Note: The suggestions in this guest essay may or may not be useful in your personal situation. It is provided here for review and discussion. There is some supporting evidence that increased heat and humidity will have an adverse affect on the Coronovirus, as does UV-C light, but each personal situation is unique, and other factors may dominate transmission likelihoods. Use your own best judgment. – Anthony
Guest essay by Leo Goldstein
- COVID-19 spreads mostly indoors by person to person and person to surface to person transmission
- Increasing humidity indoors to 50%-60% is a safe and promising method to fight COVID-19 spread
Increasing Indoor Humidity to Constrain COVID-19
Indoor temperature, humidity, and ventilation must be increased in public and private buildings in all COVID-19 affected areas. Here, indoor temperature of at least 75°F (24°C) and relative humidity levels of 50%-60% are recommended.
It is a common knowledge, confirmed by the rigorous body of research, that low temperatures and low humidity contribute to flu-like virus epidemics, while high temperatures and high humidity reduce or even prevent flu-like epidemics. Good ventilation (rapid exchange of indoor air, which potentially contains viruses, with clean outside air) is another important factor that decreases the virus spread. COVID-19 is caused by a coronavirus SARS-CoV-2. Coronaviruses and Influenza viruses are enveloped RNA viruses from the respiratory virus group. Therefore, the knowledge gained by the vast amount of prior studies of such viruses is fully applicable to the current epidemic.
People in the US and other developed countries spend most of their time indoors. The typical indoor conditions are characterized by very low relative humidity, lower than optimal temperature, and bad ventilation. Today, the seasonality of flu-like illnesses is better explained as an indirect impact of cold weather. Heating without humidification dries indoor air, and the low humidity contributes to virus survival, infectivity preservation, and transmission. There is evidence that these indoor environment conditions have been worsening over the last 10-15 years.
The suggested temperature and humidity for decreasing the spread of viruses is within most existing regulations. However, the humidity of many indoor environments typically goes unchecked and frequently falls below 30%, per Quinn and Shaman (1). Reisman and Das (2) shows that an increase of humidity in schools, from ordinary ~28% to ~45%, decreases influenza-like illnesses among children by 2.3 times in wintertime. Thus, increasing indoor humidity levels and temperatures, as stated above, would yield immediate benefits beyond just slowing the spread of COVID-19. These measures also seem to have no downsides, at least over a few weeks’ period.
There is no need to wait until summer or even mid-April. Hygrometers cost about $20 and humidifiers are also inexpensive. In the absence of a humidifier, one can leave a pot of water simmering.
Inhibition of influenza viruses by environmental conditions is closest correlated with absolute humidity, per Shaman and Kohn (3). Absolute humidity is determined by relative humidity and temperature.
There is similarity of shedding, transmission, and infection mechanisms between the relevant corona- and Influenza viruses (Dormalen et al. (3), Iljaz (9), Prussin et al. (11), Pica and Bover (12)). That allows applying results from the previous studies of such viruses the COVID-19 epidemic. The previous studies (including Dormalen et al. (5), Noti (6), Iljaz (7), Yang and Marr (8)) are unanimous that increase in humidity sharply decreases transmission, survival, and infectivity of these viruses up to ~50% through multiple biological, chemical, and physical mechanisms. The positive effects are not so uniform above 50%, but the bottom line is that increase in humidity above 50% still restraints spread of the Influenza and coronaviruses. This said, SARS-CoV-2 and SARS-CoV have been described as relatively tough viruses.
These recommendations can be implemented by individuals in their homes as well as in their businesses. However, to slow down the epidemics, they should be implemented by the whole community in the same time. Today, that means endorsement by governments.
The issue of low-quality indoor air might have been worsened recently. Energy conserving buildings are tightly sealed, have practically no passive ventilation, and often poor active ventilation. There is an energy conservation movement to keep wintertime indoor temperature at 68°F (20°C), and to drop temperatures even lower when the space is unoccupied. Repeating this cycle dries out the air even more and creates ideal conditions for Influenza and coronaviruses to survive, stay infective, and even aerosolize. It is also beneficial for computer equipment to be kept at lower temperature and humidity levels.
