COVID-19 transmission dynamics

A large proportion of COVID-19 deaths occurred in long-term care facilities or care homes for the elderly. Long-term care facilities accounted for 60% of COVID-19 deaths in Washington state, 45% in Sweden and almost one-third in the United KingdomUnited Kingdom.

As of July 2020, fecal-oral transmission has not been demonstrated nor has the occurrence of infectious SARS-CoV-2 in water environments been proven. COVID-19 shows evidence for intestinal infection and presence in stool which gives it the potential to be a waterborne disease where water sanitation is not safely managed. According to the Environmental Protection Agency standard treatment and disinfectant processes for wastewater treatmentwastewater treatment are expected to be effective. Wastewater treatment workers could potentially have a route of respiratory exposure during the pumping of wastewater through sewerage systems the transport of coronaviruses in water where there is the potential for the virus to become aerosolised.

A large proportion of COVID-19 deaths occurred in long-term care facilities or care homes for the elderly. Long-term care facilities accounted for 60% of COVID-19 deaths in Washington state, 45% in SwedenSweden and almost one-third in the United Kingdom.

Migrant agricultural workers who come to Canada live in crowded shared accommodation. A large proportion of COVID-19 cases are traced back to outbreaks at sites of migrant worker accommodation. In the province of OntarioOntario, Canada, as of July, 2020 there were more than 1244 cases of COVID-19 infected workers and three deaths in the agriculture sector. The medical officer of Windsor-Essex County Health Unit in Ontario said that 90 percent or more of COVID-19 cases are temporary foreign workers. It is difficult to determine if transmission occurs in the workplace or at the accommodation. Migrant workers that live in bunkhouses were reported to account for one-fifth of total cases in the area.

Wastewater and plumbing

Transmission of SARS-CoV-2 has been suggested to have occurred by indirect contact in an apartment building through the transmission of viral particles on air streams within the pipe network and entry into the interior of the building from the wastewater system. The study points to feces containing virus as the source from which infectious aerosols were possibly generated by turbulent flows in wastewater plumbing. The WHO previously hypothesized that empty U-traps in a plumbing system may have created a pathway for SARS-CoV-1 virus containing droplets and aerosols to enter bathrooms and spread infections in a housing complex outbreak in Hong Kong. Air pressure surges are a common cause of empty U-traps in high-rise buildings. Mechanical bathroom extract fans and favorable outdoor air conditions were also thought to increase transmission. As of September 2020 there is limited data on the infectivity of fecal droplets and aerosols for SARS-CoV-2.

Migrant agricultural workers who come to Canada live in crowded shared accommodation. A large proportion of COVID-19 cases are traced back to outbreaks at sites of migrant worker accommodation. In the province of Ontario, Canada, as of July, 2020 there were more than 1244 cases of COVID-19 infected workers and three deaths in the agriculture sector. The medical officer of Windsor-Essex County Health Unit in Ontario said that 90 percent or more of COVID-19 cases are temporary foreign workers. It is difficult to determine if transmission occurs in the workplace or at the accommodation. Migrant workers that live in bunkhouses were reported to account for one-fifth of total cases in the area.

Homeless shelters

There is a high burden of COVID-19 among sheltered homeless populations. A single shelter in Boston found 36% of the 408 residents tested positive for SARS-CoV-2 with 88% of them reporting no symptoms. A microsimulation modeling study on the clinical outcomes and costs of interventions to prevent COVID-19 in adults staying in homeless shelters found that daily symptom screening of positive individuals along with temporary housing in a non-hospital care site would reduce infections. The reduction in infections was calculated to be 37%, 72% and 51% in scenarios where reproduction numbers Re were 2.6, 1.3 and 0.9 respectively.

Household

Migrant worker accommodation

Close to 93% of COVID-19 cases in Singapore in the first 48 days occurred in dormitories for migrant workers.

Migrant agricultural workers who come to Canada live in crowded shared accommodation. A large proportion of COVID-19 cases are traced back to outbreaks at sites of migrant worker accommodation. In the province of Ontario, Canada, as of July, 2020 there were more than 1244 cases of COVID-19 infected workers and three deaths in the agriculture sector.

Homes for the elderly

A large proportion of COVID-19 deaths occurred in long-term care facilities or care homes for the elderly. Long-term care facilities accounted for 60% of COVID-19 deaths in Washington state, 45% in Sweden and almost one-third in the United Kingdom.

A research study based out of Singapore focused on transmission from adults to children. The attack rate was found to be 1.3%, 8.1% and 9.8% for children less than 5 years old, 5-9 years old and 10-16 years old respectively. The attack rate was highest when the index case was the mother at 11% compared with 6.7% or 6.3% if the index case was the father or grandparent.

