Note: Unless otherwise specified, the definitions listed below apply to the terms as used in this handbook. They may have different meanings in other contexts.

Airborne Mycobacterium tuberculosis transmission: Spread of aerosolized M. tuberculosis caused by the dissemination of infectious respiratory particles when suspended in air over long distances and time.¹

Air changes per hour (ACH): The number of times that the total air volume in a room or space is completely removed and replaced in an hour.

Air purifier or air cleaner: A portable electrical indoor device intended to remove potentially harmful particles from the circulating air, or to inactivate or destroy them.

Antimicrobial resistance (AMR): The loss of effectiveness of any anti-infective medicine, including antiviral, antifungal, antibacterial and antiparasitic medicines.

Community setting: In the context of health care, a setting (e.g. primary care facility or other health care facility at community level) where interventions aimed at maintenance, protection and improvement of health status are provided at or near to places of residence.

Congregate settings: Institutional (non-health care) settings where people reside in close proximity to each other. Congregate settings range from correctional facilities (prisons and jails) to homeless shelters, refugee camps, army barracks, hospices, dormitories and nursing homes.

Contagious (infectious) tuberculosis (TB) patient: A patient with pulmonary TB disease (confirmed or undetected) who can spread infectious respiratory particles containing viable M. tuberculosis while coughing, sneezing, talking or conducting any other respiratory manoeuvres.

Droplet transmission: The spread of an infectious agent caused by the dissemination of droplets. Droplets are primarily generated from an infected (source) person during coughing, sneezing or talking. Transmission occurs when the droplets that contain microorganisms are propelled (usually <1 m) through the air and deposited on the conjunctivae, the mouth, or the nasal, throat, or pharynx mucosa of another person. Most of the volume (>99%) comprises large droplets that travel short distances (<1 m) and do not remain suspended in the air.

General hospital: A health care institution providing medical or surgical treatment (or both), and nursing care for sick or injured people.

General population: All individuals, without reference to any specific characteristic.

Germicidal ultraviolet (UV) light (GUV): A modern term for UVG irradiation (UVGI). The word “irradiation” has been removed from the abbreviation to help alleviate end-users’ fears of ionizing radiation, which GUV does not contain.

GUV fixture or luminaire: A piece of apparatus that distributes the GUV energy emitted from one or more sources. It does not include the sources themselves but does include all the parts necessary for safe and effective operation, with the means for connecting the sources to the electricity supply² (the functional unit is composed of the fixture plus the GUV source or lamp).

Health care-associated infection (HAI): An infection occurring in a patient during the process of care in a hospital or other health care facility, which was not present or incubating at the time of admission. HAIs can also appear after discharge.

Health care facility: Any establishment (public or private) that is engaged in direct care of patients on site.

Health care setting: A setting where health care is provided (e.g. hospital, outpatient clinic or home).

Health care workers: All people engaged in actions whose primary intent is to enhance health as defined by the World Health Organization (WHO).³

Hierarchy of TB infection prevention and control (IPC) measures: TB IPC consists of a combination of measures designed to minimize the risk of M. tuberculosis transmission within populations. A three-level hierarchy of controls comprising administrative controls, environmental controls and respiratory protection, reduces exposure to M. tuberculosis and helps prevent transmission.

  • Administrative controls: These are at the top of the hierarchy of TB IPC measures. They are management measures that are intended to reduce the risk of exposure to persons with infectious TB.
  • Environmental controls: These are at the second level of the hierarchy. They prevent the spread of infectious respiratory particles and reduce their concentration.
  • Respiratory protection: These are at the third level of the hierarchy. Respiratory protection control refers to the use of personal protective equipment over the nose and mouth in situations that pose a high risk of exposure to M. tuberculosis.

Household contact of TB patient: A person who shared the same enclosed living space with the index patient for one or more nights or for frequent or extended periods during the day during the 3 months before the start of current treatment.

Infectiousness: Probability of TB transmission from an individual with TB disease (usually pulmonary TB) to a susceptible individual through aerosols containing viable M. tuberculosis while the infected individual was, for example, coughing, sneezing, or talking.

Inpatient health care setting: A health care facility where patients are admitted and assigned a bed while undergoing diagnosis and receiving treatment and care, for at least one overnight stay.

Mechanical ventilation: Ventilation created mechanically using an air supply or an exhaust fan (or both), to force air into or out of a room.

Medical masks: Medical or procedure masks that are flat or pleated and are affixed to the head with straps around the ears, the head or both. Their performance standards are tested using a set of standardized test methods – American Society for Testing Materials (ASTM) ASTM F2100, EN 14683 or equivalent – that aim to balance high filtration, adequate breathability and (optionally) fluid penetration resistance.⁴

Mixed-mode ventilation: A ventilation system that combines both mechanical and natural ventilation, providing the opportunity to choose the most appropriate ventilation mode based on the circumstances.

