Response Letter to Mungmunpuntipantip and Wiwanitkit

A central tenant in minimizing personal heat exposure is that context matters. In our article reporting on personal heat exposure in outdoor COVID-19 vaccination sites in Arizona, USA during spring 2021, we emphasized that heat mitigation strategies are highly specific to the underlying climatological and site design contexts. The Mungmunpuntipantip and Wiwanitkit letter illustrates the importance of context quite well. The authors highlight a valid concern of unintended consequences of COVID-19 transmission through changing humidity levels through heat mitigation strategies such as evaporative cooling. Systematic reviews of meteorological conditions and COVID-19 continue to hint at a “seasonality” effect for transmission and mortality; however, associations with outdoor humidity levels are far more mixed than relationships with ambient air temperature[1], [2], [3]. Evolving science confirms that most COVID-19 transmission occurs indoors where humidity likely impacts droplet and aerosol size, virus stability, and thus transport trajectory and transmission rates[4]. However, because ventilation – or a lack thereof – appears to be the most important environmental driver of indoor transmission after close proximity, those designing outdoor healthcare delivery sites should focus on ventilation before fine-tuning humidity levels. When combined with high heat, mitigating personal heat exposure is also likely more important than humidity concerns.