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ASHRAE SSPC 170 Natural Ventilation Task Group: Position Paper on Natural Ventilation in Health Care Facilities.


The current version of ANSI/ASHRAE/ASHE Standard 170 (2013) precludes natural ventilation in all spaces in health care facilities. This task group suggests it should be allowed in most health care spaces, except for operating rooms, procedure suites, sterile core areas, interventional radiology or cardiology spaces, airborne isolation areas, and protective environments. Natural or mixed-mode ventilation designs may offer some benefits. Among the most likely are energy reduction and enhanced occupant satisfaction. Less likely benefits may include enhanced indoor air quality and a more beneficial microbiome. When considering natural ventilation in health care, designers must fully address the fundamental challenges of space appropriateness, climate appropriateness, acoustics, security, and outdoor air quality. Projects implementing natural ventilation should anticipate commissioning challenges. Many commissioning and air balance providers in the United States are not experienced with natural ventilation system. Some U.S. designers or owners may be adverse to natural ventilation because of its newness, added costs, or perceived impacts on clinical outcomes.


A task group was convened as a subgroup under ASHRAE's Standing Special Project Committee 170 (SSPC 170), to investigate the applicability of natural ventilation in U.S. health care facilities (HCF) and provide recommendations to SSPC 170. This paper is a work product of the task group, representing the first recommendation to SSPC 170 on the topic. SSPC 170 is the cognizant body responsible for the maintenance of ANSI/ASHRAE/ASHE Standard 170, Ventilation for Health Care Facilities (ASHRAE 2017d). The task group convened in late 2014, and met and corresponded through 2015 and 2016. As of January 2017, SSPC 170 has not yet reviewed nor endorsed this position paper.


Natural ventilation is defined as the transport of fresh air to conditioned spaces through nonmechanical means. Prior to the advent of modern mechanical ventilation systems, natural ventilation was the norm and thus has a longstanding history in health care architecture (Nightengale 1859). With the advent of modern mechanical ventilation systems, passive ventilation through natural means has been largely eliminated in contemporary U.S. health care buildings. Provisions in current codes, standards, and state regulations limit use of operable windows or natural ventilation.

Health care spaces are cited as good opportunities for natural ventilation (Levin 2008), and the practice of naturally ventilating health care spaces has been adopted in contemporary installations outside of the United States. Modern health care facilities standards in the United Kingdom (Department of Health 2007), Germany (DIN 2009), and Norway (Burpee and McDade 2014) allow or encourage natural ventilation. World Health Organization (WHO) standards also encourage the practice (Atkinson et al. 2009). Resource-strained regions often use natural ventilation as an alternative to the expenses associated with mechanical systems.

Current State of U.S. Standards

The current version of ANSI/ASHRAE/ASHE Standard 170 precludes natural ventilation in health care facilities. Section 6.4 states the following:
...All of the air provided to a space shall be filtered in accordance
with Table 6.4, except as otherwise indicated in Section 7.1 for spaces
that allow recirculating HVAC room units.

Section 7.4 states the following:
The entire minimum outdoor air changes per hour required by Table 7.1
for the space shall meet the filtration requirements of Section 6.4.

These requirements may not have been intended to preclude natural ventilation. They may have been intended to prevent introducing unfiltered outdoor air into the space through a heating, ventilation, and air conditioning (HVAC) system. The U.S. architectural guidelines allow the use of operable windows and require consideration of natural ventilation as a means of energy savings where conditions permit (FGI 2014). However, some may interpret the ANSI/ASHRAE/ASHE Standard 170 sections to not allow natural ventilation.

Without clarification, or a modification, jurisdictions adopting ANSI/ASHRAE/ASHE Standard 170 may be prohibited from outdoor air delivered directly into a health care space. Many jurisdictions adopt the standard only in part or in principal, and may allow deviations from it.

ASHRAE's indoor air quality standard, ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, allows natural ventilation. This may provide a model for future revisions to the health care standard. In ANSI/ASHRAE Standard 62.1, filters are required for outdoor air only when outdoor air quality is unacceptable for direct introduction into a building. ANSI/ASHRAE Standard 62.1 section requires filters with a minimum efficiency reporting value (MERV) of 6 if the outdoor air standard for course particulate matter, PM10, is exceeded. ANSI/ASHRAE Standard 62.1 section requires MERV 11 filters if the outdoor air standard for fine particulate matter, [PM.sub.25], is exceeded. [PM.sub.10] and [PM.sub.25] level standards are cited to the national ambient air quality standard (EPA 2006). ANSI/ASHRAE Standard 62.1 also requires filtration of air through wetted coils. ANSI/ASHRAE Standard 62.1 section 5.8 requires MERV 8 filters on all air passing through cooling coils (ASHRAE 2013b).

