IAQ in health care settings

Engineers should use evidence-based design when engineering air systems in hospitals and health care facilities to ensure top-notch indoor air quality (IAQ).

By James Paul, PE, LEED AP, Peter Basso Associates, Troy, Mich. August 12, 2013

Learning objectives

  1. Understand data from Centers for Disease Control and Prevention in order to better design hospital air systems.
  2. Know which codes and standards pertain to indoor air quality in hospitals.

Underlying disease, organ transplants, chemotherapy, or other factors can lead to weakened immune systems, which increase the effects of disease-causing organisms in a hospital environment. For 1 in 20 inpatients, a visit to the hospital for can cause their illness to progress and, in some cases, become terminal. Hospitals by their nature contain a higher concentration of disease-causing germs and viruses than other environments.

In the past decade, the health care industry has made tremendous strides in reducing this negative trend. However, a level of threat still exists to patients, staff, and visitors. Hospital designers, builders, operators, and employees must work together to put an end to this threat. This article attempts to shed light on the role of the HVAC engineer in preventing hospital-acquired infections (HAIs).

Hospital-acquired infections

HAIs are significant contributors to the high cost of health care. According to a 2007 report, in 2002 there were more than 1.7 million HAIs in the U.S., costing between $28.4 billion and $33.8 billion and resulting in an estimated 98,987 deaths. Recently, the U.S. government has taken measures to provide additional stimulus to reduce the incidence of HAIs. The Affordable Care Act (ACA) of 2011 provides funding for public health initiatives to further improve outcomes and reduce the incidence of HAIs by doing the following: 

  • Improve infrastructure for preventing HAIs, mainly through reporting and data analysis
  • Develop new initiatives for preventing HAIs
  • Improve the National Health care Safety Network (NHSN) and electronic reporting of lab records to reduce data entry and improve record-keeping
  • Partner with state governments to increase the number of staff dedicated to HAI prevention.

The U.S. Centers for Disease Control and Prevention (CDC) seeks to provide evidence-based guidance for infection control and prevention. On the CDC website, the 15 most common diseases and organisms involved in HAIs are listed. The site also provides guidance for patients and health care professionals on how to prevent these diseases. A brief summary is shown in Table 1. Only two of the 15 disease-causing organisms described on the CDC website are airborne.

Table 1: Common diseases and organisms in hospital environments

More important, however, is the fact that the sources of HAI outbreaks are often not known. In a recent study in the American Journal of Infection Control, researchers studied a database of more than 1500 documented HAI outbreaks in order to shed light on the sources. Their findings revealed that in a significant portion of outbreaks the source of the infection could not be determined.


In 2008, ASHRAE, the American Society for Healthcare Engineering (ASHE), and the American National Standards Institute (ANSI) adopted a common standard for the ventilation of health care facilities. In 2010, the Facilities Guidelines Institute (FGI) adopted ANSI/ASHRAE/ASHE Standard 170: Ventilation of Health Care Facilities as part of its Guidelines for Construction of Healthcare Facilities. The FGI Guidelines are increasingly being adopted around the world as the standard to which health care facilities must be designed. Standard 170 is written in enforceable code language. The standard is a consensus document and is continuously updated with interpretations and new findings. It is scheduled to be re-issued in 2014 as part of the revised FGI Guidelines.

The main topics covered in the standard are systems, equipment, space ventilation, planning, construction, and system startup. Systems addressed are emergency power, ventilating, heating, and cooling. Equipment requirements include air handling unit design, cooling tower placement, humidifiers, and air distribution devices. Planning, construction, and startup requirements apply to the HVAC systems serving surgery and critical care spaces.

All the provisions of the standard must be followed; however, the main features that affect HVAC system design are: minimum filter efficiencies, selection and location of supply air outlets, minimum outdoor air and total air ventilation rates, air pressure relationships, and temperature/humidity requirements for various types of spaces and ease of maintenance/cleaning. While applying the standard it is critical that HVAC system designers understand the classification of the spaces they are designing for. This requires dialogue with the architects, infection control personnel, and the building’s users to understand how the spaces will be used.


Bacteria and viruses can survive on droplets or particles 5 microns or smaller and can remain airborne indefinitely; therefore, air filtration is a primary means of reducing the concentrations of airborne bacteria in hospitals. Studies have shown that 99.9% of airborne bacteria in hospitals are removed by having 90% to 95% efficient (MERV 14) filters in the air handling units. In air handling systems serving inpatient care areas, two filters are required, with the first filter upstream of fans and coils and the second filter downstream of the cooling coils and supply fan.

Standard 170 requires HEPA (MERV 17) filters for protective environment rooms designed for patients with high susceptibility to infections. The standard does not address the use of ultraviolet radiation lights inside of air handling units as a means for killing pathogens; however, some studies have shown that these devices can be effective in preventing microbial growth.

Air and ventilation requirements

Operating rooms, protective environment rooms, and burn units are required by the standard to have Type E, non-aspirating air devices located in the ceiling. These are typically laminar flow diffusers. Nonlaminar flow diffusers can entrain room air into their supply pattern and potentially carry airborne bacteria toward the patient. The diffusers should be located directly over the patient and the return or exhaust should be located away from the patient with two devices at or near floor level located diagonally opposite to maintain the laminar airflow.

