by Tom Stavola
(originally published on Medium)
While weather conditions will not completely eliminate SARS-CoV-2 [COVID-19], viral infectivity and transmission alter as a function of temperature, humidity, and sunlight exposure. If SARS-CoV-2 operates in a manner congruous with other viral mediated infectious diseases, and similar coronaviruses, there should be a gradual decrease in transmission and thus morbidity and mortality rates as the Northern Hemispheric warm season reaches a climax. These assertions are predicated upon extant studies which have either modelled [via simulations] or observed the effect of sunlight/temperature/humidity on analogs of SARS-CoV-2 or other viruses. Additionally, present empirical data on SARS-CoV-2 morbidity rates lends further credence to this hypothesis.
In brief, there are three primary ways in which weather modulates viral transmission and infectivity, including sunlight exposure, temperature, and humidity.
A key practical recommendation that should be instituted on a broad scale by governments, schools, hospitals, and offices is the installation of humidification systems. Indoor relative humidity should be increased to 50% for maximum viral reducing effect.
 Ultraviolet exposure via sunlight.
Ultraviolet-C plays a significant role in viral inactivation, along with Ultraviolet B and A, to a lesser extent. The following study modelled viral inactivation utilizing proxies of solar zenith angle and solar effective flux. The study found that the lower the solar zenith angle, the higher the solar effective flux, and consequently, more viral inactivation.
In simple terms, the solar zenith angle [SZA] is complementary to the solar altitude angle, with both of those values equaling 90 degrees. The smaller the solar zenith angle, the larger the solar altitude angle. In other words, the higher the Sun is with respect to the horizon, the lower the SZA, and thus increased solar effective flux and attendant viral inactivation.
Unsurprisingly, viral inactivation was greatest at lowest solar zenith angles, that is, at higher solar altitudes.
Extracted from the study — plotted here is viral inactivation at various SZA:
For reference, 0 degree SZA = 90 degree solar altitude [Sun is directly overhead, typical summer conditions up to 23.5 degrees North latitude in the northern hemisphere].
28 degree SZA = 62 degree solar altitude [typical May / August sun angle in the middle latitudes of the Northern Hemisphere, i.e., Northeastern US].
37 SZA = 53 degree solar altitude [typical March and October sun angle in the middle latitudes of the Northern Hemisphere, i.e., Northeastern US]
68 SZA = 22 degree solar altitude [typical winter sun angle in the middle latitudes of the Northern Hemisphere, i.e., Northeastern US].
Practical Implications: Higher sun angle increases UV-C, B, A receipt at ground level, increasing viral inactivation. Thus, the most favorable time for viral inactivation is between May and September in the Northern Hemisphere.
 Temperature and Humidity
It is generally well established in science that high temperatures, especially in concert with higher relative humidity values, tend to decrease viral transmission and increase viral inactivation. Viral particles become much less able to travel far distances at higher temperatures and relative humidity. While there is no general rule for all viruses, the following appears to be rather apparent.
A study on coronaviruses — using proxy viruses of transmissible gastroenteritis virus (TGEV) and mouse hepatitis virus (MHV) — found that both higher relative humidity values and higher temperatures significantly decreased viral survival.
At low relative humidity [20%] and low temperatures [4 Celsius / 39 Fahrenheit] — coronavirus appears to be at its strongest, most viable point.
This figure depicts survival of the proxy coronaviruses at 39 degrees Fahrenheit and 20% relative humidity [A], 50% relative humidity [B], and 80% relative humidity [C] — notice the persistence of the virus [on stainless steel surfaces] was greatest at the lowest humidity values:
Now, the below figure depicts the survival of the proxy coronaviruses viruses at 68 degrees Fahrenheit and 20% relative humidity [A], 50% relative humidity [B], and 80% relative humidity [C]. Note the declines in viral survival are much more expeditious — especially when we make the transition from 20% to 50% relative humidity:
Finally, the sharpest and quickest decline in viral survival occurred at 104 degrees Fahrenheit and all three humidity levels — 20% relative humidity [A], 50% relative humidity [B], and 80% relative humidity [C]:
Practical Implications: Relative Humidity values at the studied threshold of 50% significantly increased viral inactivation. Temperatures, once to the threshold of 68 degrees Fahrenheit, especially when combined with relative humidity of 50% or greater, also significantly increased the pace of viral inactivation.
