The Centers for Disease Control and Prevention (CDC) recommend that any patient with an infectious disease like the novel coronavirus (COVID-19) must be treated in an airborne infection isolation room (AIIR). An AIIR must meet several performance requirements:

  • An AIIR must be provided with at least 12 air changes per hour, all of which are exhausted directly outside.
  • Each room is kept at negative pressure, meaning that corridor air is pulled into the room and then exhausted. The purpose of this is protecting other patients, medical staff, and basically anyone in the hospital. Establishing negative pressure is very important when other buildings are used as temporary hospitals.

The CDC also advises that patients should not be placed in rooms where the air is recirculated without High-Efficiency Particulate Air (HEPA) filtration. Waiting areas for hospital emergency rooms must be exhausted directly outside, since they might contain undiagnosed patients. This minimizes the number of potentially infectious viral particles in the air.


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How Do Hospitals Manage Indoor Air Quality?

In a hospital, some of the air is conditioned and recirculated throughout the facility. In order to protect patients and hospital occupants, air handler systems rely on dilution and filtration. Dilution refers to air change rates - the air volume added or removed from a space per unit of time. Hospitals usually have high air change rates, to keep a safe and hygienic environment.

  • Hospital air handling units have a final filter with a Minimum Efficiency Reporting Value of at least MERV 14. This filter can remove 75 percent of particles ranging from 0.3 to 1.0 microns, and 95 percent of larger particles.
  • In some cases, hospitals prefer to use HEPA filters as the final filters, which trap 99.97% of particulates that measure 0.3 microns.

The ability of HEPA filters to trap the COVID-19 virus has been questioned, since each virion measures between 0.06 and 0.14 microns. However, viruses are normally expelled in droplets of saliva, which are larger particles. Based on this information and CDC recommendations, it is considered that HEPA filtration can help reduce the spread of coronavirus.

Keep in mind that no single measure can stop coronavirus with 100% effectiveness. However, transmission rates are reduced when several measures are used together. These include effective ventilation, social distancing and adequate protection equipment for medical personnel.

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To use HEPA filtration successfully in HVAC, the filters must be engineered into the HVAC installation and air handling unit.

  • HEPA filters must have a tight fit and proper saling, since air leaks around their edges make them ineffective.
  • Also, the air handling unit fans must have enough power to overcome the pressure drop caused by HEPA filters.
  • Viruses and other germs can be controlled more effectively when HEPA filtration is combined with Ultraviolet Germicidal Irradiation (UVGI).

Another design aspect to consider is the relative humidity level. The COVID-19 virus is a new strain, but previous research by ASHRAE and other institutions has found that viral transmission in general tends to decrease at 40-60% RH. Therefore, hospitals should monitor their humidity and keep it within this range. Even if the specific effect on coronavirus has not been studied yet, humidity control can help prevent infectious diseases in general.

The relative humidity inside a building depends on several factors: an effective building envelope that isolates indoor air from outdoor conditions, an adequate HVAC design, and the capacity of humidification equipment.

Plumbing Design Considerations in Hospitals

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Many design features for disease prevention are based on previous studies by organizations like ASHRAE. Although these measures have not been tested extensively for coronavirus, they help control infectious diseases in general.

Studies by the World Health Organization (WHO) have demonstrated that some infectious diseases can spread through floor drains or similar fixtures. This occurs when air pressure changes deplete plumbing traps, eliminating the barrier between indoor air and the drain system. Plumbing codes have stringent requirements for sanitary waste systems, in order to prevent trap depletion due to pressure changes.

Drains that are not regularly used can lose their trap seal due to evaporation. However, consider that this causes odor leaks, which are likely to be noticed. A simple measure to prevent evaporation in traps is dumping one gallon of water down the drain at regular intervals. Keep in mind that evaporation is faster in dry environments, so traps must be checked more frequently.

Hot water recirculation systems must be kept balanced and flowing, since germs can accumulate in stagnant warm water. Frequent handwashing is an effective measure only if the water used is germ-free. Adequate temperature control is also important, and water must not be heated excessively to prevent scalding.

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