The geography of the spread of COVID-19 seems compatible with the hypothesis that low-quality indoor environment is a large contributing factor.
Important Analysis Factor
Comparing the COVID-19 spread in the West with that in Asia, like South Korea and Japan, there is one measure that has been undertaken in the East, but not in the West: contact tracing. Japan traced individual cases of COVID-19. In the US and EU, even anonymized infection tracing was not seriously considered. That might be the case due to Big Tech’s refusal to share their users’ location data with doctors and/or government agencies, even upon the user’s consent or request.
Beyond Humidity Levels of 50%-60%
The peer reviewed research indicates that corona- and Influenza viruses lose their viability outside of the human body with an increase in relative humidity even above 50%-60%. This begs the question: should indoor relative humidity recommendations, in the areas affected by the COVID-19 epidemic, be increased even higher, perhaps to 70-90%?
The ordinary recommendations for indoor humidity levels are between 40-60%. These recommendations balance different requirements. Even in hospitals, humidity is a balancing act, intended to minimize potential growth of viruses (higher RH inhibits enveloped RNA viruses), fungi (lower RH inhibits fungi), & bacteria (vary). However, when the dominant threat is a coronavirus, other concerns can put on the back burner for a few weeks, in order to slow this epidemic.
A humidity level of over 70% is uncomfortable for many and often develops an unpleasant smell. But we can tolerate these and other discomforts, for a few weeks, to fight the spread of COVID-19.
Other Indoor Climate Parameters
There is also an option to increase the temperature and/or relative humidity in spaces, when those spaces are unoccupied. This could be done in office buildings at night and in residences during the day, when everyone is at work.
Increasing indoor ventilation may be the hardest indoor environmental recommendation to implement. Since not all windows (especially in commercial buildings) can be opened, non-traditional methods might be used. For example, where office windows cannot be opened, one might cut holes in them.
In many cases, it might be easier to use germ-killing air purification than to increase ventilation. See Ijaz (10) for air purification techniques.
Ozone generators should be re-considered. They kill germs in the air and on the surface.
Generally, there are many indoor parameters and measures that are not acceptable long term might be used for a few weeks in affected areas.
Aircrafts have very dry and cold air and a high density of passengers. They are not just virus carriers, but also incubators. (Based on Olsen (13), Booth (14).)
Notice that air cooling also decreases relative humidity. If this issue is not addressed, this epidemic might decrease in the late spring, then peak back in the summer due to elevated AC use.
Here, the term affected area is left to interpretation by the decision makers. It might include areas affected by the panic.
Peer Reviewed Studies
Ashlinn Quinn, Jeffrey Shaman, Science of Total Environment, 2017
Levels of humidity seen here are consistent with increased influenza virus survival.
Mean indoor vapor pressure (a measure of absolute humidity) was 6.7 mb in the surveyed homes during the winter season.
This corresponds to RH=30% at 75°F.
Jennifer M. Reiman, Biswadeep Das, Center for Clinical and Translational Science, Mayo Clinic, PLOS ONE, 2018
There were 2.3 times as many ILI [influenza-like illness] cases in the control rooms compared to the humidified rooms, and whether there is a causal relationship, and its direction between the number of cases and levels of influenza virus in the rooms is not known. Additional research is required, but this is the first prospective study suggesting that exogenous humidification could serve as a scalable NPI for influenza or other viral outbreaks. …
This epidemiological correlation suggests that deliberate increases in AH could be a potential NPI to reduce the spread of influenza and other viruses. One approach is to maintain relative humidity (RH) between 40–60%, the proposed optimal range for reducing growth opportunities for viruses, bacteria, and fungi . Our previous study, demonstrated that classroom humidification to RH of 40–60% may be a feasible approach to increase indoor AH to levels with the potential to reduce influenza virus survival (a target of 10mb) and transmission …
Elevated classroom humidification was maintained at an average of 9.89 mb in humidified rooms compared to 6.33 mb in control rooms
A grab bag of viruses. Control group relative humidity was 28%, while experimental groups achieved 42-45% humidity.
