The preference for specific ventilation systems is described under implementation considerations. The use of portable room-air cleaner appliances is not advised as a system to reduce M. tuberculosis transmission to health workers, persons attending health care facilities or other persons in settings with a high risk of transmission. The decision on which system to use – natural ventilation, mixed-mode ventilation, mechanical ventilation or recirculated air with HEPA filtration – depends heavily on the needs of a particular setting, climate, cost–effectiveness assessment and sustainability of resources to ensure proper design and continued adoption of rigorous standards and maintenance. The use of poorly designed or poorly maintained ventilation systems, leading to inadequate airflow, can result in health care-associated transmission of M. tuberculosis. Inadequate ventilation also increases the risk of transmission in other non-health care congregate settings such as correctional facilities, and refugee and asylum centres. Programmes need to ensure the sustained use of ventilation systems that can provide sufficient dilution and removal of infectious particles. This can be achieved through proper commissioning of ventilation systems. Settings and target population Natural ventilation is the preferred ventilation system in resource-limited settings where there is high risk of M. tuberculosis transmission. However, the use of mixed-mode ventilation, mechanical ventilation or HEPA filters may be more appropriate in settings where natural ventilation is not suitable because of the climate (e.g. in cold climates) or other constraints. Natural ventilation is also the preferred system in settings with no constant electricity supply. Resources The effective implementation and functioning of ventilation systems requires allocation of sufficient resources to:

• conduct risk assessments to assess the direction of airflow or to relocate TB wards to the upper floors of buildings or downwind of non-TB wards; and
• install and maintain such systems in many health care and non-health care congregate settings. The planning of and budgeting for ventilation systems also needs to consider the costs of regular assessment of ventilation performance and of maintenance (or upgrades for mechanical ventilation systems). As mentioned earlier, the systematic search did not yield any studies evaluating the effectiveness of portable in-room air cleaner appliances. The Guideline Development Group noted that most portable in-room air cleaner appliances enable too few ACH to adequately reduce the risk of transmission. Hence, the group opted to add a specific remark against the use of these devices until further evidence becomes available.