What IPs Need to Know About Ventilation

Infection Control Today, Infection Control Today, July/August 2021 (Vol. 25 No. 6), Volume 25, Issue 6

In the new normal after COVID-19, infection preventionists will need to become more knowledgeable about and involved in the functionality of air ventilation in health care settings.

The importance of ventilation in health care and non–health care settings has become evident amid the COVID-19 pandemic. Decreased performance of health care facility heating, ventilation, and air conditioning (HVAC) systems, including filter inefficiencies, improper installation, and poor maintenance, can contribute to the spread of health care-associated infections (HAIs), according to various research findings. Infection preventionists (IPs) who have a basic understanding of ventilation systems and associated risks might feel more confident and comfortable collaborating with architects, facility designers, and engineers. COVID-19 is 1 of many diseases transmitted from a respiratory source.

How does air play a role in disease transmission? The aerosolized transmission of disease happens through droplet and airborne means.1 Droplet transmission of disease can occur when

bacteria or viruses travel on relatively large respiratory particles that people sneeze, cough, or exhale through talking, singing, or shouting. These droplets travel short distances (usually less than 6 ft) before settling and can be loaded with infectious particles. Infectious particles that contact a person’s eyes, nose, or mouth can cause infection. If they fall on surfaces, they can be transferred onto the hand of someone who unknowingly rubs their eyes, nose, or mouth, causing possible self-contamination.

Airborne transmission is defined as infections transmitted through exposure to infectious, pathogen-containing, small droplets and particles suspended in the air over long distances that can persist in the air for long times.2 The main differences between these 2 transmission types come down to particle size and distance traveled: Droplets are thought to be larger and don’t travel as far; aerosols are smaller and can travel quite a distance. The Centers for Disease Control and Prevention (CDC) recommended isolation precautions for diseases like measles or chicken pox, which are airborne, and mumps, which is transmitted by droplet.3 IPs understand the importance of these precautions but may know less about how these measures work to protect patients and staff.

“The importance of good air quality in controlling and preventing airborne infections in health care facilities cannot be overemphasized,” wrote Anjali Joseph, PhD, the director of research at the Center for Health Design, a company that plans health care facilities, in a 2006 study. “Providing clean filtered air and effectively controlling indoor air pollution through ventilation are 2 key aspects of maintaining good air quality.”4

Knowing how important air quality and ventilation are, IPs might better converse with health care architects, designers, and facilities groups if they understand the basics of clean, filtered air and ventilation. Basic terms include the following:

