The COVID-19 pandemic has engulfed the world and India is no exception. The COVID-19 virus belongs to the Corona family to which SARS and MERS also belong. As the COVID-19 virus is still not fully understood, the behavior of SARS and MERS is considered as a reference to the extent they do not contradict the present observations. The COVID-19 virus affects the respiratory track and starts with symptoms similar to the common flu. Given that the virus is a mutated strain, there is no vaccine available to fight against this contagion.
COVID-19 Transmission Routes
The size of a coronavirus particle is in the range of 80-160 nanometers. It is transferred via infected microscopic airborne particles and contaminated aerosol droplets. Droplets and small particles of a broad spectrum of diameters get generated during the course of coughing and sneezing and, to a lesser extent, even by talking and breathing. Most large cough droplets fall on nearby surfaces and objects – such as desks and tables, where they remain active for hours and even 2-3 days. The difficult part is that the infected carrier is asymptomatic in the incubation period varying from 7 to 15 days.
Small particles (less than 5 microns) released during cough stay airborne for hours and can be transported over long distances. Small droplet nuclei or residue are formed from droplets (usually within milliseconds) in the air, which shrink in size due to the process of evaporation and desiccation in low humidity.
It is known that most of the infectious particles ranging from 0.65 to 3.3μm in the cough-generated aerosols were immediately respirable. There is also no reported data or studies to rule out the possibility of the airborne-particle route. One indication for this: Corona virus SARS-CoV-2 has been isolated from swabs taken from exhaust vents in rooms occupied by infected patients.
Apart from cough generated aerosols, the particulates suspended in the air also represents a substrate for viruses and consequently their transmission through this path. In the indoor environment, one of the major sources of dust is atmospheric dust coming in through fresh air intakes which is approximately PM2.5 and PM10. The other prominent source is the dust generated by humans and processes. Reduction of indoor dust levels is a step towards mitigation of this source of COVID-19 transmission.
The faecal transmission route for SARS-CoV-2 infections is implicitly recognized by WHO. It is known that flushing toilets are creating plumes in the air containing droplets and droplet residue when toilets are flushed with open lids. This makes it important that with the use of exhaust fans toilet air should not come into the other occupied areas. If toilet seats are equipped with lids it is recommended to flush the toilets with closed lids in order to minimize the release of droplets and droplet residues from plumes in the air. Another path for transmission is when virus-laden aerosols are deposited on apparel and on the floor and then get re-suspended into the air due to the movement of people. That is why sanitizing the floors frequently and limiting the movement of people is helpful in curtailing this route of transmission. In healthcare facilities sanitization of apparel is recommended as per protocol.
EFFECT OF ENVIRONMENTAL CONDITIONS
Relative Humidity
Relative humidity is found to affect the infectivity (the ease with which infection can take place) of virus through the respiratory route. The normal human body has excellent protection systems to prevent respiratory infections. There are several layers of filters starting with the mouth and the windpipe. The moist surfaces having mucous layer collect the larger particles before they enter the trachea and pharynx in the upper respiratory tract. In the lower respiratory tract, the bronchi & alveoli can trap smaller particles to various degrees of efficiency. When we breathe dry air the mucous membrane in the lungs become dry. The fluid over lining the cells becomes more viscous, and the little hairs called cilia, which protect our lungs from deep settling of viable and non-viable particles, cannot work and particles settle more deeply in the lungs. If we consider oxygen and CO2 transport and the blood, it is only one cell membrane that separates the airspace from the blood. So, if something goes from our lungs into the blood, we get infected.
Moisture in the air is the first arm of our immune system and we now know that our body cannot fight off foreign particles or invaders as adequately as when we’re in a dry environment. Further, the infectivity of the bacteria too increases with low humidity. Relative humidity of at least 40% is considered the threshold. Studies indicate that 80% relative humidity and above tend to neutralize the COVID-19 virus. However, too much humidity leads to higher levels of dust mites and fungi, two of the worst culprits for indoor allergy sufferers. Mold and fungi are known to exacerbate respiratory conditions such as asthma.
All things considered, the relative humidity level of 40%~70% is considered to be the most suitable environment for humans and decreases problems from pathogens.
