COVID-19 & air disinfection
Nowadays, we are facing one of the most challenging times in our modern history. The new coronavirus, believed to emerge at the end of last year, has changed the way we live from all aspects.
Experts say that our life will hardly go back to normal. Scientists and policy-makers around the world are striving to define what new normal would be. Some have proposed plans to ‘make peace’ with this coronavirus by easing the already-implemented lockdown regulation. One main problem is how we can establish the new normal while we are still waiting for the vaccine.
One of the solutions is to implement air disinfection inside our buildings. Why buildings? That is because indoor environments are prone to create an outbreak since aerosols are easily transmitted in such environments. Aerosols here are those tiny droplets filled with pathogens showered into the air when an infected person coughs, sneezes, or even simply speaks.
Even though we are still far from fully understanding the SARS-CoV-2 virus responsible for COVID-19, we commonly know it is spread through droplets.
However, an unpublished study by Fears et al. shows that SARS-CoV-2 is able to maintain its infectivity when suspended in the aerosol form for up to 16 hours. There are fears that these aerosols can be breathed in by healthy people in the same room with the infected person. Swab tests taken from the exhaust outlets in a patient room in Singapore shows a positive result. It suggests that airflows may displace coronavirus-laden droplets.
With all recent studies suggesting that this coronavirus may linger in the air for extended periods of time, the need to disinfect the air is crucial.
This idea of air disinfection reminds us of the powerful method we have long discovered in the past. And that is UV light.
Not all UV light can be used for air disinfection.
Not all UV light has the ability to inactivate germs. Most studies reveal that the most effective UV for disinfection is UV-C. Scientists have long studied the germ-killing potential of UV-C since the 19th century. Dating back to 1903, Niels Finsen, a Faroese scientist, was even awarded a Nobel Prize for Medicine for his work in using UV-C for phototherapy against lupus and tuberculosis.
Commonly, we call UV-C as germicidal UV (GUV) for this unique characteristic. UV-C is any UV light with the wavelength from 200 nm to 254 nm. We cannot get UV-C radiation from the sun as it is absorbed almost entirely by the ozone layer.
GUV has been used by many health care facilities to disinfect the air in rooms for the last 70 years as its pathogen-killing rate can reach more than 99.9%. This means of disinfection is familiarly called as UV germicidal irradiation (UVGI). It helps reduce the spread of airborne pathogens, such as measles, tuberculosis, and probably COVID-19.
The most effective and safest UVGI method
Today, people are mostly using UVGI fixtures in portable air cleaners/robots to disinfect the air. Unfortunately, this method is relatively less effective in combating airborne diseases. When we need to disinfect the air, we should make sure that we beam the air particles with the UVGI as much as possible. Portable air cleaners only have 1–2 ACH. ACH defines the rate of airflow added to or removed from the room measured in room volumes per hour.
On TV, we have seen that in many hospitals across the world, portable UVGI units are deployed in the patient rooms. This method is surely effective at destroying the germs, but only when the rooms are unoccupied. In reality, to kill pathogens in the air effectively, the disinfection should take place when the transmission is ongoing. Also, it is almost impossible to regularly empty some rooms, such as 24-hour airport lounges, medical waiting rooms, stores, restaurants, and other public spaces.
Lately, a recently published IES report suggests that upper-room UVGI is the most effective way of air disinfection in a room with around 6–12 ACH. Upper-room UVGI is safe to implement as we install UVGI fixtures overhead. IES suggested that 2.1 m should be the minimum height from the floor to the bottom of the fixture. With this height, UVGI will not harm our bodies as we will receive very little exposure. In a study shown in this IES report, the healthcare workers and patients in an upper GUV installation received no more than 1/3 of the current UV-C daily safety limit.
Even though accidental exposure to UV-C can create a mild, temporary sunburn effect on our skin, it is deemed safer than UV-A or UV-B in relation to the risk of skin cancer. UV-C has shorter wavelengths. It means that UV-C will be absorbed by the protein in the outer layer of dead skin cells. Therefore, it is less able to penetrate any living cells.
What research says
Early successes in developing the upper-room UVGI system can be traced back to the 20th century. In 1941, William F. Wells conducted a study to evaluate the effectiveness of this system in preventing the spread of measles among students in Philadelphia day schools. Results showed that 53.6% of susceptibles in schools without upper-room UVGI were infected, while only 13.3% of susceptibles were infected in schools with upper-room UVGI. This corresponds to the measure taken by McLean during the 1957-1958 flu pandemic to prevent the spread of this disease in Veterans Hospital TB. The effort showed a successful outcome as the infection rate was only 1.9% in an irradiated ward compared to an 18.9% infection rate in a non-irradiated ward.
In 2009, the Centers for Disease Control and Prevention (CDC) published the first basic guidelines on the parameters necessary to implement an upper-room UVGI system for healthcare settings. This guide produced after a satisfying result from the research conducted by the University of Colorado back in 1997 to evaluate the ability of upper-room UVGI systems in containing the spread of tuberculosis.
Use proper design to increase effectiveness.
Different types of ventilation systems will definitely affect the effectiveness of upper-room UVGI you want to employ. A proper design must ensure that the air circulation in the room reaches the optimum air change rate and promote air mixing while maintaining the indoor comfort condition. Technically, UVGI should disinfect the air as much as possible.
Another significant parameter is the UVGI dose delivered to the microorganisms, which is dependant on the UV irradiance and exposure time. The key difference between surface and air disinfection lies on this length of exposure time. Due to the rapid movement of air circulated within the room, the irradiance should be sufficiently high to achieve the appropriate amount of UVGI dose.
Other parameters include room configuration, UVGI fixture placement, and air flow’s adequacy in bringing contaminated air into the upper room.