By SK Nag
Scientists across the globe wrote to ‘WHO’ suspecting airborne infection through aerosolized contamination resulting from coughing and sneezing. ‘WHO’ did not rule out this possibility. Virus remains airborne for a longer duration piggybacking on dust particles and by sticking to the smallest moisture bubbles (known as aerosol) lingering the transmission longer than anticipated initially in a closed indoor environment. Air conditioner experts across the globe are finding it difficult to eliminate this risk in an indoor environment in a typical office where extensive dependence on Air-conditioner is present. Occupants will be at a risk if these infected aerosols are inhaled by healthy occupants. Therefore, scientists concluded that respiratory infectious pathogens are prone to be spreading faster through air conditioning infrastructure multiplying the number of infections in every recirculation of air through such mechanised systems where air is re-used multiple times. Chances of respiratory infection through inhaling respiratory droplets (> 5 micron) which infects upper respiratory parts of the human breathing system (nose and throat area) whereas droplets and aerosol (< 5micron) if inhaled can transmit much lower part of the human breathing system. The difference of droplets and aerosol are classified based on their diameter. Droplets have the floating range of maximum 2m but aerosol can remain airborne for longer time and potentially transmits the virus across the indoor environment. In this situation recirculation of return air will increase the chances of further spreading of the disease with the virus in aerosol. Although virulence of the active virus in the air is not estimated, nevertheless the existence is not ruled out.Air condition experts following operation theatre specification recommend maintaining temperature above 28 Deg Celsius and relative humidity 40 ~ 70 %. But with these two parameters will it be able to deactivate the virus, is a big question now. COVID virus is assumed to be inactive under these two control set points, and will be unscientific to comprehend at this stage, when the virus behaviour is widely unknown. So, what do we do? The requirement of Air conditioner is unavoidable and being prevalent in commercial set up globally. It is high time when the responsible engineers and scientists whether related or unrelated must come together to find a commercially viable solution to eliminate the risk of air borne (aerosolised) transmission through office air-conditioners. The office indoor ambience will be claustrophobic if there is no Ventilation or Air conditioning facility. Opening windows to replace conventional air conditioning though tried but tropical locations do not seem to have merit in such alternatives. The office space needs a comfort cooling facility as a prerequisite, whereas public facilities like parking etc. may have only Mechanical Ventilation. Mechanical ventilation maintains a comfortable Oxygen level for conventional breathing avoiding suffocating the indoor ambience, whereas Air-conditioning maintains all primary parameters for human comfort. Temperature, Relative Humidity and Oxygen content in the air are the primary designing imperatives. These parameters decide the comfort conditions. Precise design is required to make the system efficient and agile. If the system is loosely designed, it might not deliver required comfort conditions at a reasonable cost. Therefore, in order to make the system more energy efficient, design considers using recirculation of air (known as Return-Air). Return-Air after use in the space cooled loses its Oxygen content and gains its CO2 concentration substantially. Enriching with Oxygen becomes necessary to make it healthy in its subsequent use, otherwise the users inside the office will experience breathing discomfort. Prolonged utilisation of CO2 rich air may, in the long run give rise to human respiratory ailments sometimes. So in every circulation as per air change norm set by ASHRAE (American Society of Heating, Refrigerating and AirConditioning Engineers) per hour basis a minimum quantity of Fresh Air is added (5 to 10%) to the space to enhance its oxygen quality by enriching its content to satisfy the need without making them feel any discomfort. Though ASHRAE has specified various design norms which are to be complied with by designer, still those recommendations are diluted by major developers, barring a few to save capital cost of installation. Besides design, in operation too, ASHRAE has mentioned operating and maintenance guidelines in their SOP (Standard Operating Procedure). Lack of right skill to operate and maintain these systems is sometimes a big challenge for the system owner. Tier-II & III cities do not have HVAC (Heat Ventilation & Air Condition) operators in sufficient numbers. Technically, the air condition system takes care of temperature, humidity, dust particles, Oxygen (O2) and Carbon dioxide (CO2) content in a closed indoor environment for human comfort and finally containing airborne infection to eliminate cross-infection possibility. (We are limiting our discussion on comfort AC, keeping aside Precision AC & Sub-Zero AC).Temperature is a parameter which our body can sense very easily. Though temperature control alone cannot