People wear masks in Hong Kong, China, January 31, 2020. (Tyrone Siu/Reuters)

A new study from four Chinese scientists currently awaiting peer review suggests that the coronavirus could be much more likely to spread indoors than outdoors. The proctors reviewed more than 1,000 COVID-19 cases in China, classified groups of cases into “clusters” and “outbreaks,” and summarized their findings as such:

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. The first salient feature of the 318 identified outbreaks that involved three or more cases is that they all occurred in indoor environments. Although this finding was expected, its significance has not been well recognised by the community and by policy makers. Indoors is where our lives and work are in modern civilisation. The transmission of respiratory infections such as SARS-CoV-2 from the infected to the susceptible is an indoor phenomenon.

Emphasis mine. As the researchers note, the fact that most coronavirus clusters and outbreaks occurred indoors is not particularly surprising given the Chinese government’s enactment of stay-at-home orders. But while the study “does not rule out outdoor transmission of the virus,” it notes that “among our 7,324 identified cases in China with sufficient descriptions, only one outdoor outbreak involving two cases occurred.”

One potential contributing factor to the virus’s potency indoors is the poor air quality that characterizes some urban buildings, which can facilitate the spread of COVID:

Many existing buildings are crowded, poorly ventilated, and unhygienic. A comprehensive review of ventilation conditions in Chinese indoor environments by Ye et al. showed that the CO2 concentration can reach 3,500 ppm in some buildings. The design and operation of buildings have also been under pressure to reduce energy use and increase human productivity.

This study, if true, could raise important questions about the wisdom of closing public parks in urban areas, particularly those urban areas in which large groups of people live in substandard buildings.