Outdoor air usually contains very low densities of airborne bacteria. If the air handling unit’s outdoor air intake is located away from sources of contamination and is well maintained, the use of outdoor air ventilation to reduce concentrations of airborne bacteria indoors can be effective. Unfortunately, there is little empirical data showing the relationship between ventilation rates and infection rates.

Because the capital costs and energy costs to heat and cool outdoor air can be high, it is expected that the next release of Standard 170 will reduce some of the air change rates of outdoor air that have been used traditionally.

When the total air circulation rate is increased and the air handling system is equipped with high-efficiency filters, the filters will remove more airborne bacteria from the airstream. There is some theoretical research, mostly on surgical settings, showing a potential for reduction in airborne particle counts and associated surgical site infections. There is very little empirical evidence to show a linkage between increased total ventilation rates and decreased infection rates. In fact, there is actually clinical trial evidence showing an increase in surgical site infections at hospitals using laminar airflow (rather than turbulent airflow) in surgery theaters. This is significant because Standard 170 currently recommends laminar airflow in surgery theaters despite the clinical trial evidence to the contrary. Clearly, more clinical trial research is needed in order to recommend ventilation systems for surgery theaters based on facts, not just theories.

The simple approach of moving air from clean to less clean areas is the principle behind the required pressure relationships in Table 7-1 in Standard 170. The table classifies more than 70 types of areas as to whether they should be in a positive or negative pressure relationship with respect to adjacent areas. For some areas there is no requirement—the space maybe positive, negative, or neutral with respect to adjacent areas. For areas where there is a pressure relationship requirement, the return or exhaust air must be ducted.

Compared to earlier regulations, Standard 170 has reduced the requirements for adding humidification to many areas. For clinical reasons, most surgery and critical care areas require a minimum of 20% relative humidity, but all occupied areas are required to maintain humidity levels below 60%, which also helps to reduce concentrations of airborne pathogens.

Standard 170 requires air handling units and air distribution systems to have conveniently located access doors, panels, or other means for access and cleaning. For the most part, compliance with ASHRAE Standard 62.1-2010: Ventilation for Acceptable Indoor Air Quality is sufficient. This applies to the design, installation, start-up, operation, and maintenance of all air handling systems.

The future of HAIs in health care

It is clear that more empirical research is needed regarding the causes of HAI outbreaks. The organisms that cause HAIs are produced in many sources, but the role of the infection control professional should be to defeat the methods by which they are transmitted to patients, caregivers, and visitors.

Until more empirical information is available, the role of the HVAC engineer in infection control should be to use accepted standards to reduce the likelihood that the HVAC systems will promote the growth or transmission of these organisms by using accepted design and construction standards.

Every health care facility is unique. Designers of these facilities must become familiar with all the requirements of Standard 170 and should work closely with the clinical and maintenance staff to ensure that the requirements for providing a safe, reliable, and clean environment are met.

Jim Paul is a vice president with Peter Basso Assocs. He has significant experience in mechanical engineering design and is knowledgeable in the design of complex HVAC, plumbing, and fire protection systems for a variety of project types with a particularly strong emphasis on health care facility design.

Further reading

1. Estimating Health Care-Associated Infections and Deaths in U.S. Hospitals, 2002 (Klevens, Edwards, Richards, Horan, Gaynes, Pollock, Cardo); Public Health Reports, Vol. 122, March-April 2007.

2. The Direct Medical Costs of Health Care-Associated Infections in U.S. Hospitals and the Benefits of Prevention (Scott RD II), Centers for Disease Control and Prevention, March 2009.

3. TABLE 1 – Common Diseases and Organisms in Hospital Environments, (James Paul); Peter Basso Associates, compiled from:

Comparison of operating room ventilation systems in the protection of the surgical site (Memarzadeh F and Manning A), ASHRAE Transactions, V. 108, Pt. 2, 2002.

Effects of Operating Room Geometry and Ventilation System Parameter Variations on the Protection of the Surgical Suite (Memarzadeh F and Jiang Z). IAQ, Critical Operations: Supporting the Healing Environment through IAQ Performance Standards, 2004.

4. Operating Room Ventilation with Laminar Airflow Shows No Protective Effect on the Surgical Site Infection Rate in Orthopedic and Abdominal Surgery (Christan Brandt, MD, Dorit Sohr, PhD, Franz Daschner, MD, PhD, Petra Gastmeier, MD, PhD, and Henning Rüden, MD, PhD), Annals of Surgery, Vol. 248, Number 5, November 2008.

5. ASHRAE Standard 170-2008 Section 7.4.1.

6. New Theories on Plumbing and HVAC Systems (Hermans, Richard, PE, HFDP), ASHE PDC Summit, February 27, 2013.

7. Guidelines for Environmental Infection Control in Health-Care Facilities, Recommendations of CDC and the Health care Infection Control Practices Advisory Committee (HICPAC), U.S. DHHS CDC–2003.

8. Where should one search when confronted with outbreaks of nosocomial infection? (P. Gastmeier, S. Stamm-Balderjahn, S. Hansen, I. Zuschneid, D. Sohr, M. Behnke, R. Vonberg, H. Ruden), American Journal of Infection Control, Volume 34, Issue 9, pages 603-605.