achieve a human comfort situation still the intensity of comfort is palpably recognised and rated by temperature only. In reality comfort conditions achieved through complemented participation of relative humidity and oxygen saturation level. Therefore, conclusively indoor conditions for users’ comfort are controlled by all these. Although various room air conditioners (eg: window AC, split AC) are not equipped with so many control set points to maintain the air quality precisely, they are still widely used due to ease of operation and are commercially easy to acquire and install. Central Air- Conditioning plants in comparison are CapEx, OpEx & Energy intensive technology. The Central Air Conditioning plant has wide families and generations of technology. There are VRV, VAV, Central Chiller based technology available in the market. Air Condition systems are designed to adjust temperature and RH either automatically or by manual control. When oxygen level goes down, the ambience suffocates occupants. A sudden discomfort or uneasiness might result from feeling hot due to lack of oxygen in the air. This is actually controlled by an inline CO2 monitor and operating fresh air fan into the Air handling unit taking the air to the cooling coil. Besides Temperature and Oxygen there is another important element known as Humidity which has an important role to play. Humidity is moisture content in the air present. Too dry a place resulting from low humidity might result in skin irritations and breathing problems like we face in extreme cold weather where humidity is too low. High humidity conversely, will lead to fungal growth in places leading to unhealthy bacterial infections. So right monitoring of all these parameters has an important role to play. The right performance of air conditioning systems in office infrastructure maintaining a healthy atmosphere is an imperative which COVID taught us currently. That is why more than designing a system, maintaining the system is a more important engineering skill. All these discussed above have proven science to corroborate the stipulations. Another important issue which chokes the system is suspended dust particles. These particles are removed from the circulation through multiple layers of filters used in different stages of the complete circulation path. ‘In-line’ filters of varying size and nature are used at multiple locations. It starts with fresh air intake, followed by at AHU level and again at return air level. All these filters remove suspended dust particles from circulating air used to cool the place. By removing dust, it also eliminates the associated bacterial infection which may piggyback on dust particles. Filters available in the market are classified broadly in three categories. Common filters, HEPA filters and ULPA filters. Each having their own capacity of arresting particles size upto certain microns. ULPA filters can eliminate upto 0.12 micron being superior in the family, whereas HEPA can eliminate upto 0.30 micron and common filters have a much wider range of capacity depending on area of use and cost. It can eliminate maximum upto 0.5 micron. Passive control, in addition to the active control of parameters which stated above are the solution which can reduce the possibility. z Respiratory pathogens piggyback on dust particle droplets & aerosol to linger in the air for long. Therefore, Heat and Ventilation experts should address to eliminate these carriers being conducive to the spread. Active and passive control systems need to be understood by all of us to insulate ourselves from the risk. In an active system, improvement should ensure frequent cleaning of the system. Using a higher-grade filter by modifying system design capacity to exert more pressure as higher filter quality will develop tremendous head loss in the system. Flushing the system by keeping the ventilation system operational at its economic mode before and after actual use 24X7 including holidays. By reducing dust, the chances of the virus being airborne will reduce. z Velocity of air within the system to be converted to laminar flow instead of turbulent flow. Laminar flow ensures that no one comes in the line of flow. It is a controlled flow of air. z The use of UVGI will reduce the chances of contamination through air disinfection (claimed by some of the UVGI manufacturers, but effectiveness not proved yet). But that’s not all. z There has to be passive control in the Air conditioner system, which is behavioural changes in users. A paradigm shift needs to be achieved by adhering to improved and changed social etiquette. Using masks all throughout will reduce the formation of aerosol while speaking (screaming & singing create more aerosol). While sneezing and coughing, a lot of aerosols are generated and get spread very rapidly. Despite having masks, users must cover their mouths by not letting any aerosol go in the air. All aerosol and droplets should be arrested within the user’s handkerchief. So, we must learn to accept the new responsibility in an airconditioned indoor environment to control the infection possibility through passive behaviour change.
(SK Nag is Chartered Engineer, Energy Expert and industry mentor. The views expressed are personal opinion of the author. He can be reached at saibal.iim@gmail.com)
