COVID-19 is quickly destroyed by sunlight


The new coronavirus is quickly destroyed by sunlight, according to new research announced by a senior US official on Thursday, though the study has not yet been made public and awaits external evaluation.

William Bryan, science and technology advisor to the Department of Homeland Security secretary, told reporters at the White House that government scientists had found ultraviolet rays had a potent impact on the pathogen, offering hope that its spread may ease over the summer.

“Our most striking observation to date is the powerful effect that solar light appears to have on killing the virus, both surfaces and in the air,” he said.

“We’ve seen a similar effect with both temperature and humidity as well, where increasing the temperature and humidity or both is generally less favorable to the virus.”

But the paper itself has not yet been released for review, making it difficult for independent experts to comment on how robust its methodology was.

It has long been known that ultraviolet light has a sterilizing effect, because the radiation damages the virus’s genetic material and their ability to replicate.

A key question, however, will be what the intensity and wavelength of the UV light used in the experiment was and whether this accurately mimics natural light conditions in summer.

“It would be good to know how the test was done, and how the results were measured,” Benjamin Neuman, chair of biological sciences at Texas A&M University-Texarkana, told AFP.

“Not that it would be done badly, just that there are several different ways to count viruses, depending on what aspect you are interested in studying.”

Virus inactivated

Bryan shared a slide summarizing major findings of the experiment that was carried out at the National Biodefense Analysis and Countermeasures Center in Maryland.

It showed that the virus’s half-life – the time taken for it to reduce to half its amount – was 18 hours when the temperature was 70 to 75 degrees Fahrenheit (21 to 24 degrees Celsius) with 20 percent humidity on a non-porous surface.

This includes things like door handles and stainless steel.

But the half-life dropped to six hours when humidity rose to 80 percent – and to just two minutes when sunlight was added to the equation.

When the virus was aerosolized – meaning suspended in the air – the half-life was one hour when the temperature was 70 to 75 degrees with 20 percent humidity.

In the presence of sunlight, this dropped to just one and a half minutes.

Bryan concluded that summer-like conditions “will create an environment (where) transmission can be decreased.”

He added, though, that reduced spread did not mean the pathogen would be eliminated entirely and social distancing guidelines cannot be fully lifted.

“It would be irresponsible for us to say that we feel that the summer is just going to totally kill the virus and then if it’s a free-for-all and that people ignore those guides,” he said.

Previous work has also agreed that the virus fares better in cold and dry weather than it does in hot and humid conditions, and the lower rate of spread in southern hemisphere countries where it is early fall and still warm bear this out.

Australia, for example, has had just under 7,000 confirmed cases and 77 deaths—well below many northern hemisphere nations.

The reasons are thought to include that respiratory droplets remain airborne for longer in colder weather, and that viruses degrade more quickly on hotter surfaces, because a protective layer of fat that envelops them dries out faster.

US health authorities believe that even if COVID-19 cases slow over summer, the rate of infection is likely to increase again in fall and winter, in line with other seasonal viruses like the flu.

Coronaviruses are members of the Coronaviridae group and contain a single-stranded, positive- sense RNA genome surrounded by a corona-like helical envelope (Ryan 1994). The SARS virus genome consists of 29,751 base pairs. Approximately 41% of the genome is GC base pairs while 59% is TA base pairs. Coronaviruses have a size range of 0.08-0.15 microns; with a mean size of 0.11 microns (see Figure 1). Common Coronaviruses are responsible for colds and can transmit by the airborne route as well as through direct contact.

NOTE: SARS is not to be confused with Influenza A viruses (i.e. Influenza AH1N1) which are members of the Orthomyxovirus group and contain a single-stranded RNA genome enclosed in a helical envelope. The genome consists of 13,588 base pairs. Influenza viruses are responsible for flus and can transmit by the airborne route as well as through direct contact.

SARS coronavirus is one of the most hazardous infections for hospital personnel. In a study by He et al (2003) it was found that index patients were the first generation source of transmission and they infected inpatients and medical staff, creating second generation patients. The major transmission routes were close proximity airborne droplet infection and close contact infection.

There was also evidence for the likelihood of aerosol transmission of infections through the ventilation system, which spread the infection to other hospital floors. A similar report comes from Ho et al (2003), who found that Hospital outbreaks of SARS typically occurred within the first week after admission of the first SARS cases before recognition and before isolation measures were implemented.

In the majority of hospital infections, there was close contact with a SARS patient, and transmission occurred via large droplets, direct contact with infectious fluids or by contact with fomites from infectious fluids. In some instances, potential airborne transmission was reported in association with endotracheal intubation, nebulised medications and non- invasive positive pressure ventilation of SARS patients.