Contact within households is thought to be responsible for about 70% of SARS-CoV-2 transmission when community control measures are in place. In Wuhan, the use of Fangcang (field) hospitals to isolate cases outside the home reduced the reproduction number (R) from 1.18 after lockdown to 0.51. The odds that a primary case transmitted COVID-19 to others was found to be 18.7 times higher in a closed environment compared with an open-air environment.

The risk of secondary attack rate for SARS-CoV-2, which means the risk that people in close contact with an infected person become infected depends on the duration and intensity of contact. Secondary attack rate among household members for SARS-CoV-2 is between 10% and 40%. In comparison airborne viral pathogens like varicella zoster, which causes chicken pox and the measles virus have a household secondary attack rate of about 85-90%. For SARS-CoV-2, less sustained contact such as sharing a meal and passing interactions among shoppers are associated with secondary attack rates of 7% and 0.6% respectively.

The risk of secondary attack rate for SARS-CoV-2, which means the risk that people in close contact with an infected person become infected depends on the duration and intensity of contact. Secondary attack rate among household members for SARS-CoV-2 is between 10% and 40%. In comparison airborne viral pathogens like varicella zoster, which causes chicken pox and the measles virus have a household secondary attack rate of about 85-90%. For SARS-CoV-2, less sustained contact such as sharing a meal and passing interactions among shoppers are associated with secondary attack rates of 7% and 0.6% respectively.

A study in China using data from the Guangzhou Center for Disease Control and Prevention found the secondary attack rate among household contacts to be 12.4%. When those household contact were close relatives compared with contacts only living at the same residential address, secondary attack rates were 12.4% and 17.1% respectively. Risk of household infection was highest for those over 60 years old and lowest for those of less than 20 years old.

A study from researchers in Wuhan, China found the difference between secondary attack rate for children was 4% compared with 17% for adults. When index patients quarantined themselves from symptom onset, secondary attack rate was 0% compared with 17% when the index patient did not quarantine. Index cases that quarantined wore a face mask, dined separately and resided separate from others in the house. For spouses of index cases, secondary attack rate was 28%.

In England SARS-CoV-2 infection and outbreak rates were calculated for staff and students in early year, primary and secondary schools during June 2020, the first month after easing of the national lockdown. SARS-CoV-2 infections and outbreaks were uncommon across all educational settings. There was a strong correlation between number of outbreaks and regional incidence of COVID-19. Staff had an increased risk of infections compared to students and the majority of cases that were linked to outbreaks occurred in staff. The study demonstrated that control of community transmission is important to protect educational settings.

Using non dangerous tracer virus, bacteriophage MS-2, researchers found that contamination of a single doorknob or table top results in spread of viruses throughout office buildings, hotels and healthcare facilities. The tracer virus was detected on 40-60 percent of surfaces within 2 to 4 hours. Using quaternary ammonium compounds (QUATS) disinfectant containing wipes on fomites was an intervention that reduced detectible virus by 80% and the concentration of virus detected was reduced by 99%.

Ventilation is known to reduce the airborne transmission of pathogens by diluting them to lower concentrations. An increase in ventilation rate, which means an increase in the amount of clean outdoor air supplied to indoor volumes have been included in guidelines from The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), The Federation of European Heating, Ventilation and Air Conditioning associations (REHVA) and the National Health Commission of China. However many air-handling units, ducting and air supply are dimensioned to be economic and energy efficient and may not allow for large amount of outdoor air. Ventilation systems that are designed to supply large amounts of outdoor air during the pandemic may not operate economically under normal conditions and the costs may not be feasible in many cases. Air supply rate can be increased by installing portable units such as window installed fans and free-standing air handling units, filters, stand-alone air cleaners and ultraviolet germicidal irradiation units installed in rooms or in ducts.

In ventilated rooms short-distance exposure occurs when the distance between the exposed and infected person is less than 1-1.5 m and long-distance exposure is at a greater distance. Long-distance exposure mainly depends on room ventilation which includes ventilation rate and room airflow characteristics. Room airflow characteristics include air temperature, humidity, velocity and direction.

The risk of airborne cross-infection was evaluated by calculating the time-averaged intake fraction, which is the proportion of air mass exhaled by the infected person that is inhaled by the exposed person. The study found that all occupants should leave the room periodically and reduce room occupancy as much as possible. It was recommended that these strategies along with other control measures such as maximum clean air, distancing, face-to-back layout of workstations and reducing aerosol-generating activities like loud talking and singing should be applied to classrooms, offices and meeting rooms.

