In a recent study published within the Journal of the Royal Society, researchers found that indoor relative humidity (RH) modulates the severity of coronavirus disease 2019 (COVID-19) outbreaks, and intermediate RHs between 40 to 60% are robustly related to higher COVID-19 outcomes.

Study: Associations between indoor relative humidity and global COVID-19 outcomes. Image Credit: Nhemz/Shutterstock

Background

Studies in search of to link COVID-19 outbreaks and environmental conditions primarily focused on outdoor temperature, humidity, and ultraviolet light. They used differing data collection processes and statistical methodologies. The timeframes and nation-states of those studies also varied, inadvertently resulting in selection bias and reducing the generalizability of the outcomes. Further robust studies are needed to confidently elucidate the seasonal dependency of severe acute respiratory syndrome CoV-2 (SARS-CoV-2) outbreaks.

In temperate zone countries, people heat indoor spaces to take care of comfortable ambient temperatures of roughly 20–24°C, which alters the indoor RH. RH between 40% and 60% (intermediate) minimizes the danger of disease transmission, especially from respiratory viruses, corresponding to SARS-CoV-2 and influenza. The occupants of heated indoor spaces experience low RH in the course of the colder months, which likely adversely impacts their health. It increases viral stability and transmission but reduces the host’s mucous barrier integrity to pathogens.

Concerning the study

In the current study, researchers hypothesized that indoor RH may need been partially liable for the observed regional heterogeneity in global COVID-19 outcomes. They proposed that indoor, not outdoor, environment is more relevant or closely correlated with COVID-19 spread and severity. In order that they pursued answers to questions, corresponding to regional differences in COVID-19 severity and epidemic dynamics and the way the regional variability in COVID-19 related to indoor relative humidity levels. Moreover, they investigated whether the association between the 2 could withstand variations in methodologies and confounding aspects, corresponding to government response and outdoor weather conditions.

First, the team amassed a dataset with global coverage of the coronavirus outbreaks. They aggregated data by region to facilitate comparisons of temperate and tropical regions. The researchers chosen 121 countries with a minimum of fifty confirmed COVID-19 deaths and extracted their COVID-19 statistics from the Johns Hopkins University (JHU) and European Centre for Disease Prevention and Control (ECDC) datasets. They paired each country’s geographical centroid with meteorological data. RH is the relative measurement of the particular saturation vapor pressure ratio. The team computed the common indoor conditions for every country for the brand new COVID-19 deaths, the difference in these deaths, and the percent change in recent deaths for every country.

They examined the environmental conditions related to good or bad COVID-19 outcomes in a case-control fashion for more reliable quantitative predictions. The team also computed the percentages of a greater or worse COVID-19 end result based on exposure to extreme or intermediate RH conditions. Any value greater than 1.0 indicated that intermediate RH was related to fewer recent deaths and negative day-to-day change in recent deaths (higher outcomes). Finally, the team validated their findings against widely various government intervention stringencies, ‘low,’ ‘medium,’ and ‘high.’ They calculated common odds ratios with the time-lagged government responses and the time-lagged indoor RH.

Study findings

The researchers used a rigorously processed extensive global dataset of COVID-19 statistics and meteorological variables, with extrapolated and validated indoor RH levels. The dataset was highly complex and noisy in regards to the COVID-19 outbreak magnitude and reporting conditions. Yet, the identical general patterns tended to persist even when differing time lags and applying different data treatments.

The researchers, thus, noted a scientific association between COVID-19 and indoor RH. Nonetheless, as expected, the outcomes couldn’t elucidate a causal relationship between RH and respiratory viral diseases. Indeed, this relationship is complex and multifaceted from physiological and biophysical standpoints.

The researchers noted a transparent difference between COVID-19 outbreak dynamics within the three nation-states. For Northern Hemisphere (NH) countries, COVID-19 deaths spiked from March to May before step by step decreasing as summer arrived. Conversely, for SH countries, COVID-19 deaths were relatively flat until spiking from June to August (winter months). Finally, Tropical countries displayed a consistent rise in recent COVID-19 deaths across all the timescale. Together, these results demonstrated that while temperate countries in NH & SH experienced worse viral outbreaks during seasonal drops in indoor RH, the tropical zones experienced increased viral outbreaks at high indoor RH.

Intriguingly, modeling the COVID-19 outbreak versus indoor RH metrics using Huber’s T weighting (linear) function showed a negative relationship between indoor RH and COVID-19 for temperate regions and the alternative for tropical countries. The outcomes remained robust even when the team applied a non-parametric locally weighted scatterplot-smoothing (LOWESS) technique to the identical data.

No matter low, medium, or high government response stringencies, the connection between indoor RH and outbreak severity remained conserved. Similarly, various outdoor weather variables, corresponding to absolute humidity (AH), temperature, and UV, didn’t impact the observed relationship between COVID-19 outbreak severity and indoor RH. Notably, it remained consistent across different AH levels, pointing to the physical processes governed by RH, namely evaporation and condensation. Indeed, indoor RH could modulate COVID-19 spread and severity whatever the external weather conditions.

Conclusions

An in depth evaluation of the associations between regional indoor RH and COVID-19 spread and severity pointed toward a sturdy pattern: COVID-19 outcomes are less severe at indoor RH levels between 40 and 60%. Follow-up top-down studies of the population dataset could investigate the indoor RH hypothesis at higher spatial resolution. Bottom-up biophysical experimental–theoretical investigations could make clear the multifaceted mechanisms and rationalize the inconsistency in results.

Nevertheless, in comparison with other highly disruptive measures (e.g., public lockdowns), it might be easier and cost-effective to establish humidity control systems in some indoor settings. Indoor RH optimization would also achieve high compliance. So the authors proposed that maintaining indoor RH between 40% and 60% could complement existing COVID-19 countermeasures and contribute to the efforts to reduce indoor disease transmission.