Neeltje van Doremalen, et all., National Institute of Allergy and Infectious Diseases, Azaibi Tamin, et all., CDC, The New England Journal of Medicine, 2020
We found that the stability of SARS-CoV-2 was similar to that of SARS-CoV-1 under the experimental circumstances tested [including various surfaces and aerosol]. This indicates that differences in the epidemiologic characteristics of these viruses probably arise from other factors, including high viral loads in the upper respiratory tract and the potential for persons infected with SARS-CoV-2 to shed and transmit the virus while asymptomatic.
Jeffrey Shaman (Columbia University) & Melvin Kohn (John Hopkins), Proceedings of the National Academy of Sciences, 2009
This paper summarizes the field: Absolute Humidity constraints Influenza Virus Transmission (IVT) and Influenza Virus Survival (IVS). The results are applicable to coronaviruses as well.
Previous studies indicate that relative humidity (RH) affects both influenza virus transmission (IVT) and influenza virus survival (IVS). Here, we reanalyze these data to explore the effects of absolute humidity on IVT and IVS. We find that absolute humidity (AH) constrains both transmission efficiency and IVS much more significantly than RH. In the studies presented, 50% of IVT variability and 90% of IVS variability are explained by AH, whereas, respectively, only 12% and 36% are explained by RH.
Includes physics of droplets: gravitation, speed, size, and evaporation. Cough droplets with viruses evaporate faster at lower humidity. They also become smaller and stay airborne for a longer time. Absolute Humidity also better correlates with an increase in both temperature and relative humidity.
N van Doremalen, T Bushmaker, V J Munster, Laboratory of Virology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Eurosurveillance, 2013
[Aerosolized] MERS-CoV decreased only 7% in viability at 40% RH, whereas the viability at 70% RH decreased significantly with 89% (unpaired one-tailed Student’s t-test, p=0.0045). The viability of A/Mexico/4108/2009 (H1N1) virus decreased under both conditions with 95% for 40% RH and 62% for 70% RH respectively
MERS-CoV was very stable in aerosol form at 20°C – 40% RH. The decrease in viability at 20°C – 70% RH (89%) was comparable to that of A/Mexico/4108/2009 (H1N1) virus. Severe acute respiratory syndrome coronavirus (SARS-CoV) has been reported to stay viable for up to five days at 22 to 25°C and 40 to 50% RH and increase in temperature and humidity resulted in a rapid loss of viability . Although a comparison between different experimental studies should be approached cautiously, the relative stability of MERSCoV at 20°C – 40% RH and the rapid decrease in virus viability at higher temperatures and higher humidity suggests that MERS-CoV and SARS-CoV share relatively similar stability characteristics.
Suggests that coronaviruses are more sensitive to humidity than influenza viruses. This applies to aerosol forms as well as to surface born (fomite).
John D. Noti, Health Effects Laboratory Division (HELD), National Institute for Occupational Safety and Health (NIOSH), Centers for Disease Control and Prevention (CDC), PLOS ONE, 2013, Included into CDC NIOSHTIC-2, 2013
Total virus collected for 60 minutes retained 70.6–77.3% infectivity at relative humidity ≤23% but only 14.6–22.2% at relative humidity ≥43%. Analysis of the individual aerosol fractions showed a similar loss in infectivity among the fractions. Time interval analysis showed that most of the loss in infectivity within each aerosol fraction occurred 0–15 minutes after coughing.
Maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus.
Loss of infectivity is only one of the effects that decrease transmission. The infectivity is at a minimum at RH=43% (here, rounded up to 50%), then slightly increases and then drops again at RH>70%
The effect of increasing humidity on viral survival differed among several reported studies as Hemmes et al. , Hood  and Harper  concluded that survival was maximum at 10–25% RH and minimal at high >50% RH whereas, Shechmeister  and Shaffer et al.  found survival was maximal at 20–25% RH, minimal at 50% RH, and moderate at 70–80% RH.