It should be noted that compliance with ANSI/ASHRAE Standard 62.1 does not allow facilities that exclusively use natural ventilation systems. ANSI/ASHRAE Standard 62.1 section 6.1.3 states that the standard's natural ventilation procedure is only permitted to be used "in conjunction with mechanical ventilation systems."

Energy Reduction

Natural ventilation is commonly cited as an energy savings opportunity in scientific literature on commercial buildings (Brager and Bordeson 2010; CBE 2013).

On the whole, UK hospitals, which commonly employ natural ventilation, are not more energy efficient than U.S. hospitals (CBECS 2012; Short and Al-Maiyah 2009). A 2009 study of multiple design strategies in UK hospitals showed that an advanced natural ventilation (AVN) strategy held high potential for optimal energy performance. The strategy has also been proposed as an energy opportunity for refurbishments (Short et al. 2010).

Natural ventilation has the potential to save energy if implemented with stringent control protocols that limit window opening during specific inside/outdoor temperature differentials and if air intakes (windows, dampers, etc.) are controlled through control systems. If the natural ventilation system can replace mechanically conditioned air change rates, significant energy savings can be realized. Additionally, if occupant temperature comfort ranges are expanded, energy could be conserved in both heating and cooling seasons.

Occupant Satisfaction

Advocates of natural ventilation may claim a higher degree of occupant satisfaction can be realized. There has been significant research into occupant comfort in the naturally ventilated environment, which has informed ASHRAE's design standards (Brager and DeDear 2000). Such research shows that occupants of naturally ventilated spaces have an expanded comfort range; they report feeling comfortable across a wider range of temperatures, when compared to models derived from previous research on conditioned spaces. This expanded comfort range, sometimes called adaptive comfort, is incorporated into ASHRAE standards (ASHRAE 2013a) and associated tools. Members of the task group have reported feedback from facilities to this effect: "The patients like it." There may be limitations to the application of the adaptive comfort premise. Based on its source research, it may only apply to spaces where natural ventilation is under the occupants' control. However, it has been applied more liberally to spaces where natural ventilation is introduced through controlled openings (CIBSE 2000, 2005).

For most U.S. climates, even mild climates, under normal design conditions natural ventilation cannot be used exclusively to assure occupant satisfaction (McConahey 2008). In most cases, spaces will use mixed-mode ventilation systems, which combine natural ventilation and mechanical ventilation. There are two broad categories of mixed-mode designs: contingent mixed mode, where either natural ventilation or mechanical ventilation are used in a given hour, or concurrent mixed mode, where natural and mechanical ventilation systems are used together (CIBSE 2000).

Natural ventilation is often thought to be aligned with the consideration of psychological aspects of the built environment and connection to the outdoors. Research has acknowledged some evidence on the benefits of natural ventilation (Hobday and Dancer 2013). Patients may also benefit from the natural light from windows (Edwards and Torcellini 2002).

Alternative Means of Compliance

Allowing engineers multiple methodologies of compliance could add value to design outcomes. Several ASHRAE standards currently feature multiple compliance paths. ANSI/ASHRAE Standard 62.1 allows three paths to compliance: ventilation rate procedure, the natural ventilation procedure, and the indoor air quality procedure (ASHRAE 2013b). ANSI/ASHRAE/IES Standard 90.1 includes a prescriptive method as well as an energy cost budget method (ASHRAE 2013c).

The topic of natural ventilation has some popularity among engineers and architects in the U.S. It seems likely that, if it were permitted, design professionals would exercise the option.

Indoor Air Quality

Outdoor air quality may be "better" than indoor air quality. It has been asserted that sick building syndrome is 30% more frequent in air-conditioned buildings (Seppanen and Fisk 2002). This may be a result of operators eliminating outdoor air to save cost, energy, or reduce needed capacity.

There have been estimates that broader use of natural ventilation could have positive health and economic benefits (Dutton et al. 2013).

Other Benefits of Natural Air

Some studies have indicated that the bacterial composition, or microbiome, of naturally ventilated spaces may be substantially different than that of mechanically ventilated spaces. There is some speculation that the difference in microbiome maybe positive (Kemble et al. 2012; Green 2011). For example, Jessica Green's work in health care spaces indicates that the microbiome of a mechanically ventilated patient room is more "human like" and a naturally ventilated patient room has greater microbiome diversity, which is thought to be a healthier environment. While the microbiome topic is a budding area of research inquiry, there seems to be, of yet, insufficient evidence to conclude either mechanical or natural ventilation systems are more or less beneficial than the other.

