Association of regional Covid-19 mortality with indicators of indoor ventilation, including temperature and wind: insights into the upcoming winter

Background Outdoor environmental variables, such as cold temperatures and low wind speed, have been correlated with incidence and mortality from Covid-19 (caused by the SARS-CoV-2 virus). However, as Covid-19 predominantly spreads indoors, the degree to which outdoor environmental variables might directly cause disease spread is unclear. Methods World regions were considered to have reliable data if the excess mortality did not greatly exceed reported Covid-19 mortality. The relative risk of Covid-19 mortality for 142 regions as a function of median weekly temperature and wind speed was determined. For instance, Covid-19 mortality following warm weeks in a country was compared with mortality following cold weeks in the same country. Results Covid-19 mortality increases with cooling from 20 C to close to freezing (0 to 4 C, p<0.001). The relation of Covid-19 mortality with temperature demonstrates a maximum close to freezing. Below -5 C, the decrease in mortality with further cooling was statistically significant (p<0.01). With warming above room temperature (20 to 24 C), there is a nonsignificant trend for mortality to increase again. A literature review demonstrated that window opening and indoor ventilation tend to increase with warming in the range from freezing to room temperature. Conclusion The steep decline in Covid-19 mortality with warming in the range from freezing to room temperature may relate to window opening and less indoor crowding when it is comfortable outside. Below freezing, all windows are closed, and further cooling increases stack ventilation (secondary to indoor-outdoor temperature differences) and thereby tends to decrease Covid-19 mortality. Opening windows and other tools for improving indoor ventilation may decrease the spread of Covid-19. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study did not receive any funding. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The source data are available from the Johns Hopkins University (CSSE) Covid-19 github website: I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes Data are available from the first author.