Tuesday, September 28, 2021
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Covid-19 and air-conditioners: Fresh and filtered

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The chances of contracting the infection are high in enclosed areas and poorly ventilated spaces. Trapping and killing the virus through pressurisation and filtration are effective in destroying it.

By Dr KK Aggarwal

Person to person transmission is a well established route for the spread of Covid-19. And it’s not just symptomatic persons, even asymptomatic and pre-symptomatic people can spread the virus to others. This spread occurs via respiratory droplets carrying the virus and which are produced during exhalation (eg, breathing, talking, singing, coughing or sneezing) and when a person is in close contact (within six feet) of a person with Covid-19. Transmission may also occur through contaminated surfaces, but it is a less common route for the spread of the virus.

Airborne transmission of the Covid-19 virus has been a subject of debate and cannot be ruled out with certainty. The latest Scientific Brief, dated October 5, 2020, from the Centers for Disease Control and Prevention in the US, has acknowledged this mode of spread and says that “pathogens that are mainly transmitted through close contact (i.e., contact transmission and droplet transmission) can sometimes also be spread via airborne transmission under special circumstances”. The smaller droplets and particles (<5µ) containing the virus remain suspended for long hours and are consequently transported to long distances (usually greater than six feet). The size of the virus is 80-160 nm and it remains active for 48-72 hours on indoor surfaces. This is where the challenge lies as the virus becomes airborne and is carried into rooms in air conditioned buildings.

The chances of contracting the infection through airborne transmission are particularly high in enclosed spaces (buildings, public transport) and/or poorly ventilated spaces or when there is prolonged exposure to respiratory droplets produced during shouting, loud singing and exercising. There are two main transmission mediums in indoor spaces: cough generated aerosols and particulates (dust) suspended in air. Reduction of indoor aerosols and dust levels is a step towards mitigation of this source of transmission.

Several outbreaks in confined spaces such as shopping malls, worker dormitories, religious meetings and cruise ships have been reported. The Diamond Princess cruise ship outbreak, reported on February 1 as a passenger who had disembarked in Hong Kong tested positive, led to the ship being quarantined immediately in Japanese waters. This is a classic example of how closed spaces facilitate the spread of the virus. A study of the air conditioning system on the ship revealed traces of the Covid-19 virus on a variety of surfaces in cabins of both symptomatic and asymptomatic passengers up to 17 days after the cabins were vacated. It also highlights the role of poor ventilation systems in the spread of Covid-19.

Some air-conditioning systems recirculate indoor air and mix it with some fresh outside air. The air current produced by AC fans allows the virus to stay suspended in the air for a longer time and travel further than they might otherwise go. As the ship used a centralised AC and the onboard ventilation system was not designed to filter out particles as small as the coronavirus, the disease spread rapidly to other cabins during the quarantine period.

However, the same cannot be said of the air-conditioning system inside planes. According to the International Air Transport Association, the risk of transmission in planes is probably lower than in many confined spaces because modern airplanes have cabin air filtration systems equipped with HEPA filters. HEPA or high efficiency particulate air filters have similar performance to those used to keep the air clean in hospital operating rooms and industrial clean rooms. These filters are very effective at trapping microscopic particles as small as bacteria and viruses and can capture greater than 99 percent of airborne microbes in the filtered air. The cabin air system is designed to operate most efficiently by delivering approximately 50 percent outside air and 50 percent filtered, re-circulated air. This normally provides between 15 and 20 cubic feet of total air supply per minute per person in economy class. The total air supply is essentially sterile and particle free. Cabin air circulation is continuous. Total airflow to the cabin is supplied at a bulk flow rate equivalent to 20 to 30 air changes per hour.

Technological methods recommended to reduce airborne transmission of the virus are dilution, pressurisation, filtration and purification (destroying infectious agents in air). Filters trap all pathogens, forming a colony of pathogens and are a source of infection. “Trap and kill” is desirable. Hence, purification is needed.

ASHRAE or the American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends seven strategies: dilution (high ventilation rates i.e. air changes per hour), airflow patterns (how the air is flowing), pressurisation (negative pressure in isolation rooms with respect to surroundings), temperature, relative humidity, filtration and ultraviolet germicidal irradiation as strategies to reduce exposure to infectious aerosols.

The way to reduce this transmission, therefore, is to improve ventilation with outside air. This can be done through natural means or fresh air. So open the windows if possible. Else use mechanical ventilation systems. Avoid recirculation of air or achieve more fresh air (greater than 50 percent). The air conditioning and ventilation systems (air filters) for residential as well as other buildings such as hospitals, hotels and schools should be regularly inspected and cleaned to prevent transmission.

Buildings that use central ventilation should use the most efficient filters. Higher efficiency filters like HEPA should be used and higher air change rates per hour (10-15) are advisable for mechanically ventilated buildings. Use a window air conditioner that has an outdoor air intake or vent; keep the vent open.

When air conditioning or ventilation is not available, as is the case in most homes, open the windows to allow fresh air into closed spaces; alternatively, an air purifier can be used. All exhaust fans should be functional; they should be kept operating when the building is occupied. Fans should generally be avoided in closed spaces with a large number of occupants. ISHRAE (Indian Society of Heating, Refrigerating & Air Conditioning Engineers) recommends: “Fans should be operated with windows kept partly open. If an exhaust fan is located at a nearby location then it must be kept running to exhaust air for better ventilation”.

The Covid-19 virus is known to remain viable for up to four hours on copper, 24 hours on cardboard and 2-3 days on plastic or stainless steel. All environmental surfaces should be regularly cleaned and disinfected to eliminate the virus. The ideal temperature and relative humidity range is below 30 degrees Celsius and above 80 percent, respectively. This is not a comfortable situation. The room temperature should be set between 24 and 30 degrees Celsius and RH between 40 and 70 percent, according to ISHRAE. In dry climates, the RH should not be allowed to fall below 40 percent. Recirculation of indoor air in closed spaces for longer duration (as in offices, trains, ships) can transmit the infection despite the recommended physical distancing. “Good” ventilation and not just ventilation, therefore, is of paramount importance to decrease the risk of indoor infection.

But, improving ventilation alone will not protect one from Covid-19.  It is just one of the protective measures. Others include physical distancing (at least one metre), avoiding crowded public places, correct and consistent use of face masks (three layered) and meticulous hand hygiene. Such stringent measures can work as a deterrent.

—The writer is President, Confederation of Medical Associations in Asia and Oceania, and former National President, IMA

Lead Picture: UNI

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