Resilience and Contingency Planning

A facility that has both natural ventilation and mechanical ventilation systems in place could be more resilient. A long-term UK study of naturally ventilated facilities found that they would be more able to provide comfortable space changes without increasing energy use when subjected to models of climate change (Cooket al. 2013). Natural ventilation maybe used as either a primary or redundant form of ventilation. In either case, care should be taken to provide a full system design. Operable windows alone may not be a sufficient solution.

Natural ventilation systems could be used as a contingency in the event of mechanical system failure in mild climates. Such a strategy could significantly reduce capital investment in emergency power systems. One owner in our group reported that up to 60% of generators sizing is for ventilation fans or systems. Hospitals in the United Kingdom reportedly have comparatively small generator systems, a portion of which may be because of HVAC system requirements.


Natural ventilation represents a deviation from the traditional U.S. approach of a tightly controlled and conditioned space. As such, it may introduce variables into facility and system designs that must be accounted for.

Appropriateness to Specific Spaces

There are spaces in a hospital that are likely wholly inappropriate for natural ventilation in resource-developed countries. Those spaces include the following:

* Operating rooms (ORs)

* Sterile core

* Procedure suites

* Interventional radiology/cardiology

* Airborne isolation rooms

* Protective environments

* Intensive care units

There are also some spaces where natural ventilation could be readily used, such as lobbies, cafeterias, offices, and other nonclinical spaces.

A 2014 survey of international operating room HVAC standards shows that most countries treat ORs as clean spaces (Montanya et al. 2014). Filtered air is typically required. Some countries have adopted measurable levels of cleanliness, while others specify only HVAC system attributes.

Protective environments for severely immunocompromised patients also require clean air. Infections have been observed at levels of fungal contamination above 1 cfu/[m.sup.3] (Vonberg and Gastmeier 2006). High-efficiency particulate air (HEPA) filters are typically required to achieve this level of cleanliness.

Airborne isolation rooms for patients with airborne diseases are sometimes designed with natural ventilation as a source of supply air. Some have suggested this may be beneficial because ventilation rates tend to be higher in naturally ventilated spaces (Escombe et al. 2007). Air in an airborne-isolation space is normally considered contaminated and should be exhausted from the facility without mixing with any other air (CDC 2005).

If airborne isolation rooms are contained on a floor where natural ventilation is also present, designers should take precautions. The pressure in the airborne isolation room should be maintained through the range of pressures that will be present in the naturally ventilated floor.

The United States currently does not use natural ventilation anywhere in a hospital. The UK ventilation standard limits the application of natural ventilation in specialty spaces to patient wards and single patient rooms (Department of Health 2007). The German ventilation standard permits natural ventilation in most spaces, including airborne isolation rooms and minor interventional procedure rooms, excluding only operating rooms and protective environments (DIN 2009).

One task group member noted spaces that require exhaust for contaminant control could use natural ventilation. Natural ventilation air could act as the supply or makeup airstream for a mechanically exhausted space. This is common practice in hybrid natural and mechanical ventilation design.

Clinical Needs, Beyond Occupant Satisfaction

Occupants of health care spaces are thought to have unique temperature sensitivities, such that normal comfort design is inadequate. For the last 40 years, the U.S. design approach to health care spaces is to maintain spaces within a tight range of temperature and humidity. Some guidance has recommended spaces be maintained as tight as [+ or -]1.5[degrees]F (1.0[degrees]C) (Galson and Goddard 1968). The current guidance in ANSI/ASHRAE/ASHE Standard 170 indicates design temperature ranges of [+ or -]2.5[degrees]F (1.4[degrees]C) for most spaces (ASHRAE 2013d). The ranges in ANSI/ASHRAE/ASHE Standard 170, however, are minimum code requirements and should not be interpreted as operating ranges.

ASHRAE Handbook--HVAC Applications (2011a) includes a number of strong statements about the role of air conditioning in patient therapy. The text indicates temperature control goes beyond occupant satisfaction. The section Air Conditioning in Disease Prevention and Treatment begins as follows:
Hospital air conditioning plays a more important role than just the
promotion of comfort. In many cases, proper air conditioning is a
factor in patient therapy; in some instances, it is the major treatment.
Studies show that patients in controlled environments generally have
more rapid physical improvement than do those in uncontrolled
environments. Patients with thyrotoxicosis do not tolerate hot, humid
conditions or heat waves very well. A cool, dry environment favors the
loss of heat by radiation and evaporation from the patient's skin and
may save the patient's life.

