30 April 2020
COVID-19: Why we need to redefine thermal comfort

Humidification, high levels of fresh air intake are some recommendations of a document on air-conditioning and ventilation in COVID-19 times

By Sugeet Grover

The novel coronavirus disease (COVID-19) has wreaked havoc in the world. It changes a little every day, the world. While researchers upgrade their guidelines constantly, the World Health Organisation and scientists are locked in an ideological battle about ways SARS-CoV-2 can be transmitted.  The tiny organism has already made us rethink and redefine our tolerance for comfort. Let’s sweat on this a little more.

A COVID-19 guidance document on air-conditioning and ventilation and how it affected spread of infection was recently released by the Indian Society for Heating Refrigeration and Air Conditioning Engineers (ISHRAE). Here is what the document said.

On relative humidity

Dry air makes the mucous membrane in the lungs drier. This lowers the body’s defence mechanism against infections. The moisture in the air is hence an ally for our immune system. Moreover 80 per cent relative humidity (RH) and above tends to neutralise the SARS-CoV-2, according to some studies. Extremely high relative humidity levels, however, can have other ill-side effects such as mould and fungus growth.

The ISHRAE recommend that 40 to 70 per cent RH level be maintained in air to curtail chances of contracting the infection.

On temperature

Coming out of China is a research that claims that higher temperature and humidity levels reduce the transmission of COVID-19. Higher temperatures are worse for virus survival, more prominently on surfaces. The virus may remain active for 14 days at surface with 4 degrees Celsius temperature; and a day at 37° C surface temperature.

On ventilation

The guidelines claimed that air conditioning should be accompanied with fresh air intake and exhaust. Desert coolers can cool down the air by up to 15 degree centigrade.

What does this affect?

In a business-as-usual scenario, many of us would go back to our centrally air-conditioned offices. Once in a while, we would go out to take fresh air and come back. A post-COVID-19 world will need re-strategising.

Let’s take ISHRAE’s recommendation of using exhaust with air-conditioning systems. Air-conditioning predominantly works with recirculation of air, which ensures that the molecules of air once cooled down by the AC picks up heat from equipment and the human body. The AC then cools the molecules down and recirculates it in the space. (Ever received stares in your office for keeping the window or door open?). In fact, split unit ACs don’t even have a provision of fresh air intake.

High grade filters might need to be installed, but this is easier said than done. Imagine breathing through a N-95 mask, which filters out the particles and makes it easier for your lungs to work. Smaller the filter sieve size, greater the work the machine does. This also implies that the fan pulling the air has to be retrofitted.

Moreover, the return air duct could be at one end of the space. Imagine a person coughing on one end and the particles travelling to the other end before coming across a filter. Exhaust usage would also mean that a greater volume of ambient air would be needed. Ambient air temperatures in North India can reach up to 46-48° C. This would require HVAC system to work more and would burn a hole in the pockets of building operators.

Explore other forms of cooling

Radiative cooling and desiccant cooling are options that can be explored. But they will require heavy retrofits that will need to be considered for future buildings. So an immediate option might be smaller desert coolers. It should be made sure, however, that the cooler runs on fan-only mode for 30-60 minutes every day so as to dry the cooling pads.

Moisture might be the key

May and June are usually hot and dry. To maintain the RH level recommended by ISHRAE, moisture will be needed to be introduced into the air. An air conditioning system can take away moisture from the air, but a desert cooler may introduce humidity in the air. Evaporative cooling can be explored in these spaces or vessels with water can to be placed in spaces to increase humidity. The opportunity here is that as the next few months will entail dry air, introducing humidity will be beneficial in achieving thermal comfort while also making spaces cooler.

Fan it up

A fan is likely to be beneficial in all scenarios. A personal table fan in combination with other cooling devices can heavily reduce the dependence on air-conditioning. It is perhaps time that the humble fan is brought back into office spaces.

Night purge

Offices can ventilate their buildings at night even when nobody is occupying the space. The night time temperatures are lesser than day time.

Early to rise

Offices may want to start offices earlier, as the peak temperature in the interior of buildings does not coincide with peak temperatures outside. The cooling takes time to dissipate from within the building. This implies that the office can plan to operate from early hours and close before the temperatures reach uncomfortable limits.

A no-no: 24 degrees Celsius

According to the adaptive thermal comfort model that has also been adopted by National Building Code of India, one’s comfort expectations depend upon outside temperatures and the temperature that one is used to.

This essentially means that if a person gets used to an air-conditioned environment, let’s say at 24° C, for 30 days, the perception of thermal comfort would change and the person will get habituated to a lower temperature.

However, if a person is used to a naturally ventilated building, thermal comfort temperatures may go up to even 32° C. A 24° C air-conditioning setting should be a strict no-no in this scenario. An air-conditioner should not come into picture until the interior temperatures reach beyond 32° C. Employees will eventually have to make peace with a different thermal comfort standard.


Original Article: https://www.downtoearth.org.in/blog/thermal-comfort/covid-19-why-we-need-to-redefine-thermal-comfort-70808

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