Isolation and airborne precautions for suspected pulmonary tuberculosis

What it is

Tuberculosis is one of the diseases placed under airborne precautions, together with chicken pox, disseminated herpes zoster (shingles) and measles, and an N-95 mask is listed for tuberculosis, SARS and avian influenza. A short way to hold the list is three respiratory and three rashes. This is a different level of care from droplet precautions, which cover pertussis, influenza A or B, mumps, rubella and Neisseria meningitides, and from contact precautions, which cover any disease that is feco-orally transmitted, resistant bacteria such as MRSA and VRE, and skin and wound infections.

Causes and risk

Almost all infections with M. tuberculosis are due to inhalation of droplet nuclei, infectious particles of 1 to 5 microns in diameter that are aerosolized by coughing, sneezing or talking, dry while airborne and then remain suspended for long periods. This is why the air of the room is what matters: the air in a room occupied by a person with pulmonary tuberculosis may remain infectious for approximately 30 minutes even after that person has left it. Infection does not generally occur out of doors, because M. tuberculosis is killed by ultraviolet light.

Prolonged exposure and multiple aerosol inocula are usually required to establish the infection, so a brief contact carries a much smaller risk than close and repeated contact with the source. The most important determinants of infection in tuberculin-negative persons are closeness of contact and infectiousness of the source.