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Covid-19: Why we need to rethink ACs and indoor air quality

Modifications to existing HVAC systems, air filters can help reduce indoor airborne transmission

As humidity, temperature control and filtration come into focus, AC manufacturers are beginning to take the cue. Photo: Hindustan Times
As humidity, temperature control and filtration come into focus, AC manufacturers are beginning to take the cue. Photo: Hindustan Times

During a recent webinar on the importance of indoor air quality in the times of covid-19, Liam Bates, founder of air quality monitoring and solutions company Kaiterra, summed up the current conundrum when it comes to air-conditioning systems. “HVAC (heating, ventilation and air conditioning) is like a double-edged sword. It can be your best friend or it can be your worst enemy,” Bates said. “The key here is ventilation.”

If your building’s HVAC unit takes the air from a room, cools it down or heats it up, and then mixes it with a little amount of outside air before redistributing it, that is not great news. If it follows the same process, but mixes a substantial amount of outside air and then passes the air flow through high-grade filters before redistribution, the risk of infection comes down.

A February 2019 paper published in the journal Scientific Reports used computer simulations and models to show how ventilation can make a huge difference in curbing aerosol transmission of a virus. “Our model suggests that recommendation-abiding ventilation could be as effective in mitigating outbreaks as vaccinating approximately half of the population,” the paper explains. While this study was focused on influenza, the same logic would apply to the novel coronavirus.

As humidity, temperature control and filtration come into focus, AC manufacturers are beginning to take the cue. Earlier this week, electronics company Samsung unveiled a new range of “wind-free” ACs that come with a PM 1.0 filter, which uses an electrostatic charge to sterilize the air and capture ultra-fine particles down to 0.3 microns. Clean air solutions company Nirvana Being, among others, has introduced nanotech-based AC filters (made from electro spun nanofibres) that cost just 1,495 and can turn a split air-conditioner into an air sterilizer.

To understand how to move forward, it is key to understand how the air draft from air conditioners can help carry respiratory droplets. A paper published in the International Forum Of Allergy And Rhinology in July notes: “In a room setting, particle emission from the mouth or nose is influenced by its initial velocity. A sneeze, for example, can generate an extremely high velocity initially (50 m/s), but it will quickly dissipate over a short distance (5 m/s after 0.6 meter), whereas talking generates a lower velocity at 3 m/s, with the initial airflow field likely dissipating completely within 1 meter from the mouth.” While droplets with a larger diameter will fall to the surface quickly, the movement of an aerosol particle not large enough to settle will be governed by the indoor airflow as its initial velocity decreases.

“The biggest problem in a closed office space is potential transmission through the airborne route—especially when the AC is on,” says Barun Aggarwal, CEO of indoor air quality solutions provider BreatheEasy Consultants. Aggarwal is part of a task force set up by the technical committee of the Indian Society of Heating, Refrigerating & Air Conditioning Engineers (Ishrae), which came out with detailed guidelines on covid-19 and HVAC systems in April.

The guidelines include a greater focus on fresh air intake—to dilute indoor air—and low level of air drafts. “You don’t want your air conditioner to be running on full blast. You want to reduce your fan speed,” says Aggarwal.

In indoor settings, it’s suggested, you should try to maintain relative humidity at 40-60%. Upgrading filters can help too. “We have technology today that can actually filter out the covid-19 virus particle from the air as long as it is done properly,” says Aggarwal, who recommends using H13 grade filters. H13 is actually a Hepa, or high-efficiency particulate air, filter that is considered extremely efficient—a 2016 study by US space agency Nasa says Hepa filters are nearly 100% efficient at capturing a spectrum of particles, down to the smallest airborne particles. “Anybody who plans to open their offices, schools, theatres, malls or any other commercial establishment, before we have a vaccine that is scalable, needs to take these precautions,” says Aggarwal.

Ventilation and AC systems in public transport like the Metro remain an area of concern. When Indian Railways restarted limited operations earlier this year, the roof-mounted AC package unit systems in its air-conditioned coaches were overhauled to allow more fresh air into the compartments. According to a Press Trust of India report, these units now replace the air inside coaches more than 16-18 times every hour, just as they would in a hospital operation theatre. News reports suggest a similar overhaul is expected in the HVAC systems of Metro coaches before the service resumes in parts of India from 7 September.

There are other technologies too. Research has shown that irradiation, or ultraviolet-C lighting, can help “deactivate” pathogens. According to a McKinsey & Co. article published in July, HVAC systems can also incorporate ionic purifiers, ozone generators and other devices to clean the air.

For these systems to work, however, the air would have to be exposed to the sterilization process for a significant period of time. Ultraviolet-C lighting, for instance, needs a minimum exposure time of 1 minute to sterilize the air, so it’s difficult to use if there is a continuous flow of air.

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