The section goes on to advocate air-conditioning treatments for cardiac patients, individuals with head injuries, barbiturate poisoning victims, rheumatoid arthritis patients, chronic pulmonary disease patients, those needing oxygen therapy, and burn patients. The section includes two citations from 1960s clinical literature (Burch and Pasquale 1962; Walker and Wells 1961).

Based on this tradition, some owners or designers may choose not to use natural ventilation.

There has also been a traditional or popular concern over cross contamination between or among typical medical/surgical patient rooms. In some standards, codes, or design guidance typical patient rooms have been shown as negative to the corridor or equal pressure to the corridor. However, the risk of airborne transmission between typical nonisolation patient areas is quite low. The modern thinking of ANSI/ASHRAE/ASHE Standard 170 is that there is no requirement for pressure control in many of these spaces.

Climate Appropriateness

Designers must diligently determine whether a climate is appropriate for natural ventilation. A series of tests on the typical meteorological year weather data can be used for this purpose (McConahey 2008).

Application in inappropriate climates or during inappropriate hours could lead to extreme conditions in buildings, which may affect both comfort and biological growth. While the relationships between temperature, humidity, and biological growth is difficult to generalize (Memarzedeh 2011), most agree that extreme conditions should be avoided.

Risks in Unfiltered Outdoor Air

Some owners or designers may be concerned with the content of unfiltered outdoor air. That content could include pollen, pests (if natural ventilation openings permit), particles, exhaust or cooling tower discharge, or biological factors (Mohammed et al. 2013).

Noise introduced by natural ventilation openings should be considered (Germano et al. 2005). Assessment of acceptable noise levels in spaces should include a consideration of sensitivity. Occupants in zones where relaxation and rest are functional goals will have an increase sensitivity to noise. Occupants in working areas, consultation spaces, or task areas will be less sensitive. Design guidance for HVAC-related background noise can be found in Table 1 of ASHRAE Handbook--HVAC Applications (ASHRAE 2011b). Other design guidance on noise is published throughout the industry literature and can be represented by an acoustic consulting professional.

There has been concern expressed that natural ventilation might increase the prevalence of hospital-acquired infections (HAI). Because natural ventilation is rare or nonexistent in the United States and reasonably common in Europe, this concern can be cursorily tested by comparing the U.S. prevalence of HAIs (Magill et al. 2014) to those of Europe (ECDC 2013). At a cursory level, there is no indication that European ventilation practices increase HAI prevalence or risk. In both the United States and Europe, clean air systems are common in surgery rooms as a mitigation to surgical site infections. Outside the operating room, the most common infections are pneumonia, urinary tract, bloodstream, and gastrointestinal; the prevalence of these HAIs are not significantly different in the U.S. and Europe, nor is the prevalence of these infections likely to be affected by ventilation systems.

Environmental Cleaning Services

Among our members, there has been an expressed concern that openings to the outdoors could introduce dust and increase needed environmental cleaning services. General dust on indoor surfaces is caused by larger particles' natural tendency to fall out of the air, known as particle deposition. Particle concentrations are higher in outdoor air than in filtered indoor air (Burton et al. 2000). This would lead to an increased in deposited particles. The magnitude of the effect and its impact on housekeeping service needs is not known.


Commissioning of natural ventilation systems and sequences may be unfamiliar to U.S. practitioners. However, like any building system, commissioning is essential for successful operations. A 2009 case study of a Chicago university library showed some of the benefits and barriers of commissioning an advanced natural ventilation system (Lomas et al. 2009). Commissioning agents, controls system providers, and air balance contractors may not be experienced with natural ventilation designs.


Natural ventilation openings and operable windows can add both capital and operating cost to projects. Designers should fully assess cost implications.


Operable windows and natural ventilation openings may pose a security risk for unauthorized intrusion. Openings should be coordinated with the project's security planning.


Design Literature and Standards

The design approach for natural ventilation and mixed-mode ventilation includes assessments of ventilation and cooling requirements, climate, site and proximities, and outdoor air quality. Building design variables that can increase the potential for natural ventilation include mass, structure type, orientation, fenestration, and shading.

Design standards for English speakers include the following:

* A useful ten-point checklist for determining the applicability of natural ventilation and mixed-mode ventilation can be found in the ASHRAE Journal article "Mixed Mode Ventilation: Finding the Right Mix" (McConahey 2008).