Study on high humidity and influenza [not directly related to coronavirus, but again, indicates the effect of higher RH on rendering viruses inactive]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3583861/
The effect of relative humidity induced viral inactivation appears to reach a maximum around 50% RH:
Source [same study referenced earlier] “Effects of Air Temperature and Relative Humidity on Coronavirus Survival on Surfaces” https://www.condair.co.uk/humidity-health-wellbeing/scientific-studies/effects-of-air-temperature-and-relative-humidity-on-coronavirus-survival-on-surfaces
 Real-time Data
Inaccurate and underreported diagnoses issues aside, the present SARS-CoV-2 incidence from country to country suggests that warmer, more humid nations [ones that coincide with conditions noted above] tend to have significantly less SARS-CoV-2 cases. One can see the clear demarcation line between the colder middle latitudes and the tropical regions. Given the high populations of nations like the Philippines and India, for example, these case/morbidity values are very low, in comparison to nations farther north. Again, testing accuracy and frequency is a confounder, but the effect seems to be preserved across numerous nations of more equatorward latitude.[Data — approximate number of cases by country as of 3/12/2020].
What This Means and Practical Recommendations
First: a caveat is that the above is based upon analogizing to other viruses and coronaviruses/proxies. This is a novel variant of the coronavirus, so it’s not absolutely definitive that it will behave as described. However, real time data seems to indicate that it will behave as described, namely, become less transmissible as temperatures and humidity increase in the Northern Hemisphere warm season.
In the United States, average temperatures for most of the country, especially the northern half, do not reach/exceed 68F until early-mid May. Relative humidity usually does not increase significantly until mid to late May and early June. Since viral inactivation increases as sun angle, temperature, and humidity all increase, one should anticipate SARS-CoV-2 cases to decrease significantly in the period from approximately mid-May through summer in the United States. Next autumn, it is probable if not likely that SARS-CoV-2 will begin to gradually increase again.
As a practical recommendation: since higher humidity is a potent deterrent to viral survival/persistence, the interiors of all buildings, schools, hospitals, should be humidified to 50%. The relative humidity of most homes and buildings during the November-March period falls below 35% and on occasion below 25%. That environment is very conducive for viral transmission. Increasing that humidity value to 50% in homes, classrooms, offices, hospitals, and all buildings would significantly increase the pace of viral inactivation. See also, the following pilot study on influenza corroborating this hypothesis: https://www.dristeem.com/humidity-university/case-studies/case-study-humidify-to-reduce-respiratory-virus-transmission. Given the economic burden of cold season infections such as influenza and now likely SARS-CoV-2, it might be salutary, and actually cost-saving, for all schools, hospitals and offices to implement humidification systems.
SARS-CoV-2 will be a significant health hazard to a relatively small percentage of the global population, but even still, one should take all steps necessary to decrease transmission from low-risk individuals. This virus should coincide with others and demonstrate seasonality [summer decrease]. Warmer/more humid regions will probably fare better than the mid-northern latitudes with SARS-CoV-2, however, the mid-latitudes, like the USA, should expect a respite [not zero cases] from May-September, as cases gradually decline, probably reaching minimum in July.
In addition to increasing indoor humidity levels to 50%, there are numerous antiviral modalities available as prophylactics.
Potential Cost Savings
- Average cost of steam humidifier including installation: $1,000
- Annual maintenance/cleaning visits usually cost $100
- This vendor has steam humidifiers with UV lighting to keep water sterilized: https://www.condair.com/service/hygiene
- 40 classrooms per school on average
- 2,598 public schools in New Jersey
- Total cost = $114,312,000 [million] first year
- Total cost in each subsequent year = $10 million
- Total economic burden of influenza in the USA is greater than $87 billion per year
- $1.74 billion in NJ annual burden for influenza
- 31% approximate reduction in influenza cases due to humidification.
- Savings via humidification = $ 539,400,000 [million]
- Thus, cost savings in year 1 in NJ: $424,088,000 [million]
- Year 2 savings = $529,400,000 [million]
- Year 3 savings = $529,400,000 [million]
- Year 4 savings = $529,400,000 [million]
- Year 5 savings = $529,400,000 [million]
- Year 6 savings = $529,400,000 [million]
- Year 7 savings = $529,400,000 [million]
- Year 8 savings = $529,400,000 [million]
- Year 9 savings = $529,400,000 [million]
- Year 10 savings = $529,400,000 [million]
- Total 10 year cost savings = $5,188,688,000 [billion]