M. K. Ijaz, Journal of General Virology, Society for General Microbiology, 1985
The survival of airborne human coronavirus 229E (HCV/229E) was studied under different conditions … High RH [=80%] at 20 °C, on the other hand, was found to be the least favourable to the survival of aerosolized virus and under these conditions the virus half-life was only about 3 h;
Wan Yang & Linsey C. Marr, Virginia Tech, PLOS ONE, 2011
… we further model the dynamics of droplets emitted from a cough in an indoor environment and illustrate the evolution of infectious IAV [influenza A viruses] concentrations and size distributions, considering removal by gravitational settling, ventilation, and viral inactivation. We are thus able to determine the magnitude by which humidity affects airborne concentrations of infectious IAVs. … Higher RH favors removal of infectious IAVs.
Maintaining a high indoor RH and ventilation rate may help reduce chances of IAV infection.
K. H. Chan, et all., University of Hong Kong, Hindawi – Advances in Virology, 2011
Interestingly, during the outbreak of SARS in Guangzhou, clinicians kept the windows of patient rooms open and well ventilated and these may well have reduced virus survival and this reduced nosocomial transmission. SARS CoV can retain its infectivity up to 2 weeks at low temperature and low humidity environment …
Our studies indicate that SCoV is relatively more stable than the human coronaviruses 229E or OC43 … SARS CoV can survive at least two weeks after drying at temperature and humidity conditions found in an air-conditioned environment.
This is applicable to SARS-CoV dried in cold dry office conditions.
M. Khalid Ijaz, et all, American Journal of Infection Control, 2016
|Temperature||As temperature increases, survival decreases DNA viruses are more stable than RNA viruses at higher temperatures|
|RH||Enveloped viruses (most respiratory viruses, influenza) survive longer at lower RH (20%-30%) Nonenveloped viruses (adenovirus, rhinovirus, and polio virus) survive longer in higher RH (70%-90%)|
|Atmospheric gases||Ozone inactivates airborne viruses to a greater degree than bacteria or fungi|
|Light and irradiation||UV light is harmful (RH-dependent)|
|Table 3 Current and emerging technologies for decontamination of indoor air for human pathogens|
|UV irradiation Upper-room 254 nm UVC light||UV light (254 nm) at 3 levels of RH||Influenza A virus (H1N1, PR-8)||Virus susceptibility to UV increased with decreasing RH|
|UV light (254 nm)||… a coronavirus as surrogate for SARS, …||High RH did not protect viral aerosols|
|Ozone generator||Gaseous ozone and aerosolized virus were generated continuously into the chamber||Bacteriophages: single-strand RNA [like influenza & coronaviruses] and DNA, double-strand RNA and DNA||95% of virus aerosol was <2.1 µm in diameter Viruses were more susceptible to ozone at higher RH|
Aaron J. Prussin, II, et all., American Society of Microbiology, Applied and Environmental Microbiology, 2018
Shaman and Kohn (12) have concluded that the relationship is stronger with AH than with RH. According to their analysis of virus survival in aerosols (24) and transmission in guinea pigs (16), AH explains 50% and 90% of the variability in influenza virus transmission and survival, respectively … In a study examining influenza virus survival in droplets at elevated temperatures (55 to 65°C), McDevitt et al. (27) also reported that AH is a better predictor than RH for virus inactivation. …
Due to the challenges and biosafety concerns of working with the influenza virus and coronavirus, this study employed the enveloped bacteriophage Phi6, which has been suggested as a surrogate for the influenza virus (31, 32) and SARS coronavirus (33). Our results provide novel information about the complex interplay between temperature, humidity, and the survival of viruses in droplets.
Shows optimum humidity levels at 75-80% and temperature above 25 degrees Celsius.
