Science & Analysis: Outdoor Transmission of Covid-19

This is the key findings of a report for the Government assessing risk of transmission through outdoor air, water, outdoor surfaces, and food.

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Key conclusions
● The highest risk of outdoor transmission is through aerosols and droplets when people are in prolonged close, face-to-face contact within 2m. This is likely to be lower than indoor settings but remains a risk especially in crowded areas, e.g.at major sporting events, festivals and public gatherings. This risk has already been considered by the Environmental Modelling Group and has not been considered further here.
● Beyond 2m, risk is likely to progressively decrease. By 10m, risk of outdoor aerosol or droplet person-to-person transmission is Very Low with medium uncertainty.
● Based on current epidemiological evidence, the risk of long-range (>10m) aerosol or droplet person-to-person transmission outdoors is Negligible with low uncertainty, due largely to dispersion effects.
● The risk of acquiring virus from infrequently touched outdoor surfaces is Very Low to Negligible with medium uncertainty, particularly if surfaces are exposed to sunshine on a daily basis.
● Surfaces that are frequent touch points such as outer shop door handles, cash machines, outside shutters, door knockers and door bells are likely to be slightly higher risk, i.e. Low with medium uncertainty.
● Recent modelling of the solar inactivation of SARS-CoV-2 on surfaces indicates that the virus could remain infectious for long time periods when light levels are low. Modelling survival time in direct midday sunlight at the latitude of London showed that the time for 90% infectivity reduction is likely to be around 30 minutes in mid-summer but extended to 300 minutes in mid-winter (Sagripanti and Lytle 2020). The virucidal effect of UV may be halved on a cloudy day or in the shade (Ben-David and Sagripanti 2010; 2013). In practice, this means that the risk of outdoor fomite transmission (and reaerosolisation) could be elevated under UK winter conditions (December-March).
● Public toilets represent a potential SARS-CoV-2 exposure point for a number of reasons. Primary amongst these is that they contain many touch surfaces which could be contaminated with infective nasopharyngeal fluids or faecal material and to which many people are exposed in a short time period. Toilets also represent the point at which the amount of infectious virus might be greatest in waste water. Aerosol, faecal/ocular, and faecal/oral transmission risks have been hypothesised based on virus presence and evidence exists based on previous SARS-CoV outbreaks. In addition, toilets may be a contact hub point in the community where transmission can occur between users through face-to-face droplet transmission, in the toilet building itself, and in proximity. Thorough and frequent cleaning is likely to reduce risk, although this can be challenging in some remote public toilets. Toilet users, cleaning staff and plumbers may also be exposed to contaminated surfaces or sewage. The level of risk is Medium with high uncertainty.
● Once wastewater is treated, effluent discharged to receiving waters will contain very little coronavirus (few studies have detected virus in wastewater treatment effluents), and the risk of this being a route of infection is Negligible with medium uncertainty. The same risk would apply to sewage sludge applied to land and municipal solid waste: by law all biosolids (intended for application to crop-growing land) are treated and the thermophilic process will rapidly inactivate coronavirus. The level of risk is Negligible with medium uncertainty. Episodically, risks from sewage may be raised to Low or Very Low with medium uncertainty based on heavy rainfall and the triggering of combined sewage overflows, which lead to higher sewage loading of receiving waters. Pollution monitoring is therefore important.
● Recreational use of waters, particularly fresh waters many of which are not designated bathing waters (e.g. rivers, lakes and canals) presents a theoretical risk, but there is no evidence of coronavirus transmission by this route. The level of risk is Very Low to Negligible with medium uncertainty. Episodically, risk may be raised to Low or Very Low with high uncertainty based on heavy rainfall associated with the operation of combined sewage overflows discharging untreated sewage into receiving waters. Pollution monitoring is therefore important.
● Airborne droplet transmission between bathers in close proximity (<2m) is likely to be a more significant risk than from waste water sources. Waterborne transmission between bathers beyond 2m is Negligible Risk with medium uncertainty.
● Risk of infection from mains-supplied drinking water is Negligible with low uncertainty. Risks from private water supplies may locally be very low to low with high uncertainty, due primarily to contamination from septic tanks.
● The probability of exposure of UK consumers to SARS-CoV-2 via food is Very Low with high uncertainty. The uncertainty associated with this estimate is high as there is still no evidence to confirm or refute the hypothesis that people can be infected by ingesting SARS-CoV-2 in food.

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