Multimodal IPC: IPC that is implemented in an integrated way, with the aim of improving an outcome and changing behaviour.⁵ Such a strategy includes tools (e.g. bundles and checklists) developed by multidisciplinary teams that take into account local conditions. The five most common components are: system change (availability of the appropriate infrastructure and supplies to enable good IPC practices); education and training of health care workers and key players (e.g. managers); monitoring of infrastructure, practices, processes and outcomes, and provision of data feedback; reminders or communications in the workplace; and culture change within the establishment or strengthening of a safety climate.⁶

Natural ventilation: Use of natural forces to introduce outdoor air into or push it out of a building and to distribute it around a space. These forces can be wind pressure, or pressure generated by the density difference between indoor and outdoor air.⁷,⁸

Negative-pressure mechanical ventilation system: A mechanical ventilation system in which the exhaust airflow rate is higher than the supply airflow rate. This system is used to ensure that a room is at a lower pressure than the surrounding areas.

Negative-pressure room: A room in which mechanical ventilation keeps the exhaust airflow rate higher than the supply airflow rate. The room will be at a lower pressure than the surrounding areas.

Outpatient health care setting: A health care facility where patients are undergoing diagnosis and receiving treatment and care but are not admitted for an overnight stay (e.g. an ambulatory clinic or a dispensary).

Particulate respirator (N95 or FFP2): A special type of closely fitting face cover that can filter particles, to protect the wearer against inhaling infectious droplets.

The N95 respirator has a filter efficiency level of 95% or more against particulate aerosols free of oil, when tested against 0.3 μm particles. The “N” denotes that the respirator is not resistant to oil, and the “95” refers to a 95% filter efficiency.

The FFP2 respirator has a filter efficiency level of 94% or more against 0.4 μm solid particles and is tested against both an oil and a non-oil aerosol.

The performance of N95 respirators is approved by the National Institute for Occupational Safety and Health (NIOSH) of the United States Centers for Disease Control and Prevention. The performance of FFP2 respirators must comply with the essential health and safety requirements set out in European directives; that is, with “Conformité européenne” (CE).⁹

Person with presumptive TB: A person who presents with symptoms or signs suggestive of active TB disease.

Positive-pressure mechanical ventilation system: A mechanical ventilation system in which the supply airflow rate is higher than the exhaust airflow rate. This system is used to ensure that a room is at a higher pressure than the surrounding areas.

Positive-pressure room: A room in which mechanical ventilation keeps the supply airflow rate higher than the exhaust airflow rate. The room will be at a higher pressure than the surrounding areas. Under this condition, if an opening exists, air will flow from the positively pressurized space outward to the surrounding areas.¹⁰

Recirculated air filtration: Ventilation systems used in enclosed spaces, buildings, aircraft and vehicles, through which various proportions of outside air and recirculated air are mixed, conditioned and filtered before being fed into the enclosed space.

Respirator fit test: A test protocol conducted to verify that a respirator correctly fits the wearer, to minimize ambient air entering the wearer’s respiratory tract. Qualitative fit testing verifies the respirator’s fit using test agents, and is detected either qualitatively by the wearer’s sense of taste or smell or by their involuntary cough reflex (e.g. caused by irritants such as smoke), or quantitatively by an instrument. Quantitative fit-testing uses ambient aerosols or artificially generated sodium chloride aerosols, and quantitative fit-testing measures aerosol concentrations inside and outside the respirator.

Respiratory hygiene or cough etiquette: The practice of covering the mouth and nose during coughing or sneezing (e.g. by wearing a medical mask or cloth mask, or covering the mouth with a tissue, sleeve, flexed elbow or hand) to reduce the dispersal of respiratory secretions that may contain infectious particles.

Respiratory protection programme: A plan of action aimed at accomplishing an effective and sustainable use of particulate respirators by health care workers in settings that pose a high risk of M. tuberculosis transmission. The plan includes activities, responsibilities, and timelines, and outlines the means or resources that will be used. Examples of activities are policy development; education and training of health care workers; respirator fit testing; selection of respirator models and sizes; budgeting; procurement of respirators; and installation of signage about mandatory respirator use, supervision, and disposal in high-risk areas of a facility.

Respiratory separation (or isolation): Measures aimed at decreasing or eliminating the risk of airborne M. tuberculosis transmission from infectious individuals to other people seeking medical attention in a health care facility or to health care workers; such methods include use of pressure rooms, individual rooms or designated units, or timing of care procedures.

