This recommendation is applicable for health care facilities as well as other congregate spaces with a high risk of M. tuberculosis transmission. In such settings, upper-room GUV systems should be implemented as part of a standard of care. The Guideline Development Group recognized that, because of cost considerations, the implementation of this intervention may not be feasible in all settings. Low- and middle-income countries that do not have the infrastructure or capacity to fully adopt this recommendation are advised to identify areas of higher risk of transmission, and prioritize the application of this intervention accordingly. Success in the implementation of this intervention depends on appropriate installation, quality control and maintenance, to ensure that air disinfection occurs without adverse effects. Exceeding the threshold limit value33 can lead to overexposure,34 resulting in painful eye and skin irritation; hence, GUV systems must be monitored to ensure that optimal UV dose levels are achieved within a permissible limit of irradiance. The IPC measures included in these guidelines should not be considered as individual interventions, but rather as a package. The Guideline Development Group recognized the role of upper-room GUV systems, but acknowledged that overreliance on these units as a single measure for IPC – especially without testing, maintenance and validation – may actually increase the risk of exposure to M. tuberculosis, defeating the purpose of such systems. Upper-room GUV systems rely on air mixing between the upper and lower parts of a room. Thus, when implementing this intervention it is essential to consider factors that may affect the vertical air movement and transport of the infectious microorganisms to the upper portion of the room (e.g. use of simple fans to facilitate air movement in a room, temperature differential between the supply air and room air, mechanical ventilation rate and velocity of air out of ventilation diffusers). Settings and target population Upper-room GUV systems are suitable for all settings with a high risk of M. tuberculosis transmission, but particularly for those that have a significant burden of DR-TB. In biological chamber studies, the effectiveness of upper-room GUV systems has been reported to decrease as humidity increases above 50–60% (67). However, an evaluation of the efficacy of GUV for preventing transmission of M. tuberