Pathogenic organisms are present primarily in moist organic environments, but some can persist under dry conditions. In healthcare settings they can be detected in air and water, and on fomites. Evidence that some environmental pathogens cause healthcare-associated infections (HAIs) prompted the Centers for Disease Control and Prevention (CDC) to publish guidelines for environmental infection control in healthcare facilities.1 Elements of these guidelines were incorporated into the Guidelines for Infection Control in Dental Health-Care Settings - 2003.2
The strength of evidence for an environmental source of infectious agents that shows causality of a HAI is predicated on eight criteria (Box A).1
Box A. Criteria for Determining the Strength of Evidence for Environmental Sources of Infection.
- Organism must be able to survive in air or water, or on fomites.
- Organism can be cultured from in-use air, water, or fomites.
- Organism can proliferate in air or water, or on fomites.
- Acquisition of infection cannot be explained by any other recognized modes of transmission.
- Retrospective case-control studies show an association between exposure to air, water, or fomites and infection.
- Prospective case-control studies show an association between exposure to air, water, or fomites and infection.
- Prospective studies allocating exposure to air, water, or fomites to a subset of patients show an association between exposure and infection.
- Decontamination of air, water, or fomites results in elimination of infection transmission.
When considering air, water, or fomites as the source of pathogens in HAIs, one must understand the “chain of infection.”1,2 Requisites for the transmission of pathogenic organisms from an environmental source to people include (1) a source of infectious agents, i.e., contaminated air, water or fomites; (2) a susceptible host exposed to an adequate number of sufficiently virulent microorganisms; and (3) a mechanism to transfer or mode of transmission of pathogens from the source to the host.
The risk of acquiring environmentally related HAIs is greatest among immunocompromised individuals. Immunocompromised individuals are those patients and healthcare personnel (HCP) whose immune mechanisms are deficient because of hereditary or acquired immunologic disorders, chronic diseases, or immunosuppressive therapy.1 A subset of these patients, those with an absolute neutrophil count of ≤ 500 cells/mL has the greatest risk of HAIs.1