A recent blog by Dr. Erin Bromage, entitled “The Risks – Know Them – Avoid Them”, describes how viruses are transmitted in terms that a non-scientist can easily read. Here’s a summary of the most salient points:
In order to become infected, a person must take in a number of virus particles. That number is not yet known for Covid-19, but for SARS that number is as low as 1,000 virus particles.
Each breath, sneeze or cough expels droplets into the air. Droplets from an infected person carry virus particles ranging in number depending on the size of the droplets. Sneezes and coughs expel both large and small droplets, while normal speech and breathing carry mostly smaller droplets. A single sneeze can carry as many as 200,000,000 (two hundred million) virus particles. Ordinary conversation carries about 200 virus particles per minute. The number of particles expelled per minute increases significantly with louder speech and singing.
The speed of the expulsion determines how far the droplets will travel. Coughs travel at 50 mph; sneezes up to 200 mph. Large droplets follow an arc like a tennis ball and fall more quickly than small, aerosolized droplets. Aerosolized droplets can float in the air for long periods of time and are greatly affected by air flow (think, AC vent). Thus, virus particles can easily travel across a room.
Viral particles are taken in from the air through the nose, mouth and eyes. They can also be taken in when a person touches a surface where particles have fallen, then touches their face.
How does one get infected? Bromage presents this formula: Successful Infection = Exposure to Virus x Time. So, you could get infected by just being in the room with one sneeze or by five to ten minutes of close conversation. Dr. Bromage focuses on the dangers of being in noisy places where people in close quarters are speaking more loudly, like a bar, a sporting event or a meat packing plant. Singing in a group adds a danger because you inhale deeply, thus taking virus particles more deeply into the lungs.
To eliminate any mistranslation, I suggest that you read Dr. Bromage’s article. It can help you evaluate the risk before you go back to work, go shopping or get a hair cut. Also remember your increased risk increases everyone’s risk, especially our first responders. Think of others.
Tobey Miller, Fort Payne
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