  • Filtration refers to controlling air quality at the source and is defined as the physical removal of particulates from the air, a key step toward achieving acceptable indoor air quality by removing contaminants.
  1. Filters that are 90% efficient are sufficient for most patient care areas in ambulatory care facilities and hospitals, including the operating room.
  2. High-efficiency particulate air (HEPA) filters should be used for special care areas of the hospital such as surgical areas, burn units, and protective environments for patients who are immunocompromised.
  3. HEPA filters are required to be 99.97% efficient for removing particles 0.3 μm or larger (eg, COVID-19 = 0.06-0.2 μm; measles = 0.2 μm; Myobacterium tuberculosis = 1.0 μm). This means for every 10,000 particles sized 0.3 μm in diameter, HEPA filters only allow 3 particles to pass through.
  4. The principle of Brownian motion applies to HEPA filters for particles less than 0.3 μm. These smaller particles bounce off other molecules when they collide and move in random patterns. They hit the HEPA filter fibers and get stuck. So HEPA filters actually better capture these smaller particles.5
  • Ventilation guidelines are defined in terms of air volume per minute per occupant. They assume that the people in the room and their activities are responsible for most of the contaminants in the space.
  1. Supply vents are where conditioned air blows into the room, usually from the ceiling; return vents (exhaust) pull the room air in, usually near the floor.
  2. Most ventilation rates for health care facilities are stated as room air changes per hour (ACH). ACH refers to the number of times that air gets replaced in each room every hour, mixing in outside air to dilute microbial contamination.
  3. Peak efficiency for particle removal in an airborne infection isolation room (AIIR) occurs between 12 and 15 ACH, per CDC guidelines.6
  4. Ventilation rates vary with different patient care areas of a health care facility. Look at the most recent edition of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 170, Ventilation of Health Care Facilities, which is now integrated into the Facility Guidelines Institute (FGI) Guidelines for Design and Construction of Health Care Facilities.7
  5. ASHRAE Standard 170 speaks to the removal of airborne pathogens through a combination of filtration, direct exhaust of contaminated air, and dilution through introduction of outdoor air. Most IPs are familiar with ASHRAE and FGI. These resources are highly recommended and often consulted, especially when renovation and construction are planned for a facility.
  6. Be aware that the Joint Commission, through its Environment of Care standards, also audits ventilation systems for appropriate pressure relationships, air exchange rates, and filtration efficiencies because it views the proper design and maintenance of isolation rooms as vitally important in preventing the transmission of disease through the air for patients, staff, and visitors.
  7. Interestingly, studies have found that ASHRAE 170 2008 and the 2005 CDC guidelines recommendations for minimum ventilation rates of 12 ACH for hospital insolation rooms are not necessarily the optimum ACH to control infection transmission. Increasing ventilation airflow rate dilutes concentrations but does not increase ventilation effectiveness. The results of a study by Memarzadeh and Xusuggest that the most important contributing factor to contaminant transmission in an AIIR is the path between the patient and the exhaust, not the ACH.7 When this path is disrupted by airstreams, the contaminants can move to other to places in the room. If this path is not disrupted, then the contaminant is not likely to migrate, so changing the number of air exchanges does nothing for ventilation effectiveness.
  8. This is why thoughtful ventilation design and placement of exhaust and supply vents in relation to patient placement in the room are important. Simply introducing extra amounts of outdoor air hoping to dilute the room air may not be effective in providing protection from airborne infection because it does not increase ventilation effectiveness. It is very important to not block the path to the exhaust and returns.
  • Humidity is defined as the concentration of water vapor present in the air.
  1. The recommended relative humidity range for hospitals is between 40% and 60% (also check local standards). The minimum relative humidity level for an operating room should be 20% and the maximum level should be 60%, per ASHRAE Standard 170-2017, Ventilation of Health Care Facilities.8
  2. Relative humidity greater than 60% can promote growth of fungus.9
  3. Humidity control allows patients and health care workers to breathe easily, avoiding dryness of the respiratory tracts and reducing the amount of airborne dust in closed spaces.
  4. According to Stephanie Taylor, MD, an infection prevention consultant at Harvard Medical School, “At low relative humidity, indoor air was strongly associated with higher infection rates. When we dry the air out, droplets and skin flakes carrying viruses and bacteria are launched into the air, traveling far and over long periods of time.…In addition to this increased exposure to infectious particles, the dry air also harms our natural immune barriers which protect us from infections.”10
  5. Ideal humidity conditions can also improve working conditions for health care workers and help medical electrical equipment operate properly by reducing static electricity (Figure 1 demonstrates the relationship between microorganisms and humidity.)11
  • Room pressurization involves technology that controls movement of air contaminants by causing drafts between spaces, usually rooms and hallways. Pressurization can be positive or negative.
  1. In health care, certain rooms are positively or negatively pressurized with respect to surrounding areas.
  2. Negative pressure (depressurized) means airflow into the room.
  3. AIIRs are set at negative pressure with respect to adjacent areas to prevent airborne microorganisms in the room from entering hallways, corridors, and other areas.
  4. AIIRs provide negative pressure in the room (so that air flows under the door gap into the room), with an airflow rate of 6 to 12 ACH (6 ACH for existing structures; 12 ACH for new construction or renovation) and direct exhaust of air from the room to the outside of the building or recirculation of air through a HEPA filter before returning to circulation.12
  5. Positive pressure means air flows out of the room (pressurized).
  6. Protective environment rooms, used to protect neutropenic patients, are set at positive pressure to keep airborne pathogens in adjacent spaces or corridors from coming into and contaminating the airspace.
  7. The 2014 FGI Guidelines/Standard 170-2013 provides lists of rooms that should be positively or negatively pressurized with respect to surrounding areas. (Figure 2.)

• Self-closing doors are mandatory for both of these areas to help maintain the correct pressure differential.
• Variable pressure rooms (rooms where the ventilation can be manually switched between positive and negative pressure) are no longer permitted in new construction or renovation, and their use in existing facilities has been discouraged because of difficulties in assuring the proper pressure differential, especially for the negative pressure setting, and the potential for error associated with switching the pressure differentials for the room. Continued use of existing variable pressure rooms depends on collaboration between engineering and infection control. Both positive- and negative-pressure rooms should be maintained according to specific engineering specifications.13
• ASHRAE Standard 170, Ventilation of Health Care Facilities, requires each isolation room to have a permanently installed visual device or mechanism to constantly monitor the air pressure differential of the room when occupied by a patient who requires isolation. The CDC states that airflow direction should be maintained and verified, preferably daily, even when the room is unoccupied, using either a visual means of indication (eg, smoke tubes and flutter strips) or manometers.14

