Medium levels of indoor relative humidity have been shown to improve COVID-19 outcomes globally

In a recent study published in Journal of the Royal SocietyResearchers found that indoor relative humidity (RH) modifies the severity of the coronavirus disease 2019 (COVID-19) outbreak, and that average relative humidity levels between 40 to 60% are strongly associated with better outcomes for COVID-19.

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


Studies seeking to link the COVID-19 outbreak and environmental conditions have primarily focused on outdoor temperature, humidity, and UV radiation. They used different data collection processes and statistical methodologies. The time frames and geographic regions of these studies also varied, inadvertently leading to selection bias and reducing the generalizability of the results. More robust studies are needed to clarify the seasonal dependence of severe acute respiratory syndrome CoV-2 (SARS-CoV-2) outbreaks.

In temperate countries, people heat indoor spaces to maintain comfortable ambient temperatures of around 20-24°C, which changes the indoor relative humidity. An RH between 40% and 60% (medium) reduces the risk of disease transmission, especially from respiratory viruses, such as SARS-CoV-2 and influenza. Occupants of heated indoor spaces experience lower relative humidity during the colder months, which can negatively affect their health. It increases virus stability and transmissibility but reduces host mucosal barrier integrity to pathogens.

about studying

In this study, the researchers hypothesized that indoor relative humidity may be partly responsible for the observed regional variation in global COVID-19 outcomes. They suggested that the internal environment, rather than the external one, is more relevant or closely related to it The spread of COVID-19 and intensity. So they followed for answers to questions, such as regional differences in COVID-19 severity, epidemic dynamics, and how regional variation in COVID-19 relates to indoor relative humidity levels. In addition, they investigated whether the association between the two could withstand differences in methodologies and confounding factors, such as government response and outdoor weather conditions.

First, the team collected a dataset with global coverage of the coronavirus outbreak. They grouped the data by region to facilitate comparisons of temperate and tropical regions. The researchers selected 121 countries with at least 50 confirmed COVID-19 deaths and extracted their COVID-19 statistics from Johns Hopkins University (JHU) and European Center for Disease Prevention and Control (ECDC) datasets. They paired each country’s geographic centroid with meteorological data. RH is the relative measure of the actual saturation vapor pressure ratio. The team calculated each country’s internal average of new COVID-19 deaths, the variance in those deaths, and the percentage change in new deaths for each country.

They examined the environmental conditions associated with good or poor COVID-19 outcomes using a case-control method to obtain more reliable quantitative predictors. The team also calculated the odds of a better or worse COVID-19 outcome based on exposure to severe or moderate reproductive health conditions. Any value greater than 1.0 indicated that mean relative humidity was associated with fewer new deaths and a negative daily change in new deaths (better results). Finally, the team validated their findings against the stringency of widely varying government intervention, ‘low’, ‘medium’, and ‘high’. They calculated combined odds ratios with time-delayed government responses and delayed indoor relative humidity.


The researchers used a large-scale global dataset that had been rigorously processed for COVID-19 statistics and meteorological variables, with stable and validated internal RH levels. The dataset was very complex and noisy regarding the scale of the COVID-19 outbreak and reporting conditions. However, the same general patterns tend to persist even when different time periods and different data treatments are applied.

Thus, the researchers noted a systematic relationship between COVID-19 and endogenous RH. However, as expected, the results could not elucidate a causal relationship between respiratory viral diseases and reproductive health. Indeed, this relationship is complex and multifaceted from physiological and biophysical points of view.

The researchers noted a clear difference between the dynamics of the COVID-19 outbreak in the three geographic regions. For Northern Hemisphere (NH) countries, COVID-19 deaths increased from March to May before gradually declining with the arrival of summer. Conversely, for SH countries, COVID-19 deaths were relatively constant until their June–August spike (winter months). Finally, tropical countries have shown a steady rise in new COVID-19 deaths across the entire timescale. Together, these results showed that while temperate NH&SH countries experienced worse viral outbreaks during seasonal decreases in indoor relative humidity, tropical regions experienced increased viral outbreaks at higher indoor relative humidity.

Interestingly, modeling the outbreak of COVID-19 against endogenous RH measures using Huber’s (linear) T-weighting function showed a negative relationship between endogenous RH and COVID-19 for temperate regions and the opposite for tropical countries. The results remained robust even when the team applied a locally non-standard scatterplot smoothing (LOWESS) technique to the same data.

Regardless of the low, medium, or high stringency of the government response, the relationship between indoor reproductive health and outbreak severity remained maintained. Similarly, outdoor weather variables, such as absolute humidity (AH), temperature, and UV radiation, did not affect the observed relationship between the severity of the COVID-19 outbreak and indoor relative humidity. Notably, it remained constant across different levels of AH, indicating the physical processes governed by RH, namely evaporation and condensation. Indeed, indoor relative humidity can modulate the spread and severity of COVID-19 regardless of external weather conditions.


A detailed analysis of the associations between the regional indoor prevalence of coronavirus and the prevalence and severity of COVID-19 indicated a strong pattern: COVID-19 outcomes are least severe at indoor relative humidity levels between 40 and 60%. Top-down follow-up studies of the population data set can investigate the endogenous RH hypothesis at higher spatial resolution. Bottom-up experimental theoretical and physical investigations can shed light on the multifaceted mechanisms and justify the discrepancy in the results.

However, compared to other highly destructive measures (such as general lockdown), it would be much easier and cost effective to set up humidity control systems in some indoor spaces. Improving the internal RH will also result in a high level of compliance. The authors therefore suggested that maintaining indoor relative humidity between 40% and 60% could complement existing countermeasures for COVID-19 and contribute to efforts to reduce indoor transmission of the disease.

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