1.1. The concept of isolation precaution and an historical review
Isolation precaution is an important strategy in the practice of infection control. The spread of some infections can be impeded if infected patients are segregated from those who are not yet infected. Although there is no single study showing the effectiveness of isolation, there are many reports documenting the efficacy of the various components of isolation, including use of private rooms (Anderson et al., 1985), and protective equipments such as masks, gloves and gowns (Klein, Perloff & Maki, 1989; Maki, 1994; Maloney et al., 1995).
The concept of isolation can be traced back to biblical times when lepers were segregated from the rest of the populace. Towards the end of the 19th century, there were recommendations for patients with infectious diseases to be placed in separate facilities, which ultimately became known as infectious diseases hospitals (Lynch, 1949). However, in the early 1950s, many of these infectious diseases hospitals closed and the patients were moved to general hospitals. The need for proper isolation of infections in the context of these general hospitals thus became an important issue. Since then, several isolation systems evolved (NCDC, 1970; Lowbury et al., 1975; Garner & Simmons, 1983) with transmission-based precautions the most widely used, which included standard precautions (to avoid direct, unprotected contact with blood and body fluids), contact precautions, droplet precautions and airborne precautions (Gardner, 1996; Siegel et al., 2007).
Spatial separation is critically important when using isolation precautions because, as Florence Nightingale observed, many infectious diseases spread mainly through direct contact when patients are near to one another. Usually, special ventilation controls are not required; these are needed for diseases that can be transmitted over long distances by droplet nuclei (Gardner, 1996). Most diseases are not of this category. However, the infectious diseases that can be transmitted through long distance by aerosols (i.e. airborne infections) can result in large clusters of infection in a short period. Therefore, the proper isolation of these diseases is critically important.
Specific natural ventilation recommendations for isolation of airborne infections are discussed in detail in this guideline (see section 3.2).
1.2. Isolation practices for infection control
This guideline does not describe the details of the various transmission-based precautions, except for airborne precautions. Details of the other categories can be found in the relevant references (Siegel et al., 2007; WHO, 2007).
When using isolation precautions, three levels of controls must be considered (Gerberding, 1993).
The first level of control is administrative controls, which are measures taken to ensure that the entire system is working effectively. These controls include:
implementing proper procedures for triage of patients
detecting infections early
separating infectious patients from others
transporting the patients
educating the patients and staff
designating responsibilities clearly and correctly
communicating with all relevant partners.
The second level is “environmental and engineering controls”, including cleaning of the environment, spatial separation and the ventilation of spaces.
The third level of control to further decrease the risk of transmission is personal protection, which is the provision of the proper personal protective equipment (PPE) (e.g. masks, respirators).
When setting up an isolation system in the hospital, all levels of controls (administrative controls, environmental and engineering controls, and personal protection) must be given proper attention for the system to work effectively, and for the different levels to support each other.
1.3. Isolation practices for airborne infections
Airborne transmission occurs by dissemination of droplet nuclei over long distance from infectious patients (for more details on respiratory droplets, see Annex C). For pathogens to be disseminated via droplet nuclei, some requirements must be met, including:
existence of viable pathogen inside the droplet at the source;
survival of the pathogen inside the droplet after being expelled from the source, and retention of its ability to infect after exposure to physical challenges (evaporation, light, temperature, relative humidity, etc.);
attainment of sufficient infective dose to cause infection in a susceptible host; and
exposure of a susceptible host.
Infectious agents that may be dispersed over long distances by air currents and infect other susceptible individuals include Mycobacterium tuberculosis (Riley et al., 1957, 1959), rubeola virus (measles) (Bloch et al., 1985) and Varicella-zoster virus (chickenpox) (Gustafson et al., 1982). Preventing the spread of airborne infections involves implementing airborne precautions, which requires the three controls (see above in section 1.2): administrative controls; environmental and engineering controls — patient room with special air handling and ventilation; and PPE — the use of particulate respirators by health-care workers whenever possible (WHO, 2007).
