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HVAC Control of Psychometrics, Filtration, & Pressurization in the Prevention of Hospital Acquired Infections (HAI)



Hospitalization exposes patients to an array of HAI. This paper focuses on specific means by which risks of HAI can be minimized. While airborne infectious droplets pose a serious threat to

Clostridium Difficile
In 2015, Approximately 29,000 patients died within 30 days of the initial diagnosis of C. difficile.

hospital patients, employees, vendors, and visitors, I address HAI threats to patients.

Prevalence of HAI

A 2014 HAI study published in the New England Journal of Medicine describes the prevalence and distribution of HAI.

    • 4% of patients had at least 1 healthcare-associated infection
    • Pneumonia and surgical-site infection were most common, followed by gastrointestinal infection, urinary tract infection, and primary bloodstream infection
    • Device-associated infections accounted for 25.6% of all healthcare-associated infections
    • Device-associated infections and surgical-site infections accounted for 47.4% of all healthcare-associated infections
    • 6% of infections were not associated with devices or operative procedures
    • 481 pathogens contribute to 73.8% of HAI
    • More than half of all HAI occur outside ICU.

How great is the problem of HAI? The CDC reported in 2011 the deaths of approximately 75,000 hospitalized patients.

HVAC Processes Employed in the Prevention of HAI

Reference: FINALPUBLISHEDPaperonHUMIDITYandViruses509

Pressurization Control

  • Positive pressurization of surgical and storage areas prevents or limits introduction of airborne pathogens from other spaces.
  • Negative pressurization of isolation rooms prevents the spread of an infected patient’s pathogens to areas outside the isolation room.


The State of Texas has guidelines establishing minimum standards for all areas in hospitals with HVAC service. Refer to Table 3 of Texas DHS Title 25 Texas Administrative Code, Chapter 133, Hospital Licensing Rules for all pertinent regulations.


  • HEPA filters are designed to filter 99.97 percent of particles with a diameter of 0.3 microns. The size of 0.3 microns is the most evasive for a particulate filter. A HEPA filter is more efficient in trapping particles with larger or smaller diameters. When HEPA filters are installed within HVAC systems, they should be chal­lenged with a testing agent to ensure that they are meeting their designed efficiency. Dioctyl phthalate (DOP) is usually used as the challenging agent.  A DOP generator creates aerosols with a mass median diameter of 0.27 microns that are injected into the HVAC system’s airstream, upstream from the filter bank. A light-scattering photometer then determines the penetration of DOP aerosols that occurs through the filter and around its gasket.
  • Refer to ASHRAE Standard 170-2013 to select the proper level of filtration.

Source: American Society for Healthcare Engineers

Temperature and Relative Humidity Control

  • Maintaining temperatures above 60°C (140°F) for more than 60 min. will usually inactivate most viruses, though this can vary depending on the presence of organic material (e.g. blood, feces, mucus, saliva, etc.) that may surround exhaled viral particles and insulate the virus against extreme environmental changes. Higher temperatures for shorter times can be just as effective to inactivate viruses (Tang 2009). Obviously, only Superman or Supergirl could withstand such high temperature.
  • Relative humidity (RH) works in tandem with air velocity (discussed below) to suspend infectious aerosols produced by talking, vomiting, diarrhea, toilet use, breathing, and coughing.
  • Temperature and relative humidity operating ranges shall be specified by the authority having jurisdiction.

Air Movement Velocity and Room Air Change Rates

Why is air velocity an important factor? Allow this introduction to the concepts of viscosity and  Stoke’s Law:

Viscosity simply described is resistance to flow. NASA says viscosity is the “sticky” property of air. The “stickier” the air, the slower infectious aerosols (droplets) will fall through the air. If the upward velocity of air in a space is greater than the settling or terminal velocity of the aerosol, the aerosol will not fall all the way down to a patient or onto a sterile surface below.

The larger the droplet, the faster it falls and therefore upward air movement toward the return-air path is of utmost importance.

The actual cost of implementation of all the above is extremely low in comparison to the cost of patient fatalities due to HAI.

In most hospitals, HVAC electrical cost is typically around 52% of the total. Therefore, operating efficiency is important, but of higher priority is a healthy indoor environment that minimizes Hospital Acquired Infections (HAI).

The Rockwall Company stands ready to help your medical facility comply with Texas state standards and superior operating efficiency.

How may we help you?

John White
(972) 771-3514