How Dangerous Is the OUTDOOR Air for COVID-19?

Back in April, I covered evidence from the Diamond Princess Cruise Ship suggesting that COVID-19 has very little if any transmission in the open air. Despite open balconies with free flowing air during the quarantine period of the outbreak, and despite central air conditioning that derived 30% of its air from the outdoors, transmissions only occurred among individuals in the same stateroom.

Since then, two other reports have suggested that the outdoor environments are, while not 100% risk-free, exceedingly safer than indoor environments.

Indoor Transmission Is 19 Times Riskier Than Outdoor Transmission

A preprint* released on April 16, which received considerable media attention, compared indoor and outdoor transmission in Tokyo and six other prefectures of Japan. 

It used “contract tracing data” to determine whether the transmissions occurred inside or outside, but did not provide any details on how the data was obtained (presumably these data will be added by the time it is peer-reviewed, if it winds up peer-reviewed). The indoor environments included “fitness gyms, a restaurant boat on a river, hospitals, and a snow festival where there were eating spaces in tents with minimal ventilation rate.”

They examined 110 cases, 27 of which were primary cases that led to the transmission of the remaining cases. The odds of transmission indoors were 18.7 times greater than the odds of transmission outdoors. In other words, indoor transmission was 19 times riskier than outdoor transmission.

While the sample size is small, the results were statistically significant, meaning the chances of observing an effect this size or greater were there no true difference would be less than 5%. The 95% confidence interval was 6.0-57.9. This means that we can be 95% confident that indoor transmission is anywhere from 6 times riskier to 60 times riskier than outdoor transmission.

The researchers also looked at “superspreader” events, which they defined as one case spreading to three or more people, and which only accounted for just over 6% of the cases. The odds of superpreading events was 32.6 times greater indoors and the 95% confidence interval was 3.7-289.5, meaning we can be 95% confident that superspreader events are somewhere between 4 and 290 times as likely to occur indoors.

Given the small percentage of superspreading events, the best interpretation of this study is simply that indoor environments are 19 times as dangerous as outdoor environments for spreading COVID-19. 

However, the fact that the authors provide no details about how the contract tracing data was obtained and provide very few details about the spreading events suggests we should be cautious about forming conclusions from this study until the manuscript is in its final form and has been peer-reviewed.

Almost All Transmissions in China Occurred Indoors

A preprint released on April 7 used more comprehensive data from China.

Beginning on January 20, 2020, municipalities within China outside of Hubei province published written reports of most identified cases online. The capital of Hubei is Wuhan, and the largest number of cases occurred within Hubei. This dataset includes most cases that occurred outside Hubei. 

The authors collected all the case reports from outside Hubei that included sufficient information about demographics, history of exposure, and course of infection, which totaled 7,324 cases. These represented two-thirds of the 10,980 total known cases that occurred outside of Hubei up to February 11. They defined a cluster as an event where three or more infections were traced to the same infection venue within a close period of time. They defined an outbreak as a cluster where a single person appeared to cause the initial spreading. To reduce their workload and eliminate spouse-to-spouse transmissions, they limited their further analysis to clusters, which excludes transmission events involving only two people.

Within the “clusters” there were 318 outbreaks involving 1,245 people in 120 cities. A little over half of the outbreaks involved three cases; a little over a quarter involved four cases; only five outbreaks (1.6% of the total) involved ten or more cases. The largest outbreak was in a shopping mall, where 21 people were infected.

Only 27 (8.5%) of the outbreaks involved individuals who were neither family nor otherwise socially connected. 

In 26% of the outbreaks, there were multiple venues suspected to be responsible for the transmission. As a result, there were 416 venues that were suspected or confirmed to be responsible for transmission. 61% of these venues were homes (one in a villa, the others in apartments), 26% were forms of transportation, 3.4% were shopping venues, and 6.3% were other venues, such as hospitals, hotels, community events, and a thermal power plant.

Although the majority of outbreaks occurred at home or during transportation, the food and shopping outbreaks were larger. The average number of cases was 3.7 for home outbreaks, and it was similar for transportation (3.8) and entertainment (3.6), but it was a little higher for miscellaneous venues (4.4), even higher for food (4.9), and highest for shopping (8.7). No doubt, the shopping venue average was dragged upward by the 21-person outbreak at a shopping mall.

Every single one of the outbreaks were thought to have occurred indoors.

