The Link between Humidity and COVID-19 Caused Death

Objective: The objectives were to understand the relationship between humidity and COVID-19 fatality and to provide clues for improving COVID-19 patient survival rate. Methods: Humidity data and confirmed COVID-19 death cases for all fifty US states and Washington DC were collected and analyzed, and the relationship between humidity and COVID-19 mortality was derived. Results: There is a positive correlation between humidity and COVID-19 patient fatality. Conclusion: COVID-19 mortality directly and positively correlates with the humidity of the environment the COVID-19 virus experiences. Such correlation could be used to reduce COVID-19 patient fatality and save lives.


Materials and Methods
The number of total confirmed cases and death cases for COVID-19 were ob- The correlation coefficient r between humidity and death rate was calculated using Microsoft Excel function Correl based on the following Equation (2), where x stands for COVID-19 mortality and y represents average for each location. 1 Average humidity

Results
The correlation between the average humidity of the fifty states and Washington DC in late January and February along with the death cases data at three different dates in March 2020 is analyzed and the results are shown in Table 1 and Figure  1. There is a direct correlation between humidity and death rate caused by COVID-19 for all three data sets. The correlation coefficient is as high as 0.9 for the top 5 states with the most numbers of deaths. Although the correlation deceases when more states are included in the calculation, the correlation coefficient still remains as high as 0.5 even when the data for all fifty US states and Washington DC, including areas with zero reported death cases, were used ( Table 1). This is not surprising, as when the number of deaths is small, the death rate is more prone to fluctuation due to other factors such as human migration. For example, when the stay-at-home order is placed and few people travel, the correlation coefficient for all fifty US states and Washington DC goes from 0.474 on March 22 to 0.516 on March 30. It is noticeable that the two driest states (Nevada and Arizona) and the most humid state (Alaska) are three consistent outliers (Figure 1). This observed digression from the correlation for these three states suggests a general correlation between humidity and death rate may not apply to extreme humidity values.

Discussion
The objective of this study is to illustrate the relationship between humidity and COVID-19 resultant death rate and thus reduce COVID-19 mortality by controlling humidity. The most recent report on humidity and COVID-19 mortality [8] was centered on the impact of humidity on the patients' defense systems. This study is focused on the effect of humidity on the COVID-19 virus itself and is accounted for by the time difference between humidity and COVID-19 death data, about one month apart. A significantly positive correlation between humidity and COVID-19 resultant death rate was observed. The mechanism behind this observed correlation between humidity and COVID-19 caused death rate is not clear. Comparative characterization of COVID-19 (such as gene sequencing, virulence analysis, etc.) that experienced different levels of humidity can help shed light on the underlying mechanism.  Three outliers (the two driest states, Nevada and Arizona, and the most humid state, Alaska) have been observed consistently in all three data sets. One possible explanation is that the correlation between humidity and COVID-19 mortality may only be significant within a certain range of humidity as it has been found before that some viruses have a narrow ideal humidity range [4]. A more insightful explanation may rely on the mechanism study of the effect of humidity on COVID-19 virulence. I also found that the correlation increases with the rise in number of death cases. When the number of deaths is small, one or two deaths from a random event weighs heavily in the mortality calculation and thus twists the correlation. For example, a very sick COVID-19 patient may have moved from one state to another state, in which no COVID-19 deaths have been recorded yet, and later dies in the state he just moved to. The impact of human migration could explain why data collected at the end of March showed a better overall correlation than the data collected earlier since more people started to stay home towards the end of March.
This study was conducted at a large scale (statewide) but localized studies may be more informative on how to reduce COVID-19 fatality. State is an administrative unit and humidity within a state, particularly a large state, can vary a lot. Therefore, states may not be the best geographic unit to study the effect of humidity on COVID-19 fatality. Towns, communities, and even residential buildings may be better geographic units for a more comprehensive study in populated areas if localized data is available.
In the meantime, a couple of measures can be immediately taken that can potentially save lives: First, hospitals and residential houses can lower their humidity setting to a certain range. Second, vulnerable people should stay in areas of lower humidity.
The reason that I did not study the correlation between temperature and COVID-19 mortality is the obviousness that high temperature decreases transmission, livability, and viral fatality as reported by many scientists [ [8]. A comprehensive study of all environmental parameters (temperature, humidity, etc.) on COVID-19 mortality deserves further scrutiny. In addition, a more extensive collection of data, including a comprehensive analysis of other potential explanatory variables, like ages of patients, could be more informative.

Conclusion
This study is the first one to investigate the effect of humidity on the cumulative fatality of COVID-19 for all fifty US states and Washington DC using publicly accessible data. My analysis suggests that there is a direct and positive correlation between COVID-19 mortality and the humidity of the environments the virus experiences. Such findings could help with COVID-19 virulence and potentially reduce COVID-19 fatality.