By: Jerald W. Koepke, MD
This pandemic will not be over for many months. All the stay at home orders were successful in stemming the tide, but common sense says that they have left a large number of vulnerable people ready to be infected. So, another surge, or 2, or 3 is inevitable. Some interesting facts about COVID-19 that are noteworthy.
First of all, the primary mode of transmission of COVID-19 is through respiratory droplets. Respiratory microdroplets can be produced and stay suspended in the air for 30 minutes to hours after an individual coughs, sneezes or just speaks loudly. The microdroplet clouds potentially could contain virus, so respiratory precautions, masks, eye protection, and good hand washing are essential when indoors, particularly in poorly ventilated spaces.
Some estimate that as few as 1000 SARS-CoV2 viral particles are needed for an infection to take hold. Infection could occur, through 1000 viral particles you receive in one breath or from one eye-rub, or 100 viral particles inhaled with each breath over 10 breaths, or 10 viral particles with 100 breaths. It is estimated that it takes approximately 10 minutes of exposure to inhale enough virus to become infected when in an enclosed room talking with an infected person. If the infected person is coughing or sneezing, the exposure could be much greater and an infectious dose could be inhaled in a shorter period of time.
When you think of outbreak clusters, what are the big ones that come to mind? Most people would say cruise ships. But you would be wrong. Ship outbreaks don’t even land in the top 50 outbreaks to date. Ignoring the terrible outbreaks in nursing homes, we find that the biggest outbreaks are in prisons, religious ceremonies, and workplaces, such as meat packing facilities and call centers. Any environment that is enclosed, with poor air circulation and high density of people, spells trouble. The reason to highlight these different outbreaks is to show you the commonality of outbreaks of COVID-19.
All these infection events were indoors, with people closely-spaced, with lots of talking, singing, or yelling. The main sources for infection are home, workplace, public transport, social gatherings, and restaurants. This accounts for 90% of all transmission events. In contrast, outbreaks spread from shopping appear to be responsible for a small percentage of traced infections. Dining in restaurants present an ongoing risk, unless they have outdoor seating.
The Great Outdoors
These concerns essentially go away when outdoors, since these microdroplet clouds disperse into the atmosphere and the concentration of virus is next to nothing, certainly not enough to cause an infection. It takes a certain concentration of virus and the duration of exposure must be long enough for an infection to occur. Think of carbon monoxide poisoning. It does not occur outdoors.
Masks in an outdoor setting are only necessary in settings where there is a gathering of individuals in close proximity, e.g. the 16th street mall, public transportation, sporting events etc. Just passing someone carrying the virus on a sidewalk will not pose a significant risk. They would literally have to cough or sneeze directly on you to cause any meaningful exposure.
So, respiratory aerosolized droplets are the main way respiratory viruses are spread. Infection from surface contamination, although it can occur, poses less of a threat. As far as the corona viruses are concerned, they do not live long on surfaces. There was a recent editorial in the New England Journal of Medicine where researches studied the survival of corona viruses, both in aerosols, and on different types of surfaces. After 24 hours the amount of virus surviving on all surfaces decreases significantly. By 72 hours, the amount of virus on any type of surface is either non-detectable or not enough to cause an infection.