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How Low Relative Humidity Affects Us (When Sharing Air)

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Lower humidity below 40% RH acts as a channel for viruses and airborne bacteria to disperse and travel around buildings. This is because in lower relative humidity, the survival time of the virus is prolonged, and the suspension of infectious particles in dry air is also enhanced, which makes it easier for the two to combine.

When an infected person breathes, talks, coughs, or sneezes, they release into the air aerosol-like micro-mists that contain many elements, including saliva, mucus, salts, germs, and viruses. Larger droplets will fall to the ground or stay on the body surface, but the micro-mist with a size of less than 4μm - which experiments have proved - can stay in the air for several hours in the form of an aerosol.

In indoor air, the micro mist released by evaporation will quickly lose more than 90% of the moisture. When the relative humidity value is higher than 40%RH, although the elements of salt and protein will still be dissolved in the mist, they tend to become highly concentrated. They attack viruses and bacteria, rendering them inactive to reduce the risk of secondary infections.

However, below the critical level of 40%RH, further evaporative moisture loss can cause these elements to crystallize out of the solution. The internal environment of the aerosol particles changes from aggressive to protective, allowing the viruses and bacteria within to remain infectious for longer.

Lower humidity also evaporates more mist particles, reducing their size to maintain aerosol form. Therefore, dry air has the dual effects of generating more aerosol particles and prolonging the infectivity of the bacteria carried, which greatly increases the potential risk of secondary infection.

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Scientific Basis

Numerous studies and investigations have been conducted over the years demonstrating how humidity affects the transmission and survival of aerosolized germs such as cold and flu viruses. The following summarizes these studies, illustrating the benefits of maintaining an optimal humidity level for improving indoor air quality and human health.

Study: Higher humidity depletes infectious flu virus in simulated cough

Author: Noti JD et al., 2013

Citation: PLoS One., 2013;8(2):e57485

Methods: The influenza virus was "coughed up" indoors with mannequins equipped with different nebulizers at various relative humidity levels ranging from 7-73%. Air samplers were used to collect aerosol particles to understand their persistent infectivity under different indoor conditions.

Summary of results: Influenza viruses retain maximum infectivity at lower relative humidity. The virus is rapidly inactivated after coughing when the relative humidity is higher than 40% RH. Maintaining indoor relative humidity above 40% can significantly reduce the infectivity of aerosolized viruses.

Study: Optimal Relative Humidity in Inhabited Environments

By Sterling EM et al., 1985

Citation: ASHRAE Transactions, Volume 91, Part 1, CH85-13 No. 1, 1985

Methods: The health literature on relevant biological and chemical interactions were reviewed to identify optimal humidity ranges where overall health risks are likely to be reduced. For biological pollutants, pathogens causing respiratory problems, and the chemistry of their interactions, humidity has been detected to play a role in normal indoor temperatures of 19-27 degrees Celsius.

Summary of the Results: Optimal humidity for reducing human exposure to biological contaminants, pathogens, and chemical interactions occurs in the narrow range of 40-60%RH at normal room temperature.

Study: Indoor Airborne Influenza A Dynamics and Correlation with Humidity

Authors: Yang W, Marr LC

Citation: PLoS One., 1 June 2011 | Volume 6 | Issue 6 | e21481

Methods: The size, distribution, and transformation of aerosols carrying influenza A virus (IAV) released from a cough were simulated at 10-90% relative humidity levels.

Summary of results: Humidity is an important variable in the aerosol transmission of IAVs because it causes changes in aerosol particle size and affects the inactivation rate of IAVs. The virus inactivation rate increases linearly with relative humidity; at the highest relative humidity, inactivation removes up to 28% of IAV within 10 minutes.

Study: Flu and Humidity - Why a little more humidity could benefit you

By Jane A. Metz, Adam Finn

Citation: Journal of Infection (2015) 71, S54eS58

Methods: We reviewed surveys of humidity and temperature since 1960 and examined their effects on influenza virus survival, transmission, and infection rates.

Summary of results: In vitro experiments investigating influenza virus survival were mathematically modeled at different humidity levels. The results showed that the survival rate of the influenza virus increased when the humidity was low. The exciting prospect of reducing flu-related morbidity by increasing humidity in nurseries, classrooms, hospitals, nursing homes, and general public spaces is also a potential strategy for making the virus less aggressive.

——Maintaining 40-60% RH is an effective way to suppress aerosol-transmitted infection.

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