Complete mixing of expiratory airborne aerosols and room air was assumed and particle dynamics of airborne aerosols were not considered in the analysis. If the infected person were to stay in the room for a long time, the concentration of expired airborne aerosols would build up until reaching a steady state. When the infected person leaves the room, the concentration of aerosols begins to decay, but begin to build up again when the person re-enters the room. Intermittent source generation will result in lower time-averaged exposure to occupants compared with continuous exposure. Calculations showed that in a classroom scenario, if students left the room during breaks between lessons they reduced the risk of airborne cross-infection by 35% compared compared with staying in the room during breaks.

A study that assessed the dynamics of influenza droplet and aerosol transmission using wireless sensors to measure the location and close proximity of contacts in the population of a high school in the US. Simulations used empirical transmission levels observed in households. The study found that improvements in ventilation to recommended levels by American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) had a similar mitigating effect on viral transmission as vaccination covering 50-60 percent of the population. ASHRAE recommends 3 air changes per hour. The results were based on the assumption of substantial aerosol transmission

In a restaurant in Guangzhou where the air exchange rate was low (0.56-0.77 exchanges per hour) on January 24, 2020, 5 people sitting at adjacent tables to an infected person developed COVID-19. The restaurant had 21 people in an area of 45 m². The volume of clean air brought in by the ventilation system per person was estimated to be 48 L/min.

Elevators have different ventilation requirements. Washington state requires ventilation at 1 cubic foot/minute per square foot (1 foot/minute). With 1.5 feet per person in an elevator cab, a person would have about 42 L/minute of clean air, whereas a general recommended air exchange rate is 480 liters per person per minute. Computer modelling suggested that elevator cabin air could remain infectious after the infector has exited. After an infected passenger exits and a second passenger enters, they could be exposed to 25% of the viral particles that the infected passenger expelled. The CDC recommended that elevator riders should wear masks and avoid speaking.

A mathematical model for COVID-19 transmission by aerosols was applied to known outbreaks and used to present quantitative guidelines for ventilation and occupancy in the workplace. The model assumes hand-hygiene protocols are followed, that surface contamination is not the dominant transmission route and that contagious individuals are wearing face coverings to catch large droplets. In this scenario aerosols are the dominant transmission mechanism. The analysis, yet to be peer reviewed as of August 2020, lead the author to propose the following guidelines: 1) To mitigate the affects of aerosol build-up in closed spaces, ventilation and short exposure times. 2) Recirculation in HVAC systems should be avoided or use high quality filters. 3) Poorly ventilated common spaces such as bathrooms, elevators and stairwells should have increased airflow or local air scrubbers. 4) Mask use can prevent direct exposure when less than 2 m space cannot be maintained but will not prevent infection in an enclosed space regardless of the distance between occupants. The author suggested that airflow in shared spaces be at 50 m³/min ∼2000 CFM per occupant beyond the first in early phased of epidemic decay.

A literature review evaluated the COVID-19 risk associated with air-conditioning systems using literature on outbreaks of coronaviruses SARS-CoV-1, MERS-CoV and SARS-CoV-2 in indoor environments. 14 studies were used, seven involved SARS-CoV-1, six involved SARS-CoV-2 and one involved MERS-CoV. The MERS-CoV study demonstrated that the HVAC system was contaminated by viral particles. Six of the seven SARS-CoV-1 studies suspected that air-conditioning systems played a role in infection spread. In four out of six SARS-CoV-2 studies, diffusion of viral particles through HVAC was suspected or supported through computer simulation. In two of the studies transmission of SARS-CoV-2 through HVAC was excluded based on epidemiological data.

Ventilation systems have reported to be capable of transmitting or spreading virus for measles, chickenpox, flu, smallpox and the 2009 influenza A (H1N1) pandemic. Deactivation of the air recirculation mode in indoor environments has been widely recommended for controlling SARS-CoV-2, but whether this approach is possible, technically and due to cost, in all workplaces has been questioned. It is recommended that air intake systems are designed to avoid air currents in the respiratory area of occupants of indoor spaces. Other recommendations are to increase filtering efficiency of HVAC systems using nanomaterials and estimate the probability that an infectious person is inside the building.

Indoor temperature and humidity could be maintained at levels that are less favorable for transmission of SARS-CoV-2. Cold temperatures are postulated to increase viral half-lives and low relative humidity hinders viral inactivation through natural processes. Regulatory bodies in the USA suggest that in winter that indoor temperatures be maintained between 20-40 C and relative humidity be maintained between 20-60%. Relative humidity over 60% relative humidity significantly increases the likelihood of mold growth.