5. Temperature is defined as the degree of hotness or coldness measured on a definite scale.
• In health care, normal operating temperature set points should be maintained based on the existing licensing requirements for the space use and occupancy. Consult FGI for specific guidance.
• ASHRAE Standard 170 requires temperatures ranging from 68 °F to 75 °F for most patient care areas. Cool temperature standards (68 °F–73 °F [20 °C–23 °C]) are usually related to operating rooms, clean workrooms, and endoscopy suites.
• Temperatures above about 75 °F (24 °C) appear to universally lower airborne bacterial survival.15
• Virus survival, in general, decreases as temperatures rise. However, many factors, such as humidity, presence of organic matter, and whether they are DNA or RNA viruses, influence survivability. Much more research regarding temperature and its effect on viruses is ongoing.

As the above tutorial indicates, the current information on air in health care and public buildings can be overwhelming. The really heavy engineering and design details of ventilation are best left to the architects, engineers, and the facilities groups. But knowing the basics of air quality and air handling, as well as their relationship to possible disease transmission, could benefit IPs.

SHARON WARD-FORE, MS, MT(ASCP), CIC, FAPIC, is an infection prevention consultant in Chicago, Illinois. She is the Infection prevention adviser for Metrex Infection Prevention/Envista Holdings, an APIC Text clinical editor, and a contributing Editorial Advisory Board member of Infection Control Today®.


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  3. Type and duration of precautions recommended for selected infections and conditions. Centers for Disease Control and Prevention. Updated September 2018. Accessed May 12, 2021. https://www.cdc.gov/infectioncontrol/guidelines/isolation/appendix/type-duration-precautions.html
  4. Joseph A. Impact of the environment on infections in healthcare facilities. Center for Health Design. July 2006. Accessed May 14, 2021. https://www.healthdesign.org/system/files/Joseph_The_Impact_of_Environment_2006.pdf
  5. Scherzer U, Brown C. Efficiency of HEPA filters. Hamilton Medical. March 18, 2020. Accessed May 14, 2021. https://www.hamilton-medical.com/en_US/E-Learning-and-Education/Knowledge-Base/Knowledge-Base-Detail~2020-03-18~Efficiency-of-HEPA-filters~d5358f88-753e-4644-91c6-5c7b862e941f~.html
  6. Centers for Disease Control and Prevention (CDC) and Healthcare Infection Control Practices Advisory Committee (HICPAC). Guidelines for environmental infection control in healthcare facilities. recommendations of CDC and the Healthcare Infection Control Practices Advisory Committee (HICPAC). MMWR Recomm Rep. 2003;52(RR-10):1-42.
  7. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). Health care facilities resources. ASHRAE website. https://www.ashrae.org/technical-resources/bookstore/health-care-facilities-resources
  8. Memarzadeh F, Xu W. Role of air changes per hour (ACH) in possible transmission of airborne infections. Build Simul. 2012;5:15-28. doi:10.1007/s12273-011-0053-4
  9. Modes of transmission of airborne diseases. Centers for Disease Control and Prevention. 2013. Accessed May 20, 2021. https://www.cdc.gov/infectioncontrol/guidelines/environmental/background/air.html#c1
  10. Freda A. Managing operating room temperature and humidity. Health Facilities Management. June 24, 2020. Accessed May 16, 2021.https://www.hfmmagazine.com/articles/3939-managing-operating-room-temperature-and-humidity
  11. Diamond F. Q&A: Hospital airflow plays a crucial role in infection prevention. Infection Control Today®. Published online June 26, 2020. Accessed May 18, 2021. https://www.infectioncontroltoday.com/view/q-and-a-hospital-airflow-plays-a-crucial-role-in-infection-prevention
  12. Air humidification in hospitals and healthcare structures with the objective of saving energy. Carel Compendium. 2013. Accessed May 20, 2021. https://www.carel.com/documents/10191/0/%2B4000021EN/8ad03ec9-6f9f-4216-8395-b2f95325671f
  13. Guidelines for environmental infection control in health-care facilities. Centers for Disease Control and Prevention. 2003. Accessed May 12, 2021. https://www.cdc.gov/infectioncontrol/pdf/guidelines/environmental-guidelines-P.pdf
  14. Herrick M. Planning and maintaining hospital air isolation rooms. Health Facilities Management. February 1, 2017. Accessed May 25, 2021. https://www.hfmmagazine.com/articles/2671-planning-and-maintaining-hospital-air-isolation-rooms
  15. Tang JW. The effect of environmental parameters on the survival of airborne infectious agents. J R Soc Interface. 2009;6(suppl 6):S737-S746. doi:10.1098/rsif.2009.0227.focus
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