Ventilation key to mitigating risk

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Remodelling and modifying heating, ventilation, and airconditioning (HVAC) systems can help lower risk of infection. “The entry of fresh air and stale air exhaust and filteration are the two key elements,” said Dr Anurag Agrawal, pulmonologist anddirector,instituteofgenomics and Integrative Biology, Council of Scientific and Industrial Research, Delhi.

INDOOR SPREAD

Several outbreak investigation reports have shown that Covid-19 transmission is efficient in confined indoor spaces like workplaces, restaurants, parties, shopping centres, and transport, according to the European Centre of Disease Control (ECDC) guidance on ventilation of indoor spaces. In a study of 318 outbreaks in China, transmission in all cases except one occurred in indoor spaces.

Studies have shown that the “attack rate” (percentage of people who get infected in an at-risk population) depends on the amount of time spent indoors. Sixty-one people spending two and a half hours in choir practice in Washington, US, led to a 85.2% attack rate, with 52 people getting infected. A call centre in South Korea recorded an attack rate of 43.5% among 216 employees. There was no link between infection risk and the distance from the index case, which indicated the amount of time spent in contact played a bigger role than proximity. “In labs, we increase the exhaust with maximum fresh air and don’t have crowding. Currently, the lab occupancy is less than half to ensure social distancing,” said Dr Agrawal. Two outbreaks in China from recirculating air have been documented.

CLEANING THE AIR

High efficiency particulate air (HEPA) filters used in aircraft and in health care settings have demonstrated good performance with particles of the Sarscov-2 virus size (around 70−120 nm), but their role in buildings outside of health care settings in preventing transmission of infectious diseases is unclear.

A NASA study documented HEPA filters stopped particles as small as 0.1 microns, which is the approximate size of the coronavirus, but other direct research is limited, according to a Mckinsey paper titled ‘Can HVAC systems help prevent transmission of Covid-19?’

Amongstepssuggestedbythe paper to make indoor air safer are configuring ducted HVAC systems to increase the rate of exchange of fresh air to reduce re-circulation, enhancing the control of airflow, allowing smootheradjustmentsofairflow by using control systems sensitive to pressure, and installing high-performance air-purification systems.

Cheaper options for airflow management are through the useofaverticallaminarforslow, steady air flow in a straight path to push out potentially contaminated air. “Vertical laminar is used...to minimise contamination, so that the airflow systems direct air from the ceiling to the floor,” said Dr Yatin Mehra, chairman, Medanta Institute of Critical Care and Anaesthesiology, Gurugram. The takeway? Recirculated air can spread coronavirus aerosols through HVAC systems for longer distances within indoor spaces, so there’s a need to lower risk by increasingtherateofairchange, decreasing re-circulation, and increasingtheuseofoutdoorair. Still, the risk of transmission in crowded indoor remains, which makes it vital to bundle prevention measures, such as wearing masks, social distancing, washing hands, and sanitising indoor spaces.