Aside from people, airflow is the other key transmission route for disease. Ventilation and air-conditioning plays a vitally important role in controlling the spread of infection in a hospital, not only by purging spaces of airborne pathogens, but by creating negative or positive pressure relationships that either prevent air from escaping or entering a space. Isolation rooms for infected patients are negatively pressurized so that nothing can escape; operating theatres and protective spaces for the immunocompromised are positive so that nothing can enter.
Research is ongoing into the routes by which the SARS-CoV-2 virus that causes COVID-19 is transmitted. The World Health Organization’s latest guidance, published in July 2020, says that it spreads from person to person through infected secretions such as saliva, or via respiratory droplets that are expelled when an infected person coughs, sneezes, speaks or sings. These may be ingested by a person in close contact (within 1m), or they may fall onto nearby surfaces, where the virus can survive for a period of time from a few hours to several days. Airborne transmission of SARS-CoV-2 can also take place during medical procedures that generate aerosols – smaller particles that may remain suspended in the air over longer distances and time periods. Scientists continue to debate and investigate whether airborne transmission may occur in the absence of these procedures, particularly in indoor settings with poor ventilation.
The coronavirus that caused the SARS epidemic in 2002-04 was fully airborne, and this proved a transformative experience for hospital design in Asia. COVID-19 is likely to do the same for the rest of the world. “In Singapore, hospital design has evolved in quite a significant way since the SARS outbreak,” says WSP principal Lionel Neo, a specialist in engineering healthcare buildings. In the aftermath of that epidemic, Singapore’s government hospitals adopted a range of measures to improve indoor air quality, including purging systems, higher air-change rates, higher levels of filtration, and UV light and titanium coatings on cooling coils in air-handling units to kill bacteria and viruses, says Neo. “When COVID-19 hit, the systems were already in place, and hospitals were mostly considered pretty well-equipped to tackle it.”
These measures are not in place all the time – in Singapore’s tropical, humid climate, the cost and energy required to constantly refresh all the air would be enormous. Instead, systems have dual ventilation modes, so that in the case of a pandemic – or a severe air pollution event, as caused by forest fires in neighbouring Indonesia in 2019 – they can be switched to fully exhausted, isolation, or mechanical ventilation and spot cooling mode.
To achieve this, air handling is strictly zoned in new hospital designs. “That starts from floor-by-floor planning, then zone-by-zone, and then department-by-department,” says Neo. “An air-handling unit is not shared by more than one department, so that the air-handling operation can be contained within that department itself.” Ventilation systems are designed to mirror the workflow of each department – so in a sterile services unit where contaminated equipment is returned and cleaned, air flows from sterile to clean to dirty areas before being exhausted out.
Singapore’s hospitals are already fitted with prefilters with a MERV rating of at least 7, and a secondary filter of at least 14, but there are also empty slots so that HEPA filters (MERV 17 or higher) can be added during a pandemic or air pollution event, and fans are sized to handle the additional resistance that this creates. “In terms of the whole hospital project, the cost is marginal – a fraction of the total project cost,” says Neo. Increasing floor-to-floor heights has a greater knock-on effect: in more recent designs, a standard 4.3m has been raised to 5m or ideally 6m. “The extra air changes, purging and dilution that’s required pushes up the need for ceiling space, so floor-to-floor heights inevitably have to increase as well to give us a bit of headroom, especially in light of COVID. Provision for extra and well-placed mechanical ventilation risers should also be considered during the planning. When it comes to fitting out existing buildings with smaller heights, it can be quite a challenge.”