Risk of M. tuberculosis transmission: The probability of passing M. tuberculosis to another individual. This may be influenced by factors such as the frequency of contact with the source person, proximity to the person, duration of contact, use of respiratory protection, environmental factors (e.g. dilution of air, ventilation and other air disinfection), infectiousness of the source person and immune status of the exposed person.

Seal check: The person using a respirator must perform a seal check to determine whether the respirator is being properly worn. The user gently inhales and holds their breath for a few seconds. The facepiece should collapse slightly on the face. The seal check is successful if the facepiece remains collapsed while the person breathes.¹¹

Settings with a high risk of M. tuberculosis transmission: Settings where infectious TB patients or individuals with undetected or undiagnosed active TB are present. TB patients are most infectious when they are untreated (e.g. before diagnosis) or inadequately treated (e.g. if they have undiagnosed drug-resistant TB and are treated with first-line drugs). Transmission will be increased by aerosol-generating procedures (e.g. bronchoscopy or sputum induction).

TB incidence: The number of new and recurrent (relapse) episodes of TB disease (all forms) occurring in a given year.¹²

TB prevalence: The number of people with TB disease (all forms) at a given point in time.¹³

TB infection: A state of persistent immune response to stimulation by M. tuberculosis antigens with no evidence of clinically manifest TB disease. This was previously referred to as latent TB infection, (LTBI), but that term is now being discarded given that infection cannot always be considered to be dormant.

TB infection, incidence: The number of new persons identified with TB infection within a specified period of time.

TB infection, prevalence: The number of persons identified with TB infection at a given point in time.

TB symptoms: General manifestation of active pulmonary TB disease, including cough for longer than 2 weeks, with sputum production (and could have blood at times), chest pains, fatigue, loss of appetite, weight loss, fever and night sweats.

Triage: In the context of TB IPC, this refers to a simple and preliminary intervention for identifying people with signs or symptoms of TB among those seeking medical attention in health care facilities. Triage is used to fast-track the diagnosis of TB infections and facilitate further separation or other precautions, when necessary, to minimize transmission from infectious patients.

UVGI: The use of UV light C (UVC) to kill or inactivate microorganisms. UVGI is generated by germicidal lamps and can kill or inactivate microorganisms that are airborne or on directly irradiated surfaces. Low-pressure mercury-vapour lamps emit UVC. (See also Germicidal UV light (GUV) above.)

Upper-room GUV: GUV systems that are designed to generate high levels of UVC irradiance above the heads of room occupants, and to minimize UVC exposure in the lower or occupied portion of the room.

Ventilation: Provision of outdoor air into a building or a room and distribution of air within the building. The purpose of ventilation in buildings is to provide healthy air for breathing by diluting pollutants originating in the building with clean air, and by providing an airflow rate to change this air at a given rate. Ventilation is also used for odour control, containment control and climatic control (i.e. temperature and relative humidity). Ventilation may also be used to maintain pressure differentials to prevent the spread of contaminants outside a room or to prevent contaminants from entering a room.¹⁴

1 For a more global WHO definition of airborne transmission, see Natural ventilation for infection control in health care settings. Geneva: World Health Organization; 2009 (

2 International lighting vocabulary. Vienna: International Commission on Illumination; 2011 (

3 The world health report: 2006: working together for health. Geneva: World Health Organization; 2006 (

4 Technical specifications of personal protective equipment for COVID-19. Geneva: World Health Organization; 2020 (

5 Evidence-based care bundles [website]. Boston, USA: Institute for Health care Improvement; 2023 ( Pages/default.aspx).

6 Guidelines on core components of infection prevention and control programmes at the national and acute health care facility level. Geneva: World Health Organization; 2016 (

7 Differences in air density are commonly caused by temperature gradients, because warm air is less dense and tends to rise above colder air.

8 Natural ventilation for infection control in health care settings. Geneva: World Health Organization; 2009 (

9 For more details see also Technical specifications of personal protective equipment for COVID-19. Geneva: World Health Organization; 2020 (

10 Roadmap to improve and ensure good indoor ventilation in the context of COVID-19. Geneva: World Health Organization; 2021 (https://

11 How to perform a respirator seal check. Geneva: World Health Organization; 2007 (

12 Methods used by WHO to estimate the global burden of TB disease. Geneva: World Health Organization; 2021 (

13 Methods used by WHO to estimate the global burden of TB disease. Geneva: World Health Organization; 2021 (

14 See other WHO definition in Roadmap to improve and ensure good indoor ventilation in the context of COVID-19. Geneva: World Health Organization; 2021 (

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