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Conditioning for the Environment of Critical Care Hospital Operating Rooms

By David Schurk, Member ASHRAE

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©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no. 10, October 2019.

About the Author
David Schurk is national director of strategic accounts-healthcare for Carrier Corporation, Denver.

Many rooms in hospitals require special design considerations because of intensified infection concerns, high air change rates, special equipment, unique procedures, high internal loads and the presence of immunocompromised patients. But in no other health-care space does the design of the HVAC system take on more importance than in an operating room (OR), where its purpose is to minimize infection, maintain staff comfort and contribute to an environment of patient care.

ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, is considered the backbone of health-care ventilation design. The intent of the standard is to provide comprehensive guidance, comprising a set of minimum requirements that define ventilation system design that helps provide environmental control for comfort, asepsis, and odor in health-care facilities. It can also be (and is) adopted by code-enforcing agencies.

The standard defines minimum design requirements only, and due to the wide diversity of patient population and variations in their vulnerability and sensitivity, these standards do not guarantee an OR environment that will satisfactorily provide comfort and control of airborne contagions and other elements of concern. When selecting the temperature and relative humidity combination to be incorporated into the design, these standard minimums as well as the needs and desires of the surgical staff must be taken into consideration. The ASHRAE HVAC Design Manual for Hospitals and Clinics points out that the inability to maintain low OR temperature is probably the number one complaint by surgeons to facility engineers. To help mitigate this concern, OR design conditions should be developed in consultation with surgeons, anesthesiologists, infection control and nursing staff based on the classification of the surgery and any specific requirements that may result.

Temperature and Relative Humidity

While this article has an emphasis on areas requiring dehumidification, it is not meant to imply that “very-dry” air is the answer. It’s important to remember that a healthy level of indoor air hydration, in balance with the room temperature setpoint, helps promote overall environmental satisfaction and performance. One of the first determinations to make during the initial design phase is the environmental space temperature and humidity. These requirements should be considered a fundamental prerequisite in helping to promote overall occupant satisfaction and well-being. As shown in Table 1, Standard 170 requires temperatures ranging from 68°F to 75°F dry bulb (DB) (20°C to 24°C). It is important to note that these recommendations are considered minimum design values.

Just as critical as temperature, relative humidity (RH) plays a key role in maintaining a comfortable and healthy OR environment. Table 1 shows Standard 170 requiring RH to be maintained in a range from 20% to 60%. Too much humidity can decrease the perspiration rate from a person’s skin, resulting in an environment that feels relatively warmer than it actually is. It can also lead to a potential for mold and mildew growth within the built environment. On the other hand, dry air can affect people with respiratory problems, and also cause dry skin which can result in discomfort. Dry air below 40% RH has been shown to correlate with an increase in health-care associated infections.

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