Temperature
Temperature tends to be a factor that directly affects the comfort of building or hospital occupants. Comfort temperature is generally considered between 24 – 30°C, after accounting for air velocity, relative humidity and clothing. The study of the transmission of COVID-19 virus in 100 cities of China indicates that high temperature and high humidity significantly reduce the transmission of influenza.
Studies conducted at various RH levels have shown that using viral culture methods low temperatures (7–8 °C) were optimal for airborne influenza survival, with virus survival decreasing progressively at moderate temperatures (20.5–24 °C) and further decreases at higher (greater than 30 °C) temperatures.
As per some recent studies, SARS-CoV-2 has been found highly stable on surfaces for 14 days at 4 °C; one day at 37 °C and 30 minutes at 56 °C were needed to inactivate the virus.
PREVENTIVE MEASURES FOR COMMERCIAL AND INDUSTRIAL FACILITIES
Commercial establishments and industrial facilities have multiple occupancy as well as transient visitors. It is this aspect that necessitates precaution in operating their air conditioning systems.
For the purpose of guidance for operation during a pandemic like COVID-19, air conditioning is categorized based on the types of indoor units installed: (These indoor units may be connected via refrigerant or chilled water pipes to DX outdoor units, VRF outdoor units or a chiller)
The best action to limit risk of COVID-19 infection by air is to ventilate indoor environments with outdoor air as much as possible. Mechanical ventilation systems and air conditioning systems, which provide ventilation, can perform this function more effectively than simply opening the windows, because they improve the quality of the outdoor air with filtration.
Categories of indoor units:
- Multiple Cassette Units: Ceiling mounted units that can each cool up to 50 sqm and can be controlled individually or as a group.
- Multiple Hi Wall Units: Used due to ease of installation and low Cost .
- Tower Units: For larger spaces, where most occupants are not stationary thus allowing for higher drafts.
- Ducted Units: A mini central Air conditioning system that is easy to operate.
- Fan Coil Units: Installed in meeting rooms, individual office spaces
- Air Handling Units: Can provide better ventilation, filtration and Coil disinfecting.
Operating guidelines for all categories
- Air filters must be kept clean as mentioned in the section – Operation & Maintenance.
- Provide adequate ventilation with fresh air and exhaust
- Inspect and clean the indoor unit coils, as mentioned in the section – Operation & Maintenance.
- Set room temperature between 24°C and 30°C. Maintain relative humidity between 40% and 70%. (In humid climates set temperature closer to 24°C for dehumidification and in dry climates closer to or at 30°C Use Fans to increase air movement)
- Heat Recovery Wheel (HRW): It is advisable to keep this wheel in off mode to reduce cross contamination. Upon restarting, the wheel must first be sanitized.
- Toilet and kitchen exhaust fans must be kept in operating mode.
Recommendations for category 1, 2 and 3 indoor units:
If fresh air is not provided, it is advisable to introduce a fresh air duct attached to a central inline fan filter unit and distribute the fresh air by grilles into the space or near the indoor units. For cassette units the fresh air duct may be connected to the available port of the Cassette Unit. In case fresh air cannot be provided through a fan it is recommended to actively use operable windows.
A minimum fresh air volume of 3 cum/hour per person and 3.75 cum per hour per sqm (5 cfm per person and 0.6 cfm per sq ft ) is recommended.
A separate Treated Fresh Air DX Unit may be provided in the case of a multiple unit installation. This will reduce the impact of reducing available cooling capacity by supplying non treated fresh air.
Recommendations for category 4, 5 and 6 indoor units:
Fresh air must be provided by an inlet duct and fan. It is advisable to provide a MERV 13 or higher filter fitted on the AHU. If a filter of higher filtering capability is retrofitted into an existing system, care shall be taken to ensure that the fan and motor capacities are adequate to handle the higher pressure drop.
A minimum fresh air volume of 3 cum/hour per person and 3.75 cum per hour per sqm (5 cfm per person and 0.6 cfm per sq ft) must be provided . The recommendation is to maximize supply of outside air within the limits of the system.
