Did you know: the science behind COVID-19 environmental mitigation measures
Scientific studies by researchers in Singapore on the efficacy of COVID-19 mitigation measures
A*STAR has worked together with the local research ecosystem , including Temasek Foundation, Temasek Life Sciences Laboratory, Institutes of Higher Learning (IHLs) and various public agencies, on scientific studies that examine the efficacy of various COVID-19 environmental mitigation measures. These include ventilation in spaces, air filters, air ionisers, table-top dividers, and UVC lights, among others.
On top of current safe management measures such as wearing of masks, maintaining safe distancing, limiting social contacts and observing good personal hygiene, these additional solutions will help support a safer resumption of activities in Phase 3 as the nation continues to reopen in phases.
Businesses, organisations, as well as families and individuals can explore such insights and customised solutions that best fit their needs and circumstances at work, at play or at home, to supplement the current safe management measures.
Overview of preventive measures to reduce fomites, droplet and aerosol transmission
Some of these findings include:
1. Ventilation in spaces to reduce droplet and aerosol transmission
In an earlier study, A*STAR researchers, together with SingHealth doctors from the SGH Department of Infectious Diseases; and the Department of Respiratory and Critical Care Medicine, have demonstrated the importance of wearing a mask.
A*STAR researchers have since developed a computational fluid dynamics framework which accurately models the spread of droplets when a person with COVID-19 coughs in Singapore's tropical environment.
Droplet dispersion from a single cough for two persons spaced 1 m apart.
This is the first study that accurately models the characteristics of large cough droplets which evaporate partially, leaving behind smaller droplets which can then be carried even further by the wind. The findings revealed that a 100 micrometre (µm) cough droplet can travel up to 6.6m at a wind speed of 2m/s, if the person who coughs is not wearing a mask.
The simulations done by the team at A*STAR’s Institute of High Performance Computing (IHPC), take into account factors found in Singapore’s climate, such as wind, temperature and humidity levels. The findings showed the spread of droplets and aerosols are greatly dependent on environmental conditions, hence there are different risk levels for different venues, which require different mitigation measures to ensure events can take place safely.
IHPC’s simulation and modelling of cough droplets in an indoor theatre.
The simulations derived through the crunching of multiple variables and complex equations using resources at the National Supercomputing Centre, Singapore (NSCC), produced highly accurate results that depict the paths of droplets, to help advise the design of safe management measures. Read more in this scientific paper.
"Our research has shown scientific proof that it is very important to wear a mask, practise good personal hygiene and social distancing, and ensure the environment is well ventilated, as all these things help to collectively lower the risk of transmission." said Dr Lim Keng Hui, Executive Director of A*STAR’s Institute of High Performance Computing.
IMRE’s experimental setup to study the flow of droplets and aerosols.
Apart from the computer simulations, there were also physical studies designed by scientists from A*STAR’s Institute of Materials Research and Engineering (IMRE). The team used aerosol generators, laser beams and sensitive high-speed cameras to study the physical effects of airflow and the spread of droplets in real life.
"We have been working with the public agencies, the business owners, as well as venue operators to come up with science-based recommendations which will enhance mitigation measures and help reduce the cases of transmission," said Prof Loh Xian Jun, Executive Director of A*STAR’s Institute of Materials Research and Engineering.
Scientists from IMRE analysed the risk levels of different exhibition booth designs in experiments which simulated the movement of droplets and aerosols. A booth design with full plexi-glass barrier is shown here.
The combination of IHPC’s computational modelling expertise and IMRE’s experimental design and particle sensing capabilities allowed the teams to cross-validate the findings, and quantify potential aerosol exposure levels in different settings and social distancing, such as in public spaces.
Watch “Understanding the world of the invisible – How do droplets travel?”
A*STAR’s airflow modelling and simulation studies also showed that the spread of droplets and aerosols are greatly dependent on different types of ventilation environments, hence there are various risk levels associated with different venues which require different preventive measures to ensure safe management practices.
For instance, air curtains in certain venues are effective in preventing aerosols from spreading, while split air-conditioners could recirculate unfiltered air, which under some conditions might allow aerosol contaminants to build up over time.
Based on these studies, customised innovations can be applied to complement safe management measures to reduce the spread of aerosols. The research has modelled various setting through a combination of on-site tests and advanced computational modelling studies, while factoring in environmental conditions such as wind speed and direction, humidity levels and ambient air temperatures, as well as ventilation levels in indoor spaces.
Read more in this Ventilation in Spaces Factsheet.
