 µm) are typically found near highways and factories and are usually deposited in the nose, pharynx, and larynx. Fine particles (PM2.5, less than 2.5 µm) are typically found in smoke and smog, reaching deeper into the trachea, bronchioles, and alveoli.

Scientific studies have linked particle pollution to aggravated asthma, decreased lung function, increased respiratory symptoms, nonfatal heart attacks, irregular heartbeat, and premature death in people with heart or lung disease.

Adapted from: CAICE — Introduction to Aerosols

When we talk, shout, sing, cough, sneeze or simply breathe, we exhale small particles ("aerosols") made of saliva or respiratory fluid. These aerosols are the "carriers" of SARS-CoV-2 viruses and can infect when they are inhaled.

Aerosols are smaller than about 100 µm, so they can linger in the air from tens of seconds to hours, and the smaller ones can travel long distances. They infect by being inhaled through the nose or mouth, or (less likely) by deposition on the eyes. They reach different parts of the human respiratory tract depending on their size.

Figure: schematic representation of the droplet (blue) and aerosol (green, yellow, and red) infection pathways for a respiratory disease. From Milton (2020), modified by the author.

Adapted from: FAQs on Protecting Yourself from COVID-19 Aerosol Transmission

While the size of an individual SARS-CoV-2 virus is very small (120 nm or 0.12 microns), the aerosol in which respiratory viruses are contained are larger, albeit still small enough to remain suspended in air for long periods. A widely held misconception is that the virus is naked in the air, perhaps with some water. Our best guess is that the most common aerosol size is a few microns, where the viruses comprise a very small fraction of the aerosol.

Aerosol size has major implications for the ability of masks and filters to remove it from the air, how deeply it will penetrate the lungs, and determines the loss rate due to gravitational settling in indoor spaces.

Adapted from: FAQs on Protecting Yourself from COVID-19 Aerosol Transmission

Like other recommendations, these social distance rules will reduce risk, but not eliminate it. The 6 feet rule is based on the idea that large ballistic droplets fall to the ground within 6 feet, although they can travel farther in a cough or sneeze, up to 28 ft (8.5 m). The rule also helps with aerosols that do not settle to the ground because they are most concentrated close to the person who released them — like cigarette smoke near the smoker.

Indoor air is a constantly changing, turbulent environment. Even though you can't feel it, indoor air is constantly moving (typically 0.05 to 0.1 m/sec) due to body heat, sunlight warming surfaces, people moving and breathing, and HVAC airflows. This constantly changing environment dilutes plumes of aerosols. The dilution is greater with more time and space — so the farther away you can be from other people indoors, the better.

Adapted from: FAQs on Protecting Yourself from COVID-19 Aerosol Transmission

We can never be perfectly safe, only safer. You should try to avoid or reduce situations that facilitate inhaling exhaled air from others. To reduce risk, avoid crowded places, close-contact settings, confined and enclosed spaces, low ventilation areas, long duration exposures, places where people are unmasked (especially while talking, singing, or exercising), and places without air filtration.

Each one of these features potentially increases the aerosol concentration you might inhale indoors. If you must enter one of these situations, complete your tasks as quickly as possible to reduce your exposure duration and risk.

Adapted from: FAQs on Protecting Yourself from COVID-19 Aerosol Transmission

All data show that outdoors is far safer than indoors, for the same activity and distance. But that does not mean outdoors is 100% safe — cases of transmission have been traced to outdoor conversations.

Engaging in riskier activities outdoors may undo some of the benefits. Crowded outdoor locations, especially in more confined spaces (e.g. between tall buildings) under low wind and not in the sun, are the riskiest — there is less wind to disperse virus-laden aerosols, and less UV to deactivate the virus.

The risk of transmission is much lower outside because viruses released into the air rapidly become diluted through the atmosphere. Think of the smoke analogy: if people near you were smoking outdoors and you could inhale a lot of smoke, there's more risk. But outdoors is still much safer than indoors.

Adapted from: FAQs on Protecting Yourself from COVID-19 Aerosol Transmission