COVID-19 – 18.7 Times More Likely to Transfer Indoors than in Open Air

The CDC has published an article on super spreading events and their mitigation. In the report, the CDC cites a Japanese study that shows that all of their super spreading events started indoors. That study concluded that indoor locations were 18.7 times more likely to spread COVID-19 than open air locations. Dividing a 2-3 R0 level by 18.7 basically makes the number so low it is barely spreading (0.1 – 0.16).

This is another reason to be hopeful about the upcoming warm weather and to go outside every day not just for Vitamin D. It is also a reason government orders curbing outside activity may be counterproductive. Preventing indoor gatherings and encouraging outdoor social distancing may slow down outbreaks more than broad restrictions on both indoor and outdoor activities.

Warm weather has often resulted in a dramatic reduction in the spread of most viruses. There are many potential factors.

Outdoors, people are spaced out over many more times the square footage than they would normally be indoors. Each sneeze or cough is much less likely to transmit a live virus given the distance between people. The vastness of outside air acts as a filter unlike indoors where air could be recirculating that’s contagious.

Let’s go through some conceptual math. A 2,000 square foot home will have 2,000 square feet of surface area and 16,000 cubic feet of air (assuming 8 foot ceilings) for a sneeze or cough to end up.  That house’s quarter acre yard will have 10,890 square feet of surface area and 544,500 cubic feet of air (just up to 50 feet) for a sneeze or cough to land. Compare those figures to available public space square footage and the full atmosphere and the numbers are staggering, dropping the risk dramatically.

Sunlight, heat and humidity levels impact different viruses differently. Heat and sun are associated with shorter viral life outside the human body for most viruses. A Hong Kong study shows that COVID-19 does not react well to heat in a lab. Other studies show viruses like SARS (verified studies) and COVID-19 (preliminary data) do not survive well in direct sunlight. 

None of these are studies are conclusive yet, but there is correlation in the data we do have. Most of the severe outbreaks are between 30 and 50 degrees in the northern latitudes where weather has been cooler.

Correlation does not mean causation, but there must be correlation in order for there to be causation. Although not conclusive, so far the data and studies look promising that the R0 level of COVID-19 will drop in the warmer months. Should this happen, the slower transmission rate will allow herd immunity to grow in a sustainable manner, provide a reprieve to the healthcare system, and allow the healthcare system time to scale up for the cooler period when the virus R0 returns to strength.

Stay safe, stay strong!