Relative humidity can cause fewer droplets to be inhaled due to these conditions causing a slower evaporation from large droplets to small droplets. In the host, too low or too high relative humidity affects the nasal mucosa, mucous viscosity and mucociliary activity. Extremely low humidity has been associated with enabling virus settlement in human hosts and may allow greater penetration of foreign particles. To avoid dry eyes and dry nasal passages relative humidity is recommended to be greater than 30 % and 10% respectively. Viral stability is also effected by humidity. Whereas relative humidity measures how close the air is to saturation, absolute humidity is a fixed measure of water vapor content in the air. Survival and transmission potentials of influenza viruses in winter are inversely associated with absolute humidity rather than relative humidity. Under conditions of high humidity, low temperature can stabilize and protect viruses by stabilizing the lipid-containing envelope. Cold temperatures and low relative humidity have been associated with survival and transmission of some influenza viruses and an increased occurrence of respiratory tract infections.

An Italian study found that optimum thermodynamic conditions for moist air to reduce exposure risk of COVID-19 in indoor spaces would be typical summertime conditions at 15-26 C and 50% relative humidity. For the winter researchers advise that absolute humidity of the supply air should be kept higher than usual, which would imply a larger humidification in winter and a smaller dehumidification in summer. It was also recommended that indoor temperatures should be kept higher than usual. A relationship between the survival of coronaviruses and thermodynamic potential specific enthalpy of moist air was inferred. The use of a the parameter of specific enthalpy 55 kJ/kg-dry air could be used to satisfy requirements for SARS-CoV-2 inactivation and the hygrothermal comfort of people using the space.

The application of control banding, which is a qualitative method for determining the degree of risk for occupations and job tasks, has been proposed for aerosol-transmissible diseases including COVID-19. SARS-CoV-2 would be categorized as a Risk Group 3 organism as has been determined by the NIH Office of Science Policy. An employee’s exposure depends on the two variables which are air concentration of virus and length of time in contact with that concentration. The authors assume that a short exposure to a high aerosol concentration will be as likely as a longer exposure to a lower concentration. Level of exposure is determined by the likelihood that the employee will have person-to-person interactions (unlikely, possible or likely) with potentially infected people and the duration of time they are exposed (0-3 hours, 3-6 hours, >6 hours). Exposure level E1, E2 or E3 is combined with the organism risk ranking, which is R3 for COVID-19. Control band levels A, B and C are then determined. Control band B would be jobs where exposures unlikely but risk is severe or where exposures are possible or likely and the risk is moderate. Using a hierarchy where first source of infection is controlled, then pathway, then the receptor (wearing personal protective equipment), a scenario is proposed where most workers will not be required to wear personal protective equipment. Source controls are those that limit the number of sources of infectious aerosol, like conducting business by phone or employee health screening. Pathway controls include barriers, diluting the air and distancing.

Mathematical modelling was used to investigate virus exposure levels associated with one-way and two-way pedestrian traffic patterns within academic buildings. Two assumptions used were that risk of infection is a product of exposure and time an that exposure rate decreases with distance. That a small exposure to a large number of people is similar to a large exposure to a few people is an underlying assumption used to minimize exposure risk. The study highlighted that restricting foot traffic to one direction reduces exposure per unit time, it can increase the total exposure time in the hallway.

In England SARS-CoV-2 infection and outbreak rates were calculated for staff and students in early year, primary and secondary schools during June 2020, the first month after easing of the national lockdown. SARS-CoV-2 infections and outbreaks were uncommon across all educational settings. There was a strong correlation between number of outbreaks and regional incidence of COVID-19. Staff had an increased risk of infections compared to students and the majority of cases that were linked to outbreaks occurred in staff. The study demonstrated that control of community transmission is important to protect educational settings.

Using non dangerous tracer virus, bacteriophage MS-2, researchers found that contamination of a single doorknob or table top results in spread of viruses throughout office buildings, hotels and healthcare facilities. The tracer virus was detected on 40-60 percent of surfaces within 2 to 4 hours. Using quaternary ammonium compounds (QUATS) disinfectant containing wipes on fomites was an intervention that reduced detectible virus by 80% and the concentration of virus detected was reduced by 99%.

Ventilation is known to reduce the airborne transmission of pathogens by diluting them to lower concentrations. An increase in ventilation rate, which means an increase in the amount of clean outdoor air supplied to indoor volumes have been included in guidelines from The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), The Federation of European Heating, Ventilation and Air Conditioning associations (REHVA) and the National Health Commission of China. However many air-handling units, ducting and air supply are dimensioned to be economic and energy efficient and may not allow for large amount of outdoor air. Ventilation systems that are designed to supply large amounts of outdoor air during the pandemic may not operate economically under normal conditions and the costs may not be feasible in many cases. Air supply rate can be increased by installing portable units such as window installed fans and free-standing air handling units, filters, stand-alone air cleaners and ultraviolet germicidal irradiation units installed in rooms or in ducts.

