Across 25,000 cases, “limited evidence” of COVID transmission in everyday outdoor environments
The review found very few examples of outdoor transmission of COVID-19 in everyday life among c. 25,000 cases considered, suggesting a very low risk. However risk of outdoor transmission increases when the natural social distancing of everyday life is breached, and gathering density, circulation and size increases, particularly for an extended duration. There was also evidence that weather had a behavioural effect on transmission, with temperatures that encourage outdoor activity associated with lower COVID-19 transmission. Across sources, there is limited evidence of transmission of COVID-19 in outdoor environments during the natural course of everyday life. However, there is some evidence to suggest that there is a higher risk of outdoor transmission in environments where the natural social distancing that takes place when ‘milling around’ in everyday life is breached, and gathering density, circulation and size is increased, particularly where this involves an extended duration. This could include aspects of outdoor concerts, festivals and some types of physical activity and sporting events.
Among 821 cases, most COVID clusters occurred indoors, with the exception of building sites
We found many examples of SARS-CoV-2 clusters linked to a wide range of mostly indoor settings. Few reports came from schools, many from households, and an increasing number were reported in hospitals and elderly care settings across Europe. The vast majority of these clusters were associated with indoor or indoor/outdoor settings. (The only outdoor transmission events were at building sites – 4/201 events)
Of 318 COVID outbreaks, only one occurred in an outdoor environment
Three hundred and eighteen outbreaks with three or more cases were identified, involving 1245 confirmed cases in 120 prefectural cities. We divided the venues in which the outbreaks occurred into six categories: homes, transport, food, entertainment, shopping, and miscellaneous. Among the identified outbreaks, 53·8% involved three cases, 26·4% involved four cases, and only 1·6% involved ten or more cases. Home outbreaks were the dominant category (254 of 318 outbreaks; 79·9%), followed by transport (108; 34·0%; note that many outbreaks involved more than one venue category). Most home outbreaks involved three to five cases. We identified only a single outbreak in an outdoor environment, which involved two cases. Conclusions: All identified outbreaks of three or more cases occurred in an indoor environment, which confirms that sharing indoor space is a major SARS-CoV-2 infection risk.
96% of COVID clusters occur indoors, though playground transmission has been documented
Over 96% of the identified clusters (and of all cases associated with clusters) were associated with indoor environments. Households were associated with 50% of all clusters, with 2-8 cases per cluster. Entertainment and leisure activities were linked to 15% of all events, notably dining (9% of all clusters), sports and fitness (mostly indoors), parties, bar and nightclubs. Indoor shopping malls and supermarkets were linked to 32 documented clusters (5% of all events), representing another frequently documented public space setting. Only three events were associated with outdoor markets, although these clusters were relatively large (25-163 cases per cluster). (2 events occurred at playgrounds, with 20 and 26 cases per cluster.) Despite evidence showing that outdoor settings having an overall lower risk, some, typically those associated with crowds/mass gatherings (e.g. outdoor markets, rallies) have been linked to large SARS-CoV-2 clusters.
In small study, COVID transmission was 18.7 times more common indoors
Of the 110 cases examined, 27 (24.6%) were primary cases who generated secondary cases. The odds that a primary case transmitted COVID-19 in a closed environment was 18.7 times greater compared to an open-air environment. If superspreading events are defined as events where the number of secondary cases generated by a single primary case is greater than the 95th percentile of the distribution (i.e. transmission to three or more persons), then seven of the 110 cases (6.4%) were involved in such events. Six of these events (85.7%) took place in closed environments, and the odds ratio (OR) of superspreading events in closed environments was as high as 32.6.
Americans spend 90% of their time indoors, perhaps explaining much of comparatively higher indoor COVID transmission
Americans, on average, spend approximately 90 percent of their time indoors.
In computer model, wind disperses COVID particles, but remain in high concentration within 3 meters
Multiple scenarios including particle size, wind speed, source height variations as well as and combined scenarios were modeled to estimated how exposure risk changes with the above-mentioned variables. The results reveal that wind speed majorly narrows infectious plume rather than transferring the peak concentration. The particle size variation indicated that small particles, i.e.,0.01 – 2.5 μm, could reach more than 9 m away from the source in concentration range of 10 – 20 (μg/m 3). On the other hand, source height contributes to peak plume shift rather than dispersing the infected particles. This idea was further studies by using combined scenarios which indicated height difference can impact peak plume displacement rather than wind speed. In the worst-case scenario, the results indicate that the virus-laden particles can travel outdoors more than 8 m away from an infected source. The video output of the model results clearly shows the dynamic of viral peak shifts in several scenarios. The results also indicate that in specific conditions the airborne SARS-CoV-2 can be transported to 9 m away from the source.
COVID can be 99% inactivated outdoors within two hours of noon during summer in most cities in US; less so during other seasons
The (+) sign in Table 2 indicates that 99% of SARS-CoV-2 may be inactivated within the two hours period around solar noon during summer in most US cities located south of Latitude 43oN (Manchester, NH). Also 99% of the virus will be inactivated during two hours midday in several cities south of latitude 35oN in Fall (Oklahoma City, OK), but only Miami and Houston will receive enough solar radiation to inactivate 99% of the virus in spring. During winter, most cities will not receive enough solar radiation to produce 90% viral inactivation during 2-hours midday exposure (underlined values in Table 2).
COVID RNA found on 5% of samples from playground in hard hit area; unclear whether presence poses risk of infection (n=68)
The objective of the current study was to assess the presence of SARS-CoV-2 in environmental samples collected at public playgrounds and water fountains, in a country with high disease prevalence. Samples were collected from drinking fountains and high-touch recreational equipment at playgrounds. Forty-three samples were collected from playground equipment and 25 samples from water fountains. Two of the 43 (4.6%) samples from playground equipment and one (4%) sample from a drinking fountain tested positive. It is unclear whether the recovery of viral RNA on outdoor surfaces also indicates the possibility of acquiring the virus. Adherence to environmental and personal hygiene in urban settings seems prudent.
Outdoor air in urban areas is generally not infectious
The results found indicate that outdoor atmospheric concentrations of SARS-CoV-2 were very small (<0.8 copies m−3) in both northern and southern Italy. The same applies for each size range investigated with the impactor, which gave virus-laden aerosol concentrations <0.4 copies m−3. The measurements were taken in a period when the number of active cases (i.e. infected individuals) in the two regions were not at the maximum values, thereby, it is possible to assume that higher concentrations (up to a factor 2 on average for Venice) were likely be present during the period of maximum spread of contagion. The average typical threshold of about 20 virus copies is necessary to make a quantum of virus (i.e. the dose of airborne droplet nuclei that, if inhaled, is able to cause infection in 63% of susceptible persons). Considering a typical inhalation rate of about 1 m3/h, as average between rest and light exercise, the concentrations would be low to spread the contagion via airborne transmission even assuming the mentioned increase of a factor 2. Therefore, it is possible to conclude that outdoor air in residential and urban areas was generally not infectious and safe for the public in both northern and southern Italy, with the possible exclusion of very crowded sites.