Covid-19 (Novel Coronavirus) has already disrupted the lives of over a billion people in China and is continuing to spread across the globe...
Good news. This isn't an post about how covid-19 is an irresistible force that will engulf the world. In temperate climates, humidity is the key to controlling airborne diseases like covid-19. As humidity increases in spring, the transmission rate will drop. The rate of new cases of covid-19 is stabilising in China, and by the end of May, local transmission of covid-19 in Northern temperate regions will be greatly reduced. With improvements in diagnosis and treatment thanks to more time, the impact of covid-19 should be smaller next winter.
How do I know this? Well, covid-19 isn't really that novel. It's a new strain of coronavirus, and we've had those before. SARS was also a coronavirus. Many common colds are from coronavirus strains⁰. Coronaviruses, including covid-19, are spread most often by respiratory droplets¹, which is the same as influenza or cold transmission².
The flu and the common cold are seasonal diseases. SARS was also seasonal and was discovered during the winter months and then faded away at the end of the flu season by May³. The extermination of SARS was undoubtedly assisted by the efforts of many health care workers, but the strongest factor was the changing of the seasons, which reduced transmission rates to the point where it could be wiped out.
Covid-19 is Also Seasonal
Since covid-19 is new, it's fair to ask if it's seasonal, like other coronaviruses or influenza. If covid-19 is seasonal, then the worst outbreaks should be in regions currently in the winter season. Indeed, the severe outbreaks have been in China, Korea, and some farther places, like northern Italy and Iran. China, Korea, and Italy are in winter, and, where the outbreak is happening in northern Iran, they've been in the winter season⁴ as well (!!).
This difference in transmission rates due to different weather can be seen even within countries, such as Japan. Despite having more visitors from China⁵ and a higher population density, Tokyo has fewer incidences of covid-19 than the colder, less humid northern island of Hokkaido⁶, where covid-19 is still spreading.
But why is weather so important? Humans spend about 90% of our time inside protected from the weather⁷, and it's hard to believe that we're contracting viruses on our short jaunts between one climate-controlled building and another or while walking the dog.
Humidity is the unseen control for viral transmission. In cold weather, the vaporized water content of air is very low, and warming up that dry air to room temperature doesn't give it additional humidity. Most buildings do not raise the humidity to 40-60% at room temperature, which provides the best protection against airborne viruses. There are two different ways humidity reduces airborne viral transmission. First, humidity cuts the survival rate of airborne viruses from coughing or even talking. After an hour, about four fifths of viruses are killed at humidity over 43% compared with humidity under 23%⁸.
The second benefit of higher humidity at room temperature is improved immune response. At low humidity, the immune system has trouble filtering out viruses. This enables more of the virus to enter our bodies. Also, at low humidity, the membrane of our airways dry out and are damaged, which makes people more vulnerable to airborne viruses.⁹
2020 Climate Control
The studies that found this relation between humidity and reduced viral transmission were done within the last few years, so the knowledge that humidity above 40% is critical to reducing airborne viral transmission is still relatively unknown. As we fight against covid-19, now is the time to make humidifiers standard equipment for all heated buildings where people gather. Now that we know how important humidity is, we can focus on making the buildings where our children, parents, and friends gather safer. If you're planning on meeting someone, find out if the buildings you'll be in have humidifiers. If you're responsible for a public building, in addition to alcohol sanitizers at the door, make sure that the building humidity is over 40%.
If the world hears the message about the importance of humidifiers for disease control, I expect in the short term, there will be a shortage of portable humidifiers. Fortunately, it's March, and the change of the season and natural increase in temperatures will reduce the need for heating and thus humidifiers at least until fall. Let's make the most of that time, and work towards a future where any building with heating also has humidification. Imagine a future where humidity control is reducing viral transmission in hospitals, schools, offices, and theaters. It's the best way to protect ourselves against covid-19 and the next outbreak of an airborne virus.
While I enjoy researching medical studies, I am an engineer, not a doctor. See the links below for real doctor studies.
 "Coronaviruses (CoV) are a large family of viruses that cause illness ranging from the common cold..." https://www.who.int/health-topics/coronavirus
 "Coronaviruses are generally thought to be spread most often by respiratory droplets" https://www.cdc.gov/coronavirus/2019-ncov/faq.html
 "When an infected person coughs or sneezes, droplets containing viruses (infectious droplets) are dispersed into the air..." https://www.who.int/en/news-room/fact-sheets/detail/influenza-(seasonal)
 "May 6: In the United States, no new probable cases were reported in the last 24 hours, and there was no evidence of ongoing transmission beyond the initial case reports in travelers for more than 20 days. The containment in the United States has been successful." https://www.cdc.gov/about/history/sars/timeline.htm
 Qom, Iran, "Average high temperature in January: 10.2°C" https://www.weather-atlas.com/en/iran/qom-climate
 "POPULAR TOURIST SPOTS FAVORED BY CHINESE TOURISTS (PARTIAL LIST) 1. Tokyo Disneyland, 2. Mt. Fuji, 3, Tokyo Tower, 4 Hokkaido" https://www.nielsen.com/wp-content/uploads/sites/3/2019/05/outbound-chinese-tourism-and-consumption-trends.pdf
 As of March 1, Hokkaido, 72 confirmed cases, Tokyo, 45. https://jagjapan.maps.arcgis.com/apps/opsdashboard/index.html#/55c22ee976bc42338cb454765a6edf6b
 Figure 1, Nation, Percentage Time Spent Outdoors, 7.6% https://www.nature.com/articles/7500165
 "The present study allowed us to assess viral infectivity under various levels of relative humidity and showed that one hour after coughing, ∼5 times more virus remains infectious at 7–23% RH than at ≥43% RH." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3583861/
 "Low ambient humidity impairs barrier function and innate resistance against influenza infection" https://www.pnas.org/content/pnas/116/22/10905.full.pdf