Timing is also key. The longer you spend in close proximity with an infected person, the bigger the risk.
Scientists advising the UK government say that spending six seconds at a distance of 1m from someone is the same as spending one minute at a distance of 2m.
Being exposed to someone coughing is riskier. Being 2m away from a cough carries the same risk as someone talking to you for 30 minutes at the same distance.
…In conclusion, under the conditions of high temperature in the Southern Hemisphere summer, our study provides evidence ??? that lower relative humidity is associated with COVID-19 cases. It also suggests that all countries need to maintain vigilance for COVID-19, even during the summer months.
A later study on impact of temperature and humidity is worthy of consideration and integration into the above.
See:
https://www.isitzen.com/blog/2021/2/weather-impact-on-airborne-coronavirus-survival-physics-of-fluids
Very hot objects emit UV radiation (see black-body radiation). The Sun emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet where it crosses into X-rays at 10 nm. Extremely hot stars emit proportionally more UV radiation than the Sun. Sunlight in space at the top of Earth's atmosphere (see solar constant) is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1400 W/m2 in vacuum.[20]
The atmosphere blocks about 77% of the Sun's UV, when the Sun is highest in the sky (at zenith), with absorption increasing at shorter UV wavelengths. At ground level with the sun at zenith, sunlight is 44% visible light, 3% ultraviolet, and the remainder infrared.[21][22] Of the ultraviolet radiation that reaches the Earth's surface, more than 95% is the longer wavelengths of UVA, with the small remainder UVB. Almost no UVC reaches the Earth's surface.[23] The fraction of UVB which remains in UV radiation after passing through the atmosphere is heavily dependent on cloud cover and atmospheric conditions. In "partly cloudy" days, patches of blue sky showing between clouds are also sources of (scattered) UVA and UVB, which are produced by Rayleigh scattering in the same way as the visible blue light from those parts of the sky. UV-B also plays a major role in plant development as it affects most of the plant hormones.[24] During total overcast, the amount of absorption due to clouds is heavily dependent on the thickness of the clouds and latitude, with no clear measurements correlating specific thickness and absorption of UVB.[25]
The shorter bands of UVC, as well as even more-energetic UV radiation produced by the Sun, are absorbed by oxygen and generate the ozone in the ozone layer when single oxygen atoms produced by UV photolysis of dioxygen react with more dioxygen. The ozone layer is especially important in blocking most UVB and the remaining part of UVC not already blocked by ordinary oxygen in air.
Ultraviolet (UV) light is produced by the sun and by special lamps. There are three types of UV light—UVA, UVB, and UVC. UVC light has the most energy of the three types. This energy destroys the genetic material inside viruses and other microbes.
UVC light has been found to destroy viruses and other microbeson surfaces in hospitals.
Exposure to UVC light is dangerous for people.
UVC light from the sun is blocked by Earth’s atmosphere. When you go outside on a sunny day, the UV light that reaches you is UVA and some UVB. These types of UV light do not destroy viruses quickly.
Going outside on a sunny day will not quickly break down coronaviruses on skin. But it can give you a sunburn if you are not wearing sunscreen!
Some viruses are seasonal and spread more slowly in the summer. This is probably due to warmer temperatures, higher humidity, and changes in human behaviors—not because it is sunnier in the summer.
The best ways to protect yourself from COVID-19 are to wash your hands, keep your distance from other people, and clean and disinfect frequently touched surfaces.
Mechanisms for airborne transmission of pathogens
Surprisingly little progress has been made in understanding how pathogens pass from one host to the next. 28 Since the 1930s, four mechanisms of transmission have been described. These are: contact; dust; ‘respiratory droplets’ and ‘droplet nuclei’. There is confusion in the literature regarding the definition of these particles and their mechanisms of spread. For example, ‘contact’ may be used to indicate inhalation of large droplets from contagious individuals when they cough or sneeze, i.e. droplet transmission, but ‘contact’ may also refer to infectious particles transmitted directly from contaminated surfaces. Aside from particle size, the potential for transmission depends upon dynamic factors, such as number of particles produced; velocity at which they are produced; number of micro-organisms contained within the spectrum of droplet sizes; infectious longevity of those microbes; and proximity of a susceptible target…
Natural versus mechanical ventilation
There is some evidence that natural ventilation can be more effective than mechanical systems for preventing transmission. During the 1918 influenza pandemic, sick patients who were accommodated in the open air survived in greater numbers. Eighty years later, during Operation Desert Shield, respiratory tract infections were more Frequent in military personnel in air-conditioned barracks than among those housed in tents.
Background:
Infections caught in buildings are a major global cause of sickness and mortality. Understanding how infections spread is pivotal to public health yet current knowledge of indoor transmission remains poor.
Aim:
To review the roles of natural ventilation and sunlight for controlling infection within healthcare environments.
Methods:
Comprehensive literature search was performed, using electronic and library databases to retrieve English language papers combining infection; risk; pathogen; and mention of ventilation; fresh air; and sunlight. Foreign language articles with English translation were included, with no limit imposed on publication date. Findings: In the past, hospitals were designed with south-facing glazing, cross-ventilation and high ceilings because fresh air and sunlight were thought to reduce infection risk. Historical and recent studies suggest that natural ventilation offers protection from transmission of airborne pathogens. Particle size, dispersal characteristics and trans- mission risk require more work to justify infection control practices concerning airborne pathogens. Sunlight boosts resistance to infection, with older studies suggesting potential roles for surface decontamination.
Conclusions:
Current knowledge of indoor transmission of pathogens is inadequate, partly due to lack of agreed definitions for particle types and mechanisms of spread. There is recent evidence to support historical data on the effects of natural ventilation but virtually none for sunlight. Modern practice of designing healthcare buildings for comfort favours pathogen persistence. As the number of effective antimicrobial agents declines, further work is required to clarify absolute risks from airborne pathogens along with any potential benefits from additional fresh air and sunlight.
ABSTRACT
UV radiation from the sun is the primary germicide in the environment. The goal of this study was to estimate inactivation of viruses by solar exposure…
… Sunlight or, more specifically, solar UV radiation (UV) acts as the principal natural virucide in the environment. UV radiation kills viruses by chemically modifying their genetic material, DNA and RNA. The most effective wavelength for inactivation, 260 nm (55), falls in the UVC range, so-named to differentiate it from near-UV found in ground-level sunlight, i.e., the UVB and UVA portions of the spectrum, 290 to 320 nm and 320 to 380 nm, respectively (51). Nucleic acids are damaged also by UVB and UVA but with lower efficiency than by UVC radiation
… The overwhelming majority of published information on UV inactivation of viruses has been based upon exposure to UVC (UV254) radiation from a low-pressure mercury vapor (germicidal) lamp, with the primary emission at 254 nm. However, UV254 is not found in the sunlight that reaches the earth's surface; the ground-level virucidal solar UV wavelengths fall above 290 nm (16). Fortunately, the primary photochemical processes that damage the viral DNA or RNA occur at all the solar UV wavelengths, varying only in the efficiency of the different wavelengths (55). Since there are few published data that describe the survival of viruses, and none for threat viruses, following exposure to solar UV radiation, extrapolation from UV254 data will be required for most viruses. This extrapolation can be made using wavelength dependence (action spectrum) data.
… The action spectra of virus inactivation were found to be similar for all viruses regardless of genome type. Thus, one composite action spectrum was used to represent all viruses.