* ASHRAE Handbook--Fundamentals includes a chapter on ventilation and infiltration, which covers some of the basics of natural ventilation (ASHRAE 2009).

* The Chartered Institute of Building Engineers (CIBSE) offers two valuable references on natural ventilation. CIBSE AM10, Natural Ventilation in Non-Domestic Buildings is a design guide for natural ventilation, including the relevant design calculations (2005). CIBSE AM13, Mixed Mode Ventilation, is a more detailed rational guideline on mixed-mode ventilation (2000).

* The WHO document, Natural Ventilation for Infection Control in Health Care Settings, includes a section on planning and natural ventilation strategy (Atkinson et al. 2009).

* The UK health care ventilation standard, Health Technical Memorandum 03-01: Specialised ventilation for health care premises, includes guidance for applying natural ventilation (Department of Health 2007).

* The German health care ventilation standard, DIN 1946-4 Ventilation and Air Conditioning--Part 4: VAC Systems in Buildings and Rooms Used in the Health Care Sector, includes guidance for application of natural ventilation (DIN 2009). The standard is available in English.

Planning Implications

Determining the applicability of natural ventilation for a project requires a number of considerations, many of which are related to the planning of the building. Design guidelines should be consulted. Figure 1 shows a sample rational analysis proposed by CIBSE. Narrow floor plans are a key prerequisite.

Natural ventilation thorough operable windows also requires consideration of safety measures. For occupant safety and accident prevention, a maximum operable window opening may be apropos (e.g., maximum 4 in. [100 mm] window openings).


The Natural Ventilation Task Group was a special work group tasked to provide recommendations to ASHRAE SSPC 170. This position paper, developed by the task group represents the first recommendation to SSPC 170 on this topic. SSPC 170 has not yet officially reviewed nor endorsed this position paper.

The current version of ANSI/ASHRAE/ASHE Standard 170 only permits mechanical ventilation in health care facilities. This work group suggests natural ventilation should be allowed in most health care spaces, except for operating rooms, procedure suites, sterile core areas, interventional radiology or cardiology spaces, airborne isolation areas, and protective environments.

Natural or mixed-mode ventilation designs may offer some benefits. Among the most likely are energy reduction and enhanced occupant satisfaction. Less likely benefits may include enhanced indoor air quality and a more beneficial microbiome.

When considering natural ventilation in health care, designers must fully address the fundamental challenges of space appropriateness, climate appropriateness, acoustics, security, and outdoor air quality. Projects implementing natural ventilation should anticipate commissioning challenges. Many commissioning and air balance providers in the US are not experienced with natural ventilation systems.

Some U.S. designers or owners maybe averse to natural ventilation because of its newness, added costs, or perceived impacts on clinical outcomes.


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Paul Ninomura, HFDP Member ASHRAE

Heather Burpee Associate Member ASHRAE

Travis English Member ASHRAE

Jeremy Fauber Member ASHRAE

Jonnathan Flannery Member ASHRAE

Arash Guilty Associate Member ASHRAE

Lynda Herrig, HFDP Member ASHRAE

Frank Mills Life Member ASHRAE

Richard Moeller, HFDP Life Member ASHRAE

Russell Olmstead

Heather Platt Member ASHRAE

Chris Rousseau Member ASHRAE

Michael Sheerin Member ASHRAE

Paul Ninomura is a mechanical engineer at Indian Health Service, Seattle, WA. Heather Burpee is a research assistant professor at University of Washington, Seattle, WA. Travis English is director of engineering at Kaiser Permanente, Oakland, CA. Jeremy Fauber is a senior mechanical engineer at Heapy, West Chester, OH. Jonnathan Flannery is a Senior Associate Director of Advocacy at American Society for Healthcare Engineering, Chicago, IL. Arash Guilty is a associate principal and Heather Platt is a senior associate at Mazzetti, Winston-Salem, NC. L.ynda Herrig is an associate partner and Chris Rousseau is a partner at Newcomb & Boyd, Atlanta, GA. Frank Mills is a principal at Frank Mills Consulting, Preston, Lancashire, UK. Richard Moeller is a principal at GnGB, Orange County, CA. Russell Olmstead is director of Infection Prevention and Control Services at CHE Trinity Health, Detroit, MI. Michael Sheerin is a principal at TLC Engineering, Orlando, FL.
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Author:Ninomura, Paul; Burpee, Heather; English, Travis; Fauber, Jeremy; Flannery, Jonnathan; Guilty, Arash
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2017
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