Table 1. Risk assessment design strategies for hospitals include both infection control and environmental control.

These systems have been integrated over the decades with improvements in filtration processes, and in recent years with a resurgence of ultraviolet germicidal irradiation (UVGI) applications, to form a triad of technologies used by engineers to provide acceptable IAQ in health care facilities.

Although insightful, Nightingale could not have foreseen all the mounting challenges facing health care facilities in this new millennium. Today's escalating fears, from TB epidemics to bioterrorism with aerosolized anthrax spores, have generated even greater demands on this technology triad to provide a healthy indoor environment. That environment must be conducive both to patients (to nurture well-being during diagnosis treatment, and recovery), and to health care workers (to reduce exposure to patient pathogens and to limit sensitization to allergenic substances).

The strategy of choice in health care facility design is one of risk assessment that emphasizes either infection control³ or environmental control⁴. As shown in Table 1, these two approaches share many common elements but are initiated differently. Infection control begins with the ill patient and the at-risk health care staff, while environmental control starts off with the causative agent of patient illness and health care worker exposure or sensitization.

These risk assessment approaches bring to bear skilled practitioners from the medical and building sciences, including architects, engineers, epidemiologists, and industrial hygienists. These multiple disciplines are ultimately responsible for providing the effective IAQ practices in health care facilities today. Where do these practitioners go for health care facility design criteria?

Three Prevailing Guidelines and Standards

The latest edition (2001) of these AIA Guidelines has notable changes to the number of ach for selected rooms in health care facilities, as shown in Table 2, to address both diagnosed infectious patients in isolation rooms⁶, and, more broadly, undiagnosed persons, such as those with TB, in waiting areas and procedure rooms ⁷.

Total ach's, consisting of both fresh ventilation air and filtered recirculation air, are identified by specific room type along with any pressurization and exhaust requirements. These guidelines do however lack specificity on hvac equipment design requirements.

So engineers look to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbooks and Standards ^8,9 to "fill in the blanks" in the AIA Guidelines. Some state health agencies also cite ASHRAE Standard 62 "Ventilation for Acceptable Indoor Air Quality."

But Standard 62 lacks essential detail as well, such as ventilation rates for certain health care applications or procedures. ASHRAE Special Project (SP) 91 Health Care Facilities Design Guide is presently addressing coordination of AIA and ASHRAE guidelines and standards for health care facilities with drafts of a new ASHRAE publication, Hospital HVAC Design Guide, currently undergoing reviews. Note that additional OSHA standards and NIOSH criteria may also apply to employee-only accessed areas of health care facilities.

Epidemiologists and industrial hygienists look to the Centers for Disease Control (CDC) for the latest developing practices for controlling the spread of diseases. These practices appear in the CDC Guideline for Environmental Control in Health Care Facilities ¹⁰ and are often a precursor to future AIA Guidelines.

Three Major Risks

Typically, nonporous and untextured floor, wall, or ceiling surfaces are not platforms for pathogenic microbes unless the surface becomes moist, sticky, or damaged. Once dried, cleaned, or repaired, the microbes tend to disappear. Major exceptions to this are floor carpets and suspended ceilings, which can harbor significant populations of microbes and such furnishings must be precluded from use in higher risk patient areas ¹¹.

If properly disinfected, through chemical treatment, thermal eradication, UV irradiation, or metal ionization, any threat of waterborne microbes in health care potable water supplies and water sources such as cooling towers can be dramatically limited¹¹.

That returns us to the airborne side of the risk picture. One of the major airborne risks facing health care facilities is respiratory pathogens. The time-tested method for reducing the rate of airborne infection is dilution with ventilation or fresh air. What Nightingale suspected in the 1850s was quantified a century later in the 1950s by the team of Wells and Riley. The Wells-Riley equation⁶ quantifies the relationship between higher ventilation rates and lower infection rates as illustrated in Figure 1.

As the graphic illustrates though, the benefits of ventilation air diminish as the relationship asymptotes out, never allowing one to eliminate infection completely, especially within practical limits of ventilation system design, operation, and cost. So other complimentary technologies, filtration and irradiation, are brought to bear to reduce further risk of infection from respiratory pathogens.

The IAQ Technology Triumvirate

Ventilation

Conversely, immune system-compromised patients must be protected from infection, such as aspergillosis, by oversupplying these rooms with sufficient makeup air to support adequate positive pressurization. Most exhaust air fans for isolation areas are dedicated to this use, but most supply air systems are not. Hence, due to demands elsewhere in the health care facility, ventilation air supply may be subject to variation and pressurization compromised in these rooms¹².

Over the course of the last decade, a growing trend has emerged to condition ventilation air with DOAS as well. In the cooling mode, these systems decouple the highly latent loads in outside air from the balance of the space conditioning system (See sidebar).

