During periods of moderate or higher respiratory virus transmission, the NIH will utilize HVAC engineering controls for reducing potential exposure of staff to respiratory virus. The controls are based on recommendations from the CDC and the American Society of Heating, Refrigeration, and Air-conditioning Engineers. Specifically the NIH will utilize ventilation and filtration to minimize airborne concentrations of infectious aerosols.  The expectations for each of these strategies are laid out below. 

Ventilation

Locations that use recirculated air benefit from dilution of infectious aerosols by increasing the quantity of fresh air mixed with recirculated internal air. During periods of moderate or higher respiratory virus activity, the NIH will:

• Ensure existing HVAC systems maximize the use of fresh air in locations with recirculated air. Fresh air intake will be dependent on the capabilities of each individual system. 
• Aim for 5 or more air changes per hour (ACH) of clean air at minimum for offices and shared spaces. Laboratories, clinical settings, and animal care areas should have air changes in accordance with relevant guidelines for each location. 

Filtration

The NIH Office of Research Services will ensure that recirculated air is filtered to the maximum extent feasible, based on the limitations of each buildings' ventilation system.  In most cases, this filtration will involve the installation of MERV 13 filers, which can filter out up to 85% of airborne particulates.

Locations with 100% fresh air HVAC systems, such as laboratories, clinical spaces, and animal care facilities do not require increased filtration efficiency as they replace all potentially contaminated air with fresh air during each exchange.