Patients who require airborne isolation precautions should be placed in an airborne precaution room (WHO, 2007). An airborne precaution room is a room with ≥12 air changes per hour (ACH) (e.g. equivalent to ≥80 l/s for a 4×2×3 m3 room) and controlled direction of airflow, and can be used to contain airborne infections (AIA, 2001; Wenzel, 2003; Mayhall, 2004; WHO, 2007). A mechanically ventilated room is equivalent to the airborne infection isolation room described by the United States Centers for Disease Control and Prevention, which should have special features in air handling and airflow direction, including (CDC, 2003):
a negative pressure differential of ≥2.5 Pa (0.01-inch water gauge);
an airflow differential >125-cfm (56 l/s) exhaust versus supply;
clean-to-dirty airflow;
sealing of the room, allowing approximately 0.5 square feet (0.046 m2) leakage;
≥12 ACH for a new building, and ≥6 ACH in existing buildings (e.g. equivalent to 40 l/s for a 4×2×3 m3 room) for an old building; and
an exhaust to the outside, or a HEPA-filter if room air is recirculated.
The concept of natural ventilation for airborne precaution rooms was discussed in the recent World Health Organization interim guidelines (WHO, 2007). Natural ventilation can be used in airborne precaution rooms. The purpose of this document is to provide basic design guidance for the use of natural ventilation for infection control. More detailed “design guides” will follow the publication of this document.
1.4. Infection control for high-risk procedures
Airborne precautions were advised after the severe acute respiratory syndrome (SARS) epidemic for patients infected with open pulmonary tuberculosis, measles, smallpox and chickenpox. However, people also started to notice that there were situations in which other, non-airborne pathogens could be transmitted through droplet nuclei when patients had certain health-care procedures.
Presently, there is no clear definition or a precise list of high-risk health-care procedures during which some pathogens (e.g. SARS-Coronavirus, influenza) can be spread through droplet nuclei over short distances. The mechanism of this transmission is described as an opportunistic airborne transmission (Roy & Milton, 2004), and high-risk procedures may increase the potential of generating droplet nuclei because of the mechanical force of the procedure (Ip et al., 2007). Some of these procedures have been associated with a significant increase in the risk of disease transmission, and have been termed aerosol-generating procedures associated with pathogen transmission (WHO, 2007). These procedures include intubation, cardiopulmonary resuscitation, bronchoscopy, autopsy, and surgery where high-speed devices are used (WHO, 2007).
As in all areas of infection control, administrative controls, environmental and engineering controls plus the use of PPE should play a part in controlling the spread of infections during high-risk procedures.
For administrative control, it is critically important to limit these procedures to those patients who need them. Adequate staff training and the provision of safe equipment may also be important for reducing the risk. The proper use of PPE, including the use of particulate respirators, eye protection, gowns and gloves, will also provide additional protection to health-care workers. Finally, performing such procedures in a well-ventilated location, away from other patients and health-care workers, may help prevent the spread of infection. Although no studies have evaluated the impact of ventilation on reducing the risk of infectious droplet nuclei during aerosol-generating procedures, it would be best to perform these procedures in an adequately ventilated room, particularly for patients infected with known life-threatening pathogens (e.g. SARS, avian influenza).
However, it might be difficult to implement the measures stated above, especially during an emergency situation (e.g. resuscitating a collapsed patient in an outpatient department). Therefore, it is important to have in place contingency plans for such scenarios and have an emergency department that is appropriately equipped and well ventilated. Patients could then be moved rapidly to a safe location with good ventilation that is already identified for such purposes. Crowd control is also important to keep patients separate from other people. Appropriate PPE should be worn by health-care workers before starting the high-risk procedure.
1.5. Summary
In summary, although there is little evidence from studies to show an association between isolation precautions and infection control, reports and case studies indicate that some types of isolation (e.g. using private rooms and PPE) may help to prevent the spread of infection in health-care facilities.
All levels of control in an isolation system (administrative controls, environmental and engineering controls, and personal protection) are important, and should be taken into account when designing an isolation system in a hospital. Furthermore, isolation systems should be designed to prevent the spread of disease via respiratory droplets over long distances, with particular consideration paid to controlling transmission during high-risk health-care procedures (such as intubation, cardiopulmonary resuscitation, bronchoscopy, autopsy, and surgery where high-speed devices are used).
