TB is a bacterial infection caused by Mycobacterium tuberculosis that primarily attacks the lungs but can also affect other organs, including the brain, spine, or kidneys. Understanding how this bacterium travels is paramount to preventing its spread in healthcare and community settings. This article clarifies the mechanism of TB transmission, detailing why it is classified as an airborne disease and outlining the specific precautions required to contain it.
The Science of Tuberculosis Transmission
Tuberculosis transmission occurs when a person with active disease in their lungs or throat expels tiny infectious particles, known as droplet nuclei, into the air. These particles are produced when the infected person coughs, sneezes, speaks, or sings. The Mycobacterium tuberculosis bacteria are contained within these aerosolized droplets.
The physical size of these droplet nuclei determines the airborne classification. They measure between 1 and 5 micrometers in diameter, allowing them to remain suspended in the air for extended periods, sometimes for several hours. Because they can travel long distances on air currents, anyone who breathes in the air containing these particles is at risk of infection. This ability to linger and travel distinguishes TB from infections spread by larger, heavier respiratory droplets.
Airborne Precautions: Required Safety Measures
The airborne nature of Mycobacterium tuberculosis necessitates specialized infection control strategies, known as Airborne Precautions, in clinical environments. These precautions rely on the use of an Airborne Infection Isolation Room (AIIR). AIIRs are specially engineered rooms designed to maintain negative pressure, meaning air flows into the room but cannot flow out into the hospital corridor.
The contaminated air from the AIIR is exhausted directly outside or passed through a High-Efficiency Particulate Air (HEPA) filter before being recirculated. To ensure air quality, these rooms typically require six to twelve air exchanges per hour. Healthcare workers entering the room must wear a NIOSH-certified N95 respirator, a tight-fitting mask capable of filtering the tiny droplet nuclei. Standard surgical masks are insufficient because they cannot filter particles as small as those carrying TB.
When an active TB patient must be transported out of the AIIR for procedures, movement is minimized and carefully controlled. During transport, the patient must wear a surgical mask to contain the expulsion of infectious particles. This protects staff and others in public areas through which the patient passes.
Distinguishing Latent Infection from Active Disease
The need for Airborne Precautions depends on whether a patient has Latent TB Infection (LTBI) or Active TB Disease. LTBI occurs when Mycobacterium tuberculosis bacteria are present but have been contained and rendered inactive by the immune system. Individuals with LTBI are asymptomatic, do not feel sick, and cannot transmit the bacteria to others.
In contrast, Active TB Disease means the bacteria are multiplying, overcoming the body’s defenses, and causing illness. This active state is characterized by symptoms such as a persistent cough, fever, night sweats, and unexplained weight loss. Only a person with active TB disease in the lungs or throat is contagious and capable of expelling infectious droplet nuclei.
The distinction is significant for infection control because isolation is only required for active, infectious disease. A person diagnosed with LTBI requires medical treatment to prevent progression to the active stage, but they do not require isolation. Understanding this difference prevents unnecessary resource use and patient isolation.
Comparison with Droplet Precautions
The precautions for tuberculosis are stricter than those used for common infections spread via the droplet route, such as influenza or pertussis. Droplet Precautions apply to diseases transmitted by larger respiratory particles. These larger droplets are typically greater than 5 micrometers in size and are heavy, causing them to fall out of the air quickly, usually within three to six feet of the source.
In a healthcare setting, Droplet Precautions require a patient to be placed in a private room, but a negative pressure ventilation system is unnecessary. Staff need only wear a standard surgical mask, not an N95 respirator, when working near the patient. This difference reflects the distinct physics of transmission: TB travels long distances suspended in the air, while droplet-spread pathogens fall rapidly.