Hospital-acquired, or nosocomial, infections (Figure 1) have proven to be a persistent and sometimes tragic problem.

If transmission by direct contact predominates, as many experts suggest, then surface-disinfection technologies should have a major impact in reducing-infection rates. But with more than a third of all nosocomial-infections possibly involving airborne transmission at some point, the combination of surface and air disinfection should produce optimum results.

This article will examine the epidemiology and aerobiological pathways of airborne nosocomial infections and review air- and surface-disinfection technologies, including ultraviolet germicidal irradiation (UVGI). First, however,a brief synopsis of applicable guidelines,codes, and standards will provide background on current methods of nosocomial infection control.

GUIDELINES, CODES, AND STANDARDS

A variety of guidelines, codes, and standards offer details for designing health-care-facility ventilation systems.^2,3,4,5 Some guidelines address specific problems, such as tuberculosis (TB), nosocomial infections, and surgical-site infections (SSIs).^6,7,8,9 Supply-air filters typically are specified based on the American Society of Heating, Refrigerating and Air-Conditioning Engineers recommendations shown in Table 1. (MERV stands for "minimum-efficiency reporting value.")

Air recirculation is permitted in most hospital areas, including operating rooms (ORs) and intensive-care units (ICUs).³ General areas (GAs) often have recirculation rates typical of commercial office buildings, with about 15- to 25-percent outside air (OA). There are no specific requirements for filtration of recirculated air in GAs. Air-contaminant control often is accomplished with high rates of room air exchange using filtered 100percent outside air. Typically, ORs have an air-change rate (ACH) of 12 to 25, with 12 ACH typically representing 100-percent OA and 25 ACH typically representing 5 ACH of OA and 20 ACH of recirculated air. Patient and intensive-care rooms typically have an ACH of 4 to 6, with 2 ACH of OA.⁴ The American Institute of Architects² recommends 15 ACH for ORs, which appears to be the norm in the United States.

Figure 2 illustrates both the effectiveness of various rates of filtered outside air with complete air mixing and the effectiveness of recirculating the same airflow through a high-efficiency particulate-air (HEPA) filter. Note that the results are virtually identical, a fact that brings into question the universal reliance on 100-percent-OA systems, especially if, as discussed later, combined UVGI/filtration systems can approach HEPA-filter performance with lower energy costs.

Many mistakenly assume that systems designed and installed per code will produce sterile air. For analytical purposes, sterility often is assumed to mean six logs of reduction, which is the lower Y-axis limit in Figure 2. Technically, six logs of reduction implies sterility if and only if there are no survivors. Sterility of OR air may be difficult to achieve and impossible to prove.

HEPA filters typically are used in isolation rooms and other areas when air is recirculated.³ They also often are used to filter exhaust air from isolation rooms, laboratories, and other facilities. Codes and guidelines have specific requirements for the pressurization of ORs and isolation rooms that can be consulted for additional information. Finally, and perhaps most importantly, hospitals have detailed procedures for disinfection.^5,10

All of the above approaches to infection control have proven to be reliable and effective in practice. However, based on ongoing research, certain improvements may be possible, particularly regarding nosocomial infections that are wholly or partly airborne.

AIRBORNE NOSOCOMIAL EPIDEMIOLOGY
Various sources estimate that between 2 million and 4 million nosocomial infections occur annually, resulting in 20,000 to 80,000 fatalities. The cost of nosocomial infections in the United States is estimated to be about $4 billion to $5 billion annually. Table 2 lists potentially airborne nosocomial agents and the number of annual cases, based on various sources. This list is by no means complete, as practically every pathogen and mold spore may become a nosocomial agent, and new pathogens often arise. Endogenous microbes are those that exist commensally in humans, but may be transmitted to susceptible individuals, usually the immuno-compromised. Filtration efficiency and UVGI dosages for 90-percent inactivation (D₉₀) are summarized from "Aerobiological Engineering Handbook: A Guide to Airborne Disease Control Technologies."¹¹

Airborne nosocomial infections are transmitted directly or indirectly through air and may cause respiratory (primarily pneumonia) and surgical-site infections. The degree to which the transmission of nosocomial infections is airborne is unknown. One source estimates that 10 percent of nosocomial infections are airborne, while another states that 16 percent of ICU infections result from airborne-pathogen transmission.^12,13

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