Hospitals are designed to contain infectious diseases through negative and positive pressure rooms. These spaces control the movement and spread of infectious diseases using specially designed ventilation, where air pressures are regulated to move air from clean to dirty areas.

• Negative pressure rooms vent air from the room and exhaust it outside the building. High efficiency particulate air (HEPA) filters are used to help remove particles from the air. Makeup air flowing into the room is balanced so that the room remains under negative pressure. Thus, the direction of airflow under the door or when the door is opened is into the space, rather than out, helping contain infectious particles.
• Positive pressure rooms are designed to keep infectious agents out. The space is designed to have directional airflow (from one side of the room, across the occupants to the exhaust) and 12 or more air changes per hour. Clean air is supplied through HEPA filters, and air flowing through an open door or leak moves out of the space, not in.

Negative and positive pressure rooms provide a framework for limiting the spread of infectious agents, but they are not foolproof. Building design, inconsistent or inadequate operating procedures, poor maintenance, and facilities construction and renovation projects all have the potential to cause problems.

Even the best air filtration and airflow design can be defeated by inadequate maintenance or incorrect operation. A slipping fan belt on a ventilation supply fan, for instance, could alter the air balance in a positive pressure room, allowing particle-laden air from the hallway outside to flow in. Failure to correctly seal off and ventilate a construction area can send a cloud of construction dust and aspergillus spores into areas where patients are housed. Failure to fix a leaking sink can turn the cabinet below into a nursery for potentially hazardous mold.

From Hospitals to Offices, Schools, and Shops

Offices, schools, and shops don’t have negative and positive pressure rooms. In fact, people go in and out of these spaces without a thought about the air flowing in or out the doors. So, what can be done to help mitigate the spread of airborne viruses, such as COVID-19, through ventilation systems?

Scheduled HVAC maintenance is always important, but it is especially critical in the time of COVID-19 as public spaces open their doors for business and education. Typically, for recirculating ventilation systems where air is sent out through a HEPA filter before being introduced back into the system, basic maintenance would occur around the time when the filters would need to be replaced — maybe every two to three months. With more advanced filtration, however, it is more critical than ever to not let filters go beyond their specified date. A clogged or full filter will cause system issues for both the short and long term.

Besides changing out filters, systems need to be checked to ensure there is proper airflow, or air velocity, for the structure. The first step for testing airflow is to validate what the airflow should be for the system in place.

• Start by calculating the duct size and motor size to verify what CFM the system can handle.
• Once the filtration system is running, confirm the static pressure of the system to ensure there is no added drag on the motor.
• Use a flow meter to determine what the flow is going into and out of the system.
  • The airflow should be close to the specifications of the equipment.
• If the FPM numbers on the flow meter are below what is recommended for the equipment, look for obstructions in the duct work, dampers, and other areas. This is also a good time to check the motor for operation to ensure it isn’t dropping out and that it is set to the right speed of rotation.
• If the FPM numbers on the meter are above what is recommended, the motor rotation speed may be set too high, or the ducts may be too narrow for the space. Higher air velocity is difficult to manage in terms of air quality.

Hospitals have successfully kept airborne viruses out of their facility HVAC systems using a combination of good air filtration and directed airflow. As many people head back to offices, schools, and other public spaces, the same approach hospitals use to keep its occupants safe can be used in these other public spaces. As concerns about whether the COVID-19 virus can be transmitted through HVAC ventilation is at an all-time high, businesses and schools should place extra emphasis on the maintenance of their HVAC systems. Well-maintained HVAC systems in public spaces is just one way businesses and schools can help ease the transmission of COVID-19.