IAQ, infection control in hospitals
Air treatment, filtration, cleaning
All air delivery systems require filters to remove particles from the airstream to keep equipment clean and functional as well as reduce dust and contaminant distribution through the air systems. ASHRAE Standard 170-2008 identifies the minimum number and location of filter banks required with filter minimum efficiency reporting value (MERV) ratings based on space designation by function. Two filter banks are required for patient care areas. See Table 1 for filter MERV ratings and particle removal efficiency. The higher the potential for infection or the more immunodeficient a patient is (and therefore the cleaner the air required), the higher the required filter rating. Studies cited by Memarzadeh contained in the Aerobiological Engineering Handbook on Airborne Disease and Control Technologies (McGraw Hill) by W.J. Kowalski in 2005 showed multiple options for increasing the ability of specified filters to prevent spores from entering a building, including biocidal filters, electrostatic filters, carbon adsorbers, low-level ozonation, and negative air ionization.
In addition to filtration, pressurization control is used to prevent migration of fungi spores and bacteria from one area to another. Systems that use 100% outside air without any recirculation have been used successfully to minimize contaminants. HEPA filtration has also been used to improve IAQ by trapping fungi spores and most bacteria. Care must be taken in the placement of filters in equipment and air systems to keep filters dry, as a wet filter may cause spores to grow through the filter media, creating more problems than the filter was supposed to solve. The proper code-specified minimum distances and good engineering practice for filter installation from cooling coils and humidifiers must be maintained to prevent moisture collection.
As with any HVAC strategy, the design and implementation must be in line with any owner/user established project microbial and particulate IAQ goals. To achieve the goals using particulate filters, first the particle size must be known and then the minimum filter MERV rating chosen to trap and remove the particles from the airstream. ASHRAE Standard 52.2-2007: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size identifies the minimum MERV rating for preventing infiltration of fungi spores at 9 to 12, and bacteria at 13 to 16. Studies also indicate that the use of HEPA filters is most effective in positive pressure rooms such as operating rooms and protective isolation rooms. Additionally, research shows that for HEPA filters to be truly effective, they must be used with higher air change rates and controlled air velocities across the filter to maintain effectiveness, along with tightly sealed rooms. This is one of the reasons that Standard 170 does not require HEPA filters in other locations. Viruses are much smaller than bacteria and require specialized air treatment, and thus are beyond the scope of this article.
All filters must be tight-fitting within their holding frames to properly function. Air bypass around any filter increases the likelihood of airborne pathogens to be distributed into the occupied space. Type 8 holding frames for both prefilter and final filters with proper fastening clamps and devices will minimize any bypass and increase the filter effectiveness. Odors and VOCs have been successfully removed using carbon adsorbers or higher outside air dilution rates.
The use of UVGI in combination with lower MERV rating filters has shown to be an effective alternative to HEPA filters in less critical applications. UVGI has also proven effective to control microbial growth on cooling coils, and continuous UV exposure will inhibit fungal growth in airstreams; however, the effectiveness is dependent on the contact time and distance to the UV source. Multiple air passes will increase the exposure and the potential benefit of UVGI. Monitoring and reporting (alarming) when filters reach their recommended pressure drop limitations along with regularly scheduled preventive maintenance will aid in reducing the risk of infections. Proper maintenance and tight-fitting filters are a must. Monitoring through differential pressure sensors to the BAS is a cost-effective method to provide alerts to proper filter maintenance.
Air-handling systems providing air to patient care areas should have interior surfaces that are cleanable, non-eroding, and do not contribute to microbial growth. Consider tight-fitting and sealed casings to prevent unwanted air infiltration and prevent positive pressure air leakage. The components should be arranged to prevent moisture carry-over and have double-sloped drain pans to prevent any standing water or moisture traps. The pan drains should be properly sized, trapped, and installed to allow water to drain properly. Filters and other components need to be accessible and maintainable. Space should be provided between components to allow for cleaning.
Does that sound too easy? Yes, but due to cost constraints, often some of the features are omitted, resulting in lesser quality and certainly less IAQ—and a potential risk of higher future infection rates.
Any new construction or renovation project will require air-moving equipment and ductwork. All equipment should remain clean before and after it arrives on the job site. All ductwork should be sealed from the fabrication shop through the installation to avoid construction dust from entering and contaminating the duct. Air handling systems should be operated only with the scheduled filters in place including all final filters. This action will keep the downstream duct clean and lessen the risks of introducing contaminants upon start-up.
Once systems are operational, a continual program to measure IAQ is necessary to identify and remediate any IAQ issues due to odors, VOCs, airborne pathogens, and other related issues.
The many codes and standards governing healthcare can be confusing. Stepping back and focusing on the goal of maintaining a high IAQ level and minimizing the spread of airborne pathogens through the HVAC system will allow a good, concise design to be established and minimize the potential for increased infections.
Designing and specifying good quality equipment that is cleanable and maintainable, as well as a sustainable system that is reliable and with the appropriate level of monitoring instrumentation, will help meet the goal. It is important to have the owner perform a risk assessment on each project and to implement infection control practices. Collaboration and communication between the HVAC engineer and facility manager along with the rest of the project team play important roles in creating a safe patient care environment.
J. Patrick Banse is senior mechanical engineer with Smith Seckman Reid. He has more than 30 years of experience in the consulting engineering field with the last 25 years in healthcare design and engineering. He is responsible for HVAC, plumbing, and fire protection design for hospital and healthcare projects. Banse is a member of the Consulting-Specifying Engineer editorial advisory board.
ASHRAE. ASHRAE Standard 62.1-2010, Ventilation for Acceptable Indoor Air Quality.
ASHRAE. ASHRAE Standard 52.2-2007, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size.
ASHRAE. ASHRAE Standard 170-2008, Ventilation of Health Care Facilities.
Facility Guidelines Institute. 2010 Guidelines for Design and Construction of Health Care Facilities.
Memarzadeh, Farhad. The Environment of Care and Health Care-Associated Infections. An Engineering Perspective. ASHE Monograph
Mills, Frank. Indoor Air Quality Standards in Hospitals. Business Briefings: Hospital Engineering & Facilities Management, 2003.