In ventilated rooms short-distance exposure occurs when the distance between the exposed and infected person is less than 1-1.5 m and long-distance exposure is at a greater distance. Long-distance exposure mainly depends on room ventilation which includes ventilation rate and room airflow characteristics. Room airflow characteristics include air temperature, humidity, velocity and direction.

The risk of airborne cross-infection was evaluated by calculating the time-averaged intake fraction, which is the proportion of air mass exhaled by the infected person that is inhaled by the exposed person. The study found that all occupants should leave the room periodically and reduce room occupancy as much as possible. It was recommended that these strategies along with other control measures such as maximum clean air, distancing, face-to-back layout of workstations and reducing aerosol-generating activities like loud talking and singing should be applied to classrooms, offices and meeting rooms.

Complete mixing of expiratory airborne aerosols and room air was assumed and particle dynamics of airborne aerosols were not considered in the analysis. If the infected person were to stay in the room for a long time, the concentration of expired airborne aerosols would build up until reaching a steady state. When the infected person leaves the room, the concentration of aerosols begins to decay, but begin to build up again when the person re-enters the room. Intermittent source generation will result in lower time-averaged exposure to occupants compared with continuous exposure. Calculations showed that in a classroom scenario, if students left the room during breaks between lessons they reduced the risk of airborne cross-infection by 35% compared compared with staying in the room during breaks.

A study that assessed the dynamics of influenza droplet and aerosol transmission using wireless sensors to measure the location and close proximity of contacts in the population of a high school in the US. Simulations used empirical transmission levels observed in households. The study found that improvements in ventilation to recommended levels by American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) had a similar mitigating effect on viral transmission as vaccination covering 50-60 percent of the population. ASHRAE recommends 3 air changes per hour. The results were based on the assumption of substantial aerosol transmission

In a restaurant in Guangzhou where the air exchange rate was low (0.56-0.77 exchanges per hour) on January 24, 2020, 5 people sitting at adjacent tables to an infected person developed COVID-19. The restaurant had 21 people in an area of 45 m². The volume of clean air brought in by the ventilation system per person was estimated to be 48 L/min.

Elevators have different ventilation requirements. Washington state requires ventilation at 1 cubic foot/minute per square foot (1 foot/minute). With 1.5 feet per person in an elevator cab, a person would have about 42 L/minute of clean air, whereas a general recommended air exchange rate is 480 liters per person per minute. Computer modelling suggested that elevator cabin air could remain infectious after the infector has exited. After an infected passenger exits and a second passenger enters, they could be exposed to 25% of the viral particles that the infected passenger expelled. The CDC recommended that elevator riders should wear masks and avoid speaking.

A mathematical model for COVID-19 transmission by aerosols was applied to known outbreaks and used to present quantitative guidelines for ventilation and occupancy in the workplace. The model assumes hand-hygiene protocols are followed, that surface contamination is not the dominant transmission route and that contagious individuals are wearing face coverings to catch large droplets. In this scenario aerosols are the dominant transmission mechanism. The analysis, yet to be peer reviewed as of August 2020, lead the author to propose the following guidelines: 1) To mitigate the affects of aerosol build-up in closed spaces, ventilation and short exposure times. 2) Recirculation in HVAC systems should be avoided or use high quality filters. 3) Poorly ventilated common spaces such as bathrooms, elevators and stairwells should have increased airflow or local air scrubbers. 4) Mask use can prevent direct exposure when less than 2 m space cannot be maintained but will not prevent infection in an enclosed space regardless of the distance between occupants. The author suggested that airflow in shared spaces be at 50 m³/min ∼2000 CFM per occupant beyond the first in early phased of epidemic decay.

A literature review evaluated the COVID-19 risk associated with air-conditioning systems using literature on outbreaks of coronaviruses SARS-CoV-1, MERS-CoV and SARS-CoV-2 in indoor environments. 14 studies were used, seven involved SARS-CoV-1, six involved SARS-CoV-2 and one involved MERS-CoV. The MERS-CoV study demonstrated that the HVAC system was contaminated by viral particles. Six of the seven SARS-CoV-1 studies suspected that air-conditioning systems played a role in infection spread. In four out of six SARS-CoV-2 studies, diffusion of viral particles through HVAC was suspected or supported through computer simulation. In two of the studies transmission of SARS-CoV-2 through HVAC was excluded based on epidemiological data.

Ventilation systems have reported to be capable of transmitting or spreading virus for measles, chickenpox, flu, smallpox and the 2009 influenza A (H1N1) pandemic. Deactivation of the air recirculation mode in indoor environments has been widely recommended for controlling SARS-CoV-2, but whether this approach is possible, technically and due to cost, in all workplaces has been questioned. It is recommended that air intake systems are designed to avoid air currents in the respiratory area of occupants of indoor spaces. Other recommendations are to increase filtering efficiency of HVAC systems using nanomaterials and estimate the probability that an infectious person is inside the building.

