By now, you’re probably aware that Gov. Tim Walz has signed an executive order requiring individuals to wear masks when they are inside public locations, like grocery stores or gas stations. The order encourages people to wear a mask at all times when out in public, but it doesn’t require it when in open air locations. This begs the question, just how dangerous is indoor air compared to the outdoors for contracting COVID-19?
To understand this, we turn to a study that examined COVID transmission in Japan. The study used contact tracing where COVID transmission occurred either inside or outside, but there was limited data on how this transmission was proven, although this data is expected to come later. Regardless, the indoor environments analyzed included fitness gyms, a restaurant boat, hospitals and a festival where eating occurred inside tents.
Researchers analyzed 110 cases, 27 of which were primary cases that led to the transmission of the other cases. After examining the transmission data, researchers found that indoors transmission likelihood was 19 times greater than the odds of catching the virus outdoors. Now, this is just one study and the size is relatively small, but the findings were statistically significant, leading researchers to say with 95 percent confident that indoor transmission was between 6 and 60 times more likely than outdoors transmission.
Analyzing “Superspreaders”
The team also looked at what they defined as “superspreader” events, which involved one confirmed case spreading to at least three other individuals. While this only happened in about six percent of cases, the likelihood of a superspreading event was 32.6 times more likely at indoor locations. Adjusted for the confidence interval, researchers can say with 95 percent certainty that superspreader events are between 4 and 290 times more likely to occur indoors.
Again, because there’s little details about proving the contact tracing data, we should wait until more information comes out to really solidify our conclusions, but it certainly seems that indoors transmission is much more likely than at outdoor events. A similar study out of China found that only one out 7,324 cases is believed to have been transmitted outdoors, during a face-to-face conversation between two individuals. This study relied on a physician’s judgment on where the transmission likely occurred, but those are still astoundingly high numbers for indoors transmission.
At the end of the day, we really do need to be smart about wearing masks when we’re in indoor places where a number of people are around us. This can help reduce the transmission of the virus and buy us time for a vaccine or a deeper understanding of how to best care for sick individuals.