Natalie Pica, Nicole M Bouvier, Mount Sinai School of Medicine, Science Direct, 2012
► Respiratory viruses are spread from person to person via various modes of transmission, including direct and indirect contact, droplet spray, and aerosol
Modes of person-to-person transmission of respiratory viruses
|Contact transmission||In both modes of contract transmission (direct and indirect), contaminated hands play an important role in carrying virus to mucous membranes.|
|Direct transmission||Virus is transferred by contact from an infected person to another person without a contaminated intermediate object (fomite).|
|Indirect transmission||Virus is transferred by contact with a contaminated intermediate object (fomite).|
|Droplet spray transmission||Virus transmits through the air by droplet sprays (such as those produced by coughing or sneezing); a key feature is deposition of droplets by impaction on exposed mucous membranes.|
|Aerosol transmission||Virus transmits through the air by aerosols in the inspirable size range or smaller; aerosol particles are small enough to be inhaled into the oronasopharynx and distally into the trachea and lung.|
(Adapted from Centers for Disease Control and Prevention (CDC); URL: http://www.cdc.gov/influenzatransmissionworkshop2010/).
airborne routes (droplet spray and aerosol) seemed to be more important in SARS coronavirus spread
|Influenza viruses||Orthomyxoviridae||Contact, droplet spray and/or aerosol (conflicting data)|
|SARS coronavirus||Coronaviridae||Droplet spray and aerosol, possibly contact|
In the early 1960s, Schulman and Kilbourne developed an influenza virus transmission model in mice. Although mouse-to-mouse transmission is relatively inefficient, they still observed a significant decrease in transmission efficiency with increasing relative humidity (RH) …
Transmission of viruses via airborne routes may be affected by ambient humidity, which affects not only the virus’ stability but also respiratory droplet size, as water content evaporates. In turn, droplet size influences whether the particle will quickly settle to the ground or remain airborne long enough to be inhaled into the respiratory tract of a susceptible host. For influenza virus, mathematical modeling suggests that RH is an important variable in airborne transmission of influenza virus; high RH favors removal of infectious particles both by increasing the settling of large, water-laden droplets and by hastening virus inactivation
Sonja J. Olsen, Ph.D., The New England Journal of Medicine, 2003
This paper suggests that aircraft might be not only carriers, but incubators of the coronaviruses because of the human density and low humidity levels.
Timothy F. Booth, et all, Oxford Journal of Infectious Disease, 2005
Previous studies of human coronavirus 229E (a common cold virus) showed that experimental aerosols could persist and retain viability for as long as 6 days at 20C and 50% relative humidity . These conditions are representative of typical indoor environments. Although such experiments may overestimate the ability of a virus to survive in real environments (for example, they do not take into account air turnover rates in buildings), one would expect SARS CoV to have similar airborne survival characteristics, given that these viruses are in the same family and have broadly similar physicochemical properties
Popular Medical Advice
Leah Binder, Forbes, 2019
Since that study was published, there is now more research in peer-reviewed literature observing a link between dry air and viral infections, such as the flu, colds and measles, as well as many bacterial infections, and the National Institutes of Health (NIH) is funding more research. Taylor finds one of the most interesting studies from a team at the Mayo Clinic, which humidified half of the classrooms in a preschool and left the other half alone over three months during the winter. Influenza-related absenteeism in the humidified classrooms was two-thirds lower than in the standard classrooms—a dramatic difference.
Emily Bamforth, Cleveland.com, 2020
Humidity could be helpful in relieving the symptoms of coronavirus, as well as preventing it from spreading
Should temperature or humidity in buildings be adjusted to prevent the transmission of COVID-19? • There is no data to suggest that adjusting the temperature or humidity of a building would be an effective way to reduce transmission of COVID-19. The Health Department does not recommend that buildings increase humidity levels to control COVID-19 transmission.
Such obscurantism might have contributed to New York becoming the new COVID-19 epicenter. New York and New Jersey have more than half of the 68,581 COVID-19 cases, confirmed in the US by March 26, 2020.
Conflict of Interest Statement
The Author declares absence of conflicts of interest.
Note from Anthony: I’ve been sick myself the last few days, but I can’t tell if it was flu or seasonal allergy issues, or COVID-19. Time will tell.
What I can tell you though, is that I got one of these, and it has helped my sneezing and coughing. It has a UV-C light to kill germs, and that type of UV light may kill the Coronavirus according to this article.
There are also models that add heat and humidity to the indoor air:
As suggested in the article above by Leo Goldstein, there may be value against COVID-19 in increased humidity, increased temperature, and use of UV-C light in a circulating air filter
Best of luck in these trying times to all!