Indoor temperature and humidity could be maintained at levels that are less favorable for transmission of SARS-CoV-2. Cold temperatures are postulated to increase viral half-lives and low relative humidity hinders viral inactivation through natural processes. Regulatory bodies in the USA suggest that in winter that indoor temperatures be maintained between 20-40 C and relative humidity be maintained between 20-60%. Relative humidity over 60% relative humidity significantly increases the likelihood of mold growth.

Relative humidity can cause fewer droplets to be inhaled due to these conditions causing a slower evaporation from large droplets to small droplets. In the host, too low or too high relative humidity affects the nasal mucosa, mucous viscosity and mucociliary activity. Extremely low humidity has been associated with enabling virus settlement in human hosts and may allow greater penetration of foreign particles. To avoid dry eyes and dry nasal passages relative humidity is recommended to be greater than 30 % and 10% respectively. Viral stability is also effected by humidity. Whereas relative humidity measures how close the air is to saturation, absolute humidity is a fixed measure of water vapor content in the air. Survival and transmission potentials of influenza viruses in winter are inversely associated with absolute humidity rather than relative humidity. Under conditions of high humidity, low temperature can stabilize and protect viruses by stabilizing the lipid-containing envelope. Cold temperatures and low relative humidity have been associated with survival and transmission of some influenza viruses and an increased occurrence of respiratory tract infections.

An Italian study found that optimum thermodynamic conditions for moist air to reduce exposure risk of COVID-19 in indoor spaces would be typical summertime conditions at 15-26 C and 50% relative humidity. For the winter researchers advise that absolute humidity of the supply air should be kept higher than usual, which would imply a larger humidification in winter and a smaller dehumidification in summer. It was also recommended that indoor temperatures should be kept higher than usual. A relationship between the survival of coronaviruses and thermodynamic potential specific enthalpy of moist air was inferred. The use of a the parameter of specific enthalpy 55 kJ/kg-dry air could be used to satisfy requirements for SARS-CoV-2 inactivation and the hygrothermal comfort of people using the space.

The application of control banding, which is a qualitative method for determining the degree of risk for occupations and job tasks, has been proposed for aerosol-transmissible diseases including COVID-19. SARS-CoV-2 would be categorized as a Risk Group 3 organism as has been determined by the NIH Office of Science Policy. An employee’s exposure depends on the two variables which are air concentration of virus and length of time in contact with that concentration. The authors assume that a short exposure to a high aerosol concentration will be as likely as a longer exposure to a lower concentration. Level of exposure is determined by the likelihood that the employee will have person-to-person interactions (unlikely, possible or likely) with potentially infected people and the duration of time they are exposed (0-3 hours, 3-6 hours, >6 hours). Exposure level E1, E2 or E3 is combined with the organism risk ranking, which is R3 for COVID-19. Control band levels A, B and C are then determined. Control band B would be jobs where exposures unlikely but risk is severe or where exposures are possible or likely and the risk is moderate. Using a hierarchy where first source of infection is controlled, then pathway, then the receptor (wearing personal protective equipment), a scenario is proposed where most workers will not be required to wear personal protective equipment. Source controls are those that limit the number of sources of infectious aerosol, like conducting business by phone or employee health screening. Pathway controls include barriers, diluting the air and distancing.

Mathematical modelling was used to investigate virus exposure levels associated with one-way and two-way pedestrian traffic patterns within academic buildings. Two assumptions used were that risk of infection is a product of exposure and time an that exposure rate decreases with distance. That a small exposure to a large number of people is similar to a large exposure to a few people is an underlying assumption used to minimize exposure risk. The study highlighted that restricting foot traffic to one direction reduces exposure per unit time, it can increase the total exposure time in the hallway.

In EnglandEngland SARS-CoV-2 infection and outbreak rates were calculated for staff and students in early year, primary and secondary schools during June 2020, the first month after easing of the national lockdown. SARS-CoV-2 infections and outbreaks were uncommon across all educational settings. There was a strong correlation between number of outbreaks and regional incidence of COVID-19. Staff had an increased risk of infections compared to students and the majority of cases that were linked to outbreaks occurred in staff. The study demonstrated that control of community transmission is important to protect educational settings.

