Live virus can be found in the respiratory tract, urine, and feces. It is typically spread via large droplets released when sneezing or coughing, or by aerosolized particles–droplets of moisture that are small enough to remain floating in the air for hours. Small amounts of aerosols are released when breathing, medium amounts when speaking or breathing heavily, large amounts when speaking loudly or singing, and even larger amounts when coughing. It can also be spread through aerosols released when a toilet is flushed. Infection happens through the mucosal membranes in the nose, throat, and eyes. Virus particles can reach these membranes either via inhalation or from being touched by contaminated hands.
Viral infection operates on a sliding scale for both the infector and infectee. The number of live virus particles contained in respiratory droplets or aerosols is proportional to the viral load in your body–how much the virus has already replicated. Viral load cannot be inferred from the presence of symptoms or their severity. It usually peaks in the first week after symptom onset, but can be at infectious levels several days before symptoms begin to show. In order to become infected, the Accumulated Exposure must reach an Infectious Dose of live virus particles in a short enough window of time. This infectious dose varies from person to person based on their immune system health. Rodent studies and expert opinions estimate that the infectious dose could be as low as 500 SARS-CoV2 infectious viral particles. If the accumulated exposure is much higher than the infectious dose it could lead to a more severe infection. This has been proven to be the case with SARS, MERS, and influenza, as well as initial animal studies with COVID-19.
A single sneeze or cough can deliver an infectious dose. Several minutes of conversation could do the same. Louder conversation is riskier. Singing even more so. The longer the exposure, the greater the accumulated dose, and the higher the risk.
A 2 meter / 6 feet distance is not by itself sufficient to eleminate risk. The cloud of particles from a sneeze can travel up to 7-8 meters or 26 feet.
Infectious particles can be blocked both by physical barriers and by environmental factors.
Masks are better for blocking large droplets than they are for aerosols. Well-fitted N95 respirators can block 95% of aerosols, leading to ~70% reduction in infection risk. Surgical masks can provide a 2-3x reduction in viral copy numbers. Cloth masks range from 26-98% depending on the fit, fabric, and number of layers. The key point to remember is that even if a mask blocks 50% of particles, that halves the total accumulated exposure. Both parties wearing masks that block 80% of particles lowers the accumulated exposure by a factor of 25.
There is very little evidence of outdoor transmission of the coronavirus. . This is mainly due to the large amount of air flow dispersing particles extremely quickly. Heat, ultraviolet light, and humidity all help to either keep the particles from spreading or kill the virus in-transit. However, unlikely is not impossible. One study of 7,324 identified cases identified only one outdoor outbreak involving 2 cases, involving a long conversation in close proximity.
A single sneeze or cough can deliver an infectious dose. Several minutes of conversation could do the same. Louder conversation is riskier. Singing even more so. The longer the exposure, the greater the accumulated dose, and the higher the risk.