Nosocomial transmission was effectively halted by enforcement of standard routines, contact and droplet precautions in all clinical areas, and additional airborne precautions in high-risk areas.

Table 1 summarizes the studies that have been performed on Coronaviuses under UV exposure and also shows the genomic prediction of the UV rate constant in the final row. The last two studies (Kariwa 2004 and Darnell 2004) seem to be anomalous but it is unclear from the data why these results indicate such an unusually high UV resistance, but have been included for completeness. All the data in Table 1 except for the Duan (2003) study were used in the development of the genomic model of ssRNA viruses shown in Figure 2. Based on the ssRNA genomic model the UV rate constant for SARS Coronavirus computes to be 0.3289 m2/J and this gives a D90 value of 7 J/m2, which is in fairly good agreement with the first three studies shown in Table 1.
Figure 2: Genomic model of 27 ssRNA viruses representing 62 data sets (Kowalski et al 2009). The SARS virus (NC_004718) is highlighted in red and the average of the four Coronavirus studies are highlighted in green.

General Explanation of the Disease

COVID-19 is the respiratory disease caused by the SARS-CoV-2 virus that has caused outbreaks worldwide. The SARS-CoV-2 is a new variant in the betacoronavirus family (Fisher 2020).

It transmits by direct contact or contact with fomites and can be suspended in air as well, as are the related betacoronaviruses SARS, MERS, and the four known Human coronaviruses – OC43, 229E, NL63, and HKU1. The majority of infection transmissions are believed to be by droplet spray from coughing and sneezing and by direct contact or contact with fomites.

Confirmation That Ultraviolet is Effective

Ultraviolet light can be an effective measure for decontaminating surfaces that may be contaminated by the SARS-CoV-2 virus by inducing photodimers in the genomes of microorganisms. Ultraviolet light has been demonstrated to be capable of destroying viruses, bacteria and fungi in hundreds of laboratory studies (Kowalski 2009).

The SARS-CoV-2 virus has not yet been specifically tested for its ultraviolet susceptibility but many other tests on related coronaviruses, including the SARS coronavirus, have concluded that they are highly susceptible to ultraviolet inactivation.

This report reviews these studies and provides an estimate of the ultraviolet susceptibility.

It is estimated that the SARS-CoV-2 virus can survive on surfaces for up to 9 days, based on its similarity to SARS and MERS. Standard disinfectants are effective against SARS-CoV-2 but as an extra level of

protection, and to shield against errors in the manual disinfection process, ultraviolet light can be used to disinfect surfaces and equipment after the manual chemical disinfection process is completed.

ASHRAE recommends ultraviolet germicidal irradiation as one strategy to address COVID-19 disease transmission (ASHRAE 2020).

COVID-19 is highly contagious and so any residual contamination, no matter how small, can pose a threat to healthcare workers and patients.

The PurpleSun E300 Focused Multivector Ultraviolet (FMUV) system with Shadowless DeliveryTM (see Figure 1) is an automated system that has proven to reduce surface contamination by 96% and can address contamination left behind by current manual chemical cleaning which was shown to only reduce contamination by 36% (Armellino 2020).

The PurpleSun E300 system has demonstrated elimination of 99%-99.99% of bacteria and fungi as listed in Table 2 in laboratory tests within 90 seconds (Petraitis 2017). Similar reductions could be expected against the COVID-19 coronavirus in 90 seconds as well.

Figure 1: The PurpleSun E300 FMUV system in PACT configuration for transport or storage (Left), CUBE configuration for surrounding smaller equipment (Center), and RECTAN mode for surrounding larger equipment (Right).

Scientific Rationale

Coronaviruses are members of the Coronaviridae group and contain a single-stranded, positive-sense RNA genome surrounded by a corona-like helical envelope (Ryan 1994). Approximately 100 sequences of the SARS-CoV-2 genome have been published and these suggest there are two types, Type I and Type II, of which the latter came from the Huanan market in China while the Type I strain came from an unknown location (Zhang 2020).

The genome consists of 29,751 base pairs (NC_045512.2) and the genome is about 80% homologous with SARS viruses (NCBI 2020, Fisher 2020). Coronaviruses have a size range of 60-140nm, with a mean size of 0.10 microns (Zhu 2020).

Table 1 summarizes the results of studies that have been performed on Coronaviruses under ultraviolet light exposure, with the specific species indicated in each case.

The D90 value indicates the ultraviolet dose for 90% inactivation.

Although there is a wide range of variation in the D90 values, this is typical of laboratory studies on ultraviolet susceptibility.

The range of D90 values for coronaviruses is 7-241 J/m2 the mean of which is 67 J/m2, should adequately represent the ultraviolet susceptibility of the SARS-CoV- 2 (COVID-19) virus.


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