SARS-CoV-2 Infectious Dose
Some experts estimate that exposure to as few as 1000 SARS-CoV-2 viral particles can cause an infection. This dose of virus could occur by inhaling 1000 infectious viral particles in a single breath, 100 viral particles in 10 breaths, or 10 viral particles in 100 breaths.
A single cough releases about 3,000 respiratory droplets that travel at 50 miles per hour. Most droplets are large, and quickly fall to the ground, but many remain airborne and can travel across a room in a few seconds. A single sneeze releases about 30,000 droplets traveling up to 200 miles per hour. Most droplets are small and travel great distances. The droplets from a single cough or a sneeze emitted by an infected person may contain as many as 200,000,000 virus particles.
Unlike sneezing and coughing which release huge amounts of viral material, the respiratory droplets released from breathing only contain low levels of virus. A single breath releases 50 to 5000 droplets. Most of these droplets travel at low velocity and fall to the ground quickly. Because breath is expelled at low force, viral particles residing in the lower respiratory areas are not expelled. Breathing may release as few as 20 to 30 viral particles per minute.
Speaking increases the release of respiratory droplets about tenfold more than breathing; 200 virus particles per minute. A person could become infected after ten minutes of face to face talking with an infected person.
Infection with SARS-CoV-2 depends not only on the infectious dose of virus but also exposure time. If an infected person coughs or sneezes directly towards someone, they can become infected with 1000 viral particles in a few minutes. If someone enters a room shortly after an infected person coughs or sneezes, they can become infected after taking a few breaths. If an infected person was breathing and emitting 20 viral particles per minute, but not sneezing or coughing, another person would need to remain in the same room for at least 50 minutes to inhale an infectious dose of 1000 viral particles.
At least 40% of community spread SARS-CoV-2 infections are transmitted by asymptomatic people, who can shed virus up to 5 days before symptom onset. Any time an individual spends more than 10 minutes within a face-to-face situation with an asymptomatic person, they are at risk of infection. Sharing a space with an asymptomatic person for an extended period increases the risk of infection. A high density of people inhabiting an enclosed space with poor air circulation is especially risky. This explains why many of the largest outbreaks have involved:
- Meat packing plants
- Long term care facilities
- Churches
- Warehouses
- Call centers
- Business conferences
- Social gatherings such as funerals and birthdays
- Restaurants
- Public transportation
Social distancing guidelines are designed to minimize the risk of exposure. As the guidelines are relaxed, everyone needs to remember the principle of viral exposure over time. When assessing the risk of resuming normal activities, individuals should consider a facilities’ air volume, people density, and the time spent. Briefly entering a large store with high air volume and low people density poses a low risk of receiving an infectious dose of SARS-CoV-2. However, an employee of that same store who works extended hours and contacts many people each day has a much higher risk of receiving an infectious dose.
Reference
Bromage E. The Risks-Know Them-Avoid Them. https://www.erinbromage.com/post/the-risks-know-them-avoid-them