In buildings without mechanical ventilation systems it is recommended to actively use operable windows. Add a TFA (treated fresh air) unit if recommended Fresh Air intake impacts cooling performance. Install UVGI (Ultraviolet germicidal irradiation) for larger Ducted Units and AHUs to keep Coils continuously clean and disinfected. It is advisable to inspect the AHUs and ducts for Air tightness and low leakage.
Portable room cleaners
At the outset, it is important to note that a portable air cleaner will only clean the air that passes through the cleaner. The air cleaner will not be able to protect people from direct (droplets) and indirect (surface contamination) exposure but can help reduce the indoor concentration of contaminants through the action of dilution. Thus, over a period of time, the air in a space can be cleansed with the right air cleaners.
There are many technologies used in portable air cleaners. With regards to removing virus and bacteria, specifically, technologies vary from passive filtration to active filtration with all sorts of claims. With any active filtration technology, it is important to ensure the single pass efficiency and the possibility of any harmful byproducts that may be released due to the active nature of the cleaning. Some of the technologies used are Ionization, Bi-polar ionization, PCO, ESP, ozone generators etc. The efficacy of some of these is not yet clearly proven and in fact some of these technologies may have contra-indications. UVGI, if deployed correctly has proven to be useful in inactivating bio-aerosols. Passive technologies include HEPA filtration that can remove particles down to 0.1 micrometers or even smaller particles. An H13 certified or equivalent HEPA filter should be able to remove these viruses from the air that passes through the cleaner.
One must be aware of the number of air changes when installing portable air cleaners in a given space. Ideally, one must design air cleaners with about 3-4 air changes per hour. The higher the air changes, the better the efficiency of cleaning. One must be careful when reading the CADR numbers on most commercial products as they will give either the highest number (based on highest fan speed) which will be very noisy or will give the free-flow air handling capacity of the fan motor (i.e. without filters installed), rather than the actual airflow rate (with all filters installed).
Hence, one must analyze the Air Flow, in Cum per hour ( or equivalent CFM ) of the unit at different speeds and choose the machine with the right Air Flow, based on room size and fan speed, to ensure proper filtration and comfort , that includes a desired level of quietness.
Operation & Maintenance
Most Commercial establishments have remained closed during the lockdown. These establishments will need maintenance for both engineering and health safety. The air-conditioned spaces of establishments under prolonged lockdown will pose health hazards due to fungus and molds in the ducts and open spaces depending on the humidity and temperature prevailing within. Further there may be bird droppings, and excreta of rodents as well increased level of insects. The system not be designed for sufficient fresh air intake and ventilation. The following steps are recommended for the start-up of air conditioning system.
- The user or the owner should get the area sanitized
- Study the fresh air and exhaust system adequacy as per the guidelines and inform the user to modify the system if found inadequate.
- Carry the preventive maintenance on all the units as per manufacturer’s guidelines. This should include disinfecting and cleaning of:
- Filters, grilles, diffusers & internal surfaces: it is recommended to use 5% Cresol solution (containing 50% Cresol and 50% Liquid soap solution). Mix 1 liter of this solution in 9 liters of water. The surface shall be sprayed with this solution, left for 10 minutes and then washed / wiped clean with water / cloth. (the above methodology is only for washable filters)
- Condensate drain pan: Disinfecting / treatment of condensate drain pan is suggested using UV treatment or 1% sodium hypochlorite dosing. This will apply only if the HVAC equipment is working on a re-circulatory mode.
- Coils: Follow standard recommendations of coil cleaning and then sanitize using the same protocol as that of the filters specified above
- In case the area has ducted air distribution, it is advisable to clean the ducts by an appropriate method that may include sanitization.
- The following process is recommended at start-up:
- Open all the doors and windows of the space.
- Ensure that all cleaning protocols as advised above are complete
- Run the fresh air system at the maximum intake of air setting.
- Start and run the exhaust systems if available.
- Start the air conditioning system in fan mode only, without filters and run it for minimum of two to four hours with doors open and exhaust system operational.
- Install the clean & sanitized filters g. Start the AC in normal mode and run for two hours with doors open and then close the doors and windows.
6. The fresh air and ventilation system should be kept on throughout the off cycle and on the weekend and holidays in air circulation mode.