In large outside air fraction applications, like health care facilities, conventional systems handling the combined fresh and return airstreams can leave humidity loads unmet, especially at part-load conditions. Indoor humidity levels rise as a result and encourage microbial activity.

DOAS can neutralize outside air humidity loads, allowing conventional cooling technologies to handle the predominately sensible load downstream. DOAS technologies include cool and reheat, heat pipe or run-around heat exchange equipped cooling coils, dual path cooling coils, enthalpy exchangers, and desiccant dehumidifiers¹³.

Filtration

Based on ASHRAE's new Minimum Efficiency Reporting Value (MERV) rating system for filters in Standard 52.2-1999, "Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size," mechanical arrestance of fungi (spores) requires filters with MERVs of 9 to 12 (with up to 90% efficiency in the 3 to 10 micron range).

Mechanical arrestance of bacteria requires filters with MERVs of 13 to 16 (with up to 95% efficiency in the 0.3 to 1.0 micron range). Mechanical arrestance of high percentages of viruses requires high efficiency particulate arrestance (HEPA) filtration, beyond the range and outside the scope of the ASHRAE MERV rating tests.

The lower MERV-rated filters can be effectively applied as prefilters to reduce the larger particle loading on HEPA filters and extend the service life of the more expensive HEPA media. An essential aspect of proper filtration application is matching the health care space usage and contaminant control requirements with the proper filter selection ¹⁵.

Irradiation

Though not presently sanctioned by health care construction and design guidelines, the "precursor" CDC infection control guidelines do recognize that "as a supplemental air-cleaning measure, UVGI is effective in reducing the transmission of airborne bacterial and viral infections in hospitals ... but it has only minimal inactivating effect on fungal spores"¹⁰ and bacterial spores¹⁶ .

In isolation rooms and some common areas like waiting rooms, UVGI is applied to upper air zones and its effectiveness is dependent on sufficient irradiation intensity, satisfactory air movement from the lower air zone, adequate airborne microbe exposure time, moderate room air humidity, and susceptibility of that microbe to UV. In AHUs, UVGI is applied in close proximity to the filter banks and the cooling coils. The UV emitters must have a line of sight to the targeted microbial amplification sites such as wet coils, drain pans, and moisture-exposed filters.

Finding the Right Combination

When all three technologies are applied together, UVGI can target vulnerable bacteria (excluding spores) and viruses in AHU microbial amplification sites and in ill patient containment areas; filtration can remove fungi (spores) from incoming outdoor air and provide the bulk of bacteria and virus removal in recirculated air; and ventilation can deliver dilution air from outdoors to further decrease concentrations of all airborne microbes promulgating indoors.

Conclusions

These technologies allow multiple medical and building science practitioners to meet current and future guidelines for health care facility design, construction, and operation, including disease and infection control. Much work still lies ahead, though, to determine the preferred mix of technologies to optimize the overall economics while minimizing health risks associated with the resultant IAQ provided in health care facilities. ES

EDITOR'S NOTE: Some images associated with this article do not transfer to the Internet. To review the images, please refer to the print version of this issue.

"It's not the heat, it's the humidity!"

Back in 1997, the concept of the Ventilation Load Index (VLI)¹⁹ was introduced to provide a quick measure of the annual sensible and latent cooling loads in ton-hrs/cfm of outside air brought indoors to a space neutral condition (75 degrees F/50% rh). The VLI was a response to the increasing ventilation rates of ASHRAE Standard 62 and, in particular, the emerging problems with humidity control²⁰ during the cooling season for buildings with large volumes of outside air, like health care facilities.

As shown in Figure 2 for three selected major cities in the United States, the separation of the sensible and latent components of outside air helped engineers "visualize the humidity control problem" by quantifying the individual magnitudes of the ventilation cooling loads associated with ambient temperature and moisture, while further fostering the introduction of DOAS technologies for ventilation air pretreatment as a solution.

At the same time as the VLI was introduced, ASHRAE was giving moisture loads present in outside air long overdue recognition in the 1997, and now 2001, edition of the ASHRAE Handbook of Fundamentals. The 1993 Handbook (Chapter 24) contained only cooling design drybulb temperatures (1%, 2.5%, and 5% summer conditions; replaced by 0.4%, 1%, and 2% annual conditions in 1997).

The 1997 edition (Chapter 26) introduced the design dewpoint temperature and design humidity ratio. This design humidity ratio, which occurs at lower drybulb temperatures, had been the long overlooked "other peak cooling condition." In fact, in nonarid climates, the cooling load resulting from outside air is larger at the design humidity ratio than at the design drybulb temperature as shown for Atlanta in Figure 3.

Works Cited