2. Air ionisers and air filters to help reduce aerosols from the air in enclosed or poorly ventilated indoor spaces
While masks are able to effectively block the spread of droplets when a person sneezes or coughs, they do not eliminate aerosols that may have escaped into the air. IMRE researchers have published a scientific paper that examined the effectiveness of plant-based ionisers and air filters, and found them to be very useful in reducing aerosol concentrations in the air, especially in poorly ventilated areas. While air filters are able to trap the aerosols floating in the air, plant and natural fibre ionisers will cause the aerosols and droplets to congregate and fall to surfaces such as the floor, on walls and surfaces. Hence, the use of these plant ionisers should be in conjunction with regular cleaning of surfaces, floors and walls to avoid the risk of fomite transmission.
In the study, the following solutions were found to be effective:
- Ioniser (Plant-based)
Potted plants (watered) fitted with electronic ioniser devices, to enable them to emit ions from their leaves. IMRE, together with Temasek Life Sciences Lab, found that a large ioniser plant is able to cover a wide area of up to 30m3.
- Ioniser (Natural Fibre)
Natural fibre ropes coiled into a ball of less than 20cm in diameter, perched on a stand containing a negative ion stimulator. Similar to plant-based ionisers, these generate ions which charge up aerosols and cause them to be deposited on the walls and floors as fomites, rather than remaining in the air.
- Air Filters
Layered antimicrobial filters can be wrapped around the back of fans or air-conditioners, to reduce concentration of aerosols of size less than 0.1 micrometers. A*STAR’s Singapore Institute of Manufacturing Technology (SIMTech) has developed an anti-microbial and water-repellent coating that can be applied on air filters, which demonstrated bacterial filtration efficiency (BFE) of 97% and above, and viral filtration efficacy (VFE) of over 95% and above, as coated. In the studies using the filters, IMRE showed that in a typical office meeting room with the size of about 117m3, the use of two fans fitted with air filters with anti-microbial coating can bring down the time needed to mitigate aerosols from 15 minutes to 10.5 minutes. This is equivalent to an ACH of 8, which effectively keeps aerosol concentration low even if there are constant injections of aerosols into the air (induced by repeated coughing or constant talking). As a general reference, the Center for Disease Control and Prevention (CDC) in the USA recommends a minimum ACH of 6 for patient-care areas including hospitals, and a minimum ACH of 10 for isolation wards1.
Comparison chart that shows the effectiveness of various types of ionisers in removing aerosols in the air.
Read more in this Air Filters and Air Ionisers Factsheet.
3. Reducing droplet transmission using table-top dividers
In addition to safe distancing measures, table-top dividers can be used to reduce the exchange of speech droplets and provide a safer environment when people remove their masks for meals. These dividers were tested in food court settings, in both air-conditioned and non-air-conditioned environments. To reduce the risk of fomite transmission, dividers should also be sanitised regularly, and the ventilation of the venue should be enhanced as well.
The research team at IMRE has tested the efficacy of using table-top dividers and found that U-shaped dividers that stand at a minimum height of 60cm were able to achieve optimal reduction of droplets spread in an air-conditioned setting with Central Air Conditioning systems. A minimum height of 70cm for U-shaped dividers were found to be the most effective for outdoor settings. Criss-crossed dividers were also effective, but dividers with bottom or audio gaps were found to be less effective.
Read more in Table Top Dividers Factsheet.
4. Disinfecting surfaces using UVC solutions
Irradiance mapping with transient safe, buffer and effective zones marked in this Hawker Center scene.
While UVC light is found to be effective for the disinfection of viruses, UVC poses potential health risks such as eye and skin injury. To further safeguard users from such risks, researchers used computational methods to identify safe boundaries without risking human intervention.
UVC has been traditionally used for disinfection purposes; however, there is minimal data guiding the use of different UVC devices against SARS-CoV-2 and other coronaviruses. The research team at A*STAR’s Singapore Bioimaging Consortium (SBIC) and SIMTech has ascertained the appropriate UV doses for viral inactivation without causing damage to the surface materials. In the study, the team has demonstrated that over 99.9% of coronaviruses is inactivated within 300 seconds under UVC exposure2 . IHPC has also leveraged its expertise in optical simulation to develop clearly defined safety zones and guidelines that will make it safer for humans to operate existing UVC solutions.
Using findings from the studies, UVC disinfection has been trialed in pilots with the aim of strengthening hygiene quality in public spaces. This includes installations at migrant worker dormitories and community care facilities to improve disinfection of common areas.
UVC solutions would require thoughtful implementation to ensure that risks to users are minimised because of the potential damage to eyes and skin.
Read more in this UVC Solutions Factsheet.
1 Source: Ventilation of Health Care Facilities, taken from ASHRAE Standard Committee (2018)”
2 Based on tests performed by A*STAR’s SBIC using 254 nm Mercury lamp and 277 nm UVC LED. Viruses tested include human coronaviruses 229e and OC43 for the surrogates.