Only one of the 7,324 cases were thought to have transmitted outdoors. This transmission event only involved two people, so it wasn’t included in the clusters or outbreaks. It involved a conversation between two people outdoors. Notably, a conversation would involve face-to-face spread of air droplets. As I’ve noted here and here, three minutes of talking is equivalent to one cough. 

One of this study’s limitations is that it relies on the judgment of the physicians who described the cases. Without any kind of technological contact tracing, this relies on the physician’s judgment about where the transmission probably occurred, and the physician could have been wrong in many cases. However, a major strength of this paper is that the dataset is so comprehensive.

The overall conclusion of this study is that transmission appears to occur almost exclusively indoors, and the rare outdoor transmissions appear to involve close contact such as a face-to-face conversation.

Modeling a Windy, Urban Environment

A preprint published on April 23 provides a model of how to think about the safety of an outdoor environment.

In this paper, Chilean researchers modeled the spread of a sneeze in a windy urban center. 

According to their model, with a wind of moderate intensity, 400-900-micrometer sneeze droplets can travel 5 meters (16.4 feet) in 2.3 seconds, while 100-200-micrometer droplets can travel 11 meters (36 feet) in 14 seconds. Shortly after these timeframes, the droplets will fall to the ground.

Although some smaller particles are sufficiently small to stay in the air for longer, termed aerosols, as I covered in the last issue, aerosols do not seem to contain virus with strong replication capabilities, and in an outdoor environment they are exceedingly unlikely to carry a sufficient infectious dose of the virus to any given individual.

Under non-windy circumstances, most sneeze droplets disperse less than 1 meter (3.3 feet) and fall to the ground in less than 3 seconds. 

Therefore, the worst case scenario for an individual walking alone in an outdoor environment would be to be within 36 feet of someone who sneezed in their direction in the last 15 seconds with the wind carrying the sneeze droplets right to them. Note that this does not mean that the sneeze droplets would necessarily contain an infectious dose.

Talking face to face for an extended period of time (with three minutes of talking equally one cough) and being in a large crowd where you have no idea who is coughing or sneezing on you, would represent the absolute most dangerous outdoor conditions.

However, the paucity of documentation of outdoor spreading suggests that, notwithstanding the risks of crowds and wind-carried sneeze droplets, simply avoiding indoor environments with strangers or with friends and family who are infected would eliminate most of the risk.

The Bottom Line

Here’s the bottom line:

  • Most of the risk of contracting COVID-19 can be eliminated by avoiding indoor public environments and close contact indoors with friends or family who are infected.
  • Outdoors, the primary risks are being in a crowd of people with close contact where you don’t know who might cough or sneeze on you, talking face-to-face with someone for an extended period of time (with three minutes of talking being the equivalent of one cough right in your face), and being downwind of a sneeze that occurred within the last 15 seconds up to 36 feet away.
  • Wearing a mask while indoors among strangers, outdoors when speaking to someone face to face with less than six feet apart, outdoors when the wind is blowing and there are strangers within 36 feet, or outdoors when risking coming within six feet of strangers, can eliminate most of this risk.
  • When the wind is not blowing and one is not within six feet of strangers, or when it is windy but one is at least 50 feet away from strangers (giving a margin of safety over 36 feet) the outdoor air would seem exceedingly safe even without a mask.

The paucity of documentation of clear transmission outdoors makes it likely that most people will be quite safe outdoors simply staying away from crowds and avoiding face-to-face unmasked discussions with strangers. Even outdoor crowds and unmasked face-to-face discussions with strangers do not seem anywhere near as dangerous as indoor public environments.

In conclusion, get some sunshine and fresh air! It’s needed for mental and physical health on so many levels. Just take basic safety precautions, and focus most of your energy on avoiding the risks of indoor spaces, not outdoor spaces.

Stay safe and healthy,


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I am not a medical doctor and this is not medical advice. I have a PhD in Nutritional Sciences and my expertise is in conducting and interpreting research related to my field. Please consult your physician before doing anything for prevention or treatment of COVID-19, and please seek the help of a physician immediately if you believe you may have COVID-19.


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*  The term “preprint” is often used in these updates. Preprints are studies destined for peer-reviewed journals that have yet to be peer-reviewed. Because COVID-19 is such a rapidly evolving disease and peer-review takes so long, most of the information circulating about the disease comes from preprints.