A mathematical model for COVID-19 transmission by aerosols was applied to known outbreaks and used to present quantitative guidelines for ventilation and occupancy in the workplace. The model assumes hand-hygiene protocols are followed, that surface contamination is not the dominant transmission route and that contagious individuals are wearing face coverings to catch large droplets. In this scenario aerosols are the dominant transmission mechanism. The analysis, yet to be peer reviewed as of August 2020, lead the author to propose the following guidelines: 1) To mitigate the affects of aerosol build-up in closed spaces, ventilation and short exposure times. 2) Recirculation in HVAC systems should be avoided or use high quality filters. 3) Poorly ventilated common spaces such as bathrooms, elevators and stairwells should have increased airflow or local air scrubbers. 4) Mask use can prevent direct exposure when less than 2 m space cannot be maintained but will not prevent infection in an enclosed space regardless of the distance between occupants. The author suggested that airflow in shared spaces be at 50 m³/min ∼2000 CFM per occupant beyond the first in early phased of epidemic decay.

A mathematical model for COVID-19 transmission by aerosols was applied to known outbreaks and used to present quantitative guidelines for ventilation and occupancy in the workplace. The model assumes hand-hygiene protocols are followed, that surface contamination is not the dominant transmission route and that contagious individuals are wearing face coverings to catch large droplets. In this scenario aerosols are the dominant transmission mechanism. The analysis, yet to be peer reviewed as of August 2020, lead author to propose the following guidelines: 1) To mitigate the affects of aerosol build-up in closed spaces, ventilation and short exposure times. 2) Recirculation in HVAC systems should be avoided or use high quality filters. 3) Poorly ventilated common spaces such as bathrooms, elevators and stairwells should have increased airflow or local air scrubbers. 4) Mask use can prevent direct exposure when less than 2 m space cannot be maintained but will not prevent infection in an enclosed space regardless of the distance between occupants. The author suggested that airflow in shared spaces be at 50 m³/min ∼2000 CFM per occupant beyond the first in early phased of epidemic decay.

Indoor temperature and humidity could be maintained at levels that are less favorable for transmission of SARS-CoV-2. Cold temperatures are postulated to increase viral half-lives and low relative humidity hinders viral inactivation through natural processes. Regulatory bodies in the USA suggest that in winter that indoor temperatures be maintained between 20-40 C and relative humidity be maintained between 20-60%. Relative humidity over 60% relative humidity significantly increases the likelihood of mold growth.

Mathematical modelling was used to investigate virus exposure levels associated with one-way and two-way pedestrian traffic patterns within academic buildings. Two assumptions used were that risk of infection is a product of exposure and time an that exposure rate decreases with distance. That a small exposure to a large number of people is similar to a large exposure to a few people is an underlying assumption used to minimize exposure risk. The study highlighted that restricting foot traffic to one direction reduces exposure per unit time, it can increase the total exposure time in the hallway.

In England SARS-CoV-2 infection and outbreak rates were calculated for staff and students in early year, primary and secondary schools during June 2020, the first month after easing of the national lockdown. SARS-CoV-2 infections and outbreaks were uncommon across all educational settings. There was a strong correlation between number of outbreaks and regional incidence of COVID-19. Staff had an increased risk of infections compared to students and the majority of cases that were linked to outbreaks occurred in staff. The study demonstrated that control of community transmission is important to protect educational settings.

Relative humidity can cause fewer droplets to be inhaled due to these conditions causing a slower evaporation from large droplets to small droplets. In the host, too low or too high relative humidity affects the nasal mucosa, mucous viscosity and mucociliary activity. Extremely low humidity has been associated with enabling virus settlement in human hosts and may allow greater penetration of foreign particles. To avoid dry eyes and dry nasal passages relative humidity is recommended to be greater than 30 % and 10% respectively. Viral stability is also effected by humidity. Whereas relative humidity measures how close the air is to saturation, absolute humidity is a fixed measure of water vapor content in the air. Survival and transmission potentials of influenza viruses in winter are inversely associated with absolute humidity rather than relative humidity. Under conditions of high humidity, low temperature can stabilize and protect viruses by stabilizing the lipid-containing envelope. Cold temperatures and low relative humidity have been associated with survival and transmission of some influenza viruses and an increased occurrence of respiratory tract infections.

Under conditions of high humidity, low temperature can stabilize and protect viruses by stabilizing the lipid-containing envelope. Cold temperatures and low relative humidity have been associated with survival and transmission of some influenza viruses and an increased occurrence of respiratory tract infections.

The application of control banding, which is a qualitative method for determining the degree of risk for occupations and job tasks, has been proposed for aerosol-transmissible diseases including COVID-19. SARS-CoV-2 would be categorized as a Risk Group 3 organism as has been determined by the NIH Office of Science Policy. An employee’s exposure depends on the two variables which are air concentration of virus and length of time in contact with that concentration. The authors assume that a short exposure to a high aerosol concentration will be as likely as a longer exposure to a lower concentration. Level of exposure is determined by the likelihood that the employee will have person-to-person interactions (unlikely, possible or likely) with potentially infected people and the duration of time they are exposed (0-3 hours, 3-6 hours, >6 hours). Exposure level E1, E2 or E3 is combined with the organism risk ranking, which is R3 for COVID-19. Control band levels A, B and C are then determined. Control band B would be jobs where exposures unlikely but risk is severe or where exposures are possible or likely and the risk is moderate. Using a hierarchy where first source of infection is controlled, then pathway, then the receptor (wearing personal protective equipment), a scenario is proposed where most workers will not be required to wear personal protective equipment. Source controls are those that limit the number of sources of infectious aerosol, like conducting business by phone or employee health screening. Pathway controls include barriers, diluting the air and distancing.

Using non dangerous tracer virus, bacteriophage MS-2, researchers found that contamination of a single doorknob or table top results in spread of viruses throughout office buildings, hotels and healthcare facilities. The tracer virus was detected on 40-60 percent of surfaces within 2 to 4 hours. Using quaternary ammonium compounds (QUATS) disinfectant containing wipes on fomites was an intervention that reduced detectible virus by 80% and the concentration of virus detected was reduced by 99%.

Ventilation is known to reduce the airborne transmission of pathogens by diluting them to lower concentrations. An increase in ventilation rate, which means an increase in the amount of clean outdoor air supplied to indoor volumes have been included in guidelines from The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), The Federation of European Heating, Ventilation and Air Conditioning associations (REHVA) and the National Health Commission of China. However many air-handling units, ducting and air supply are dimensioned to be economic and energy efficient and may not allow for large amount of outdoor air. Ventilation systems that are designed to supply large amounts of outdoor air during the pandemic may not operate economically under normal conditions and the costs may not be feasible in many cases. Air supply rate can be increased by installing portable units such as window installed fans and free-standing air handling units, filters, stand-alone air cleaners and ultraviolet germicidal irradiation units installed in rooms or in ducts.

In ventilated rooms short-distance exposure occurs when the distance between the exposed and infected person is less than 1-1.5 m and long-distance exposure is at a greater distance. Long-distance exposure mainly depends on room ventilation which includes ventilation rate and room airflow characteristics. Room airflow characteristics include air temperature, humidity, velocity and direction.

The risk of airborne cross-infection was evaluated by calculating the time-averaged intake fraction, which is the proportion of air mass exhaled by the infected person that is inhaled by the exposed person. The study found that all occupants should leave the room periodically and reduce room occupancy as much as possible. It was recommended that these strategies along with other control measures such as maximum clean air, distancing, face-to-back layout of workstations and reducing aerosol-generating activities like loud talking and singing should be applied to classrooms, offices and meeting rooms.

Complete mixing of expiratory airborne aerosols and room air was assumed and particle dynamics of airborne aerosols were not considered in the analysis. If the infected person were to stay in the room for a long time, the concentration of expired airborne aerosols would build up until reaching a steady state. When the infected person leaves the room, the concentration of aerosols begins to decay, but begin to build up again when the person re-enters the room. Intermittent source generation will result in lower time-averaged exposure to occupants compared with continuous exposure. Calculations showed that in a classroom scenario, if students left the room during breaks between lessons they reduced the risk of airborne cross-infection by 35% compared compared with staying in the room during breaks.

A study that assessed the dynamics of influenza droplet and aerosol transmission using wireless sensors to measure the location and close proximity of contacts in the population of a high school in the US. Simulations used empirical transmission levels observed in households. The study found that improvements in ventilation to recommended levels by American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) had a similar mitigating effect on viral transmission as vaccination covering 50-60 percent of the population. ASHRAE recommends 3 air changes per hour. The results were based on the assumption of substantial aerosol transmission

Using non dangerous tracer virus, bacteriophage MS-2, researchers found that contamination of a single doorknob or table top results in spread of viruses throughout office buildings, hotels and healthcare facilities. The tracer virus was detected on 40-60 percent of surfaces within 2 to 4 hours. Using quaternary ammonium compounds (QUATS) disinfectant containing wipes on fomites was an intervention that reduced detectible virus by 80% and the concentration of virus detected was reduced by 99%.

A study that assessed the dynamics of influenza droplet and aerosol transmission using wireless sensors to measure the location and close proximity of contacts in the population of a high school in the US. Simulations used empirical transmission levels observed in households. The study found that improvements in ventilation to recommended levels by American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) had a similar mitigating effect on viral transmission as vaccination covering 50-60 percent of the population. ASHRAE recommends 3 air changes per hour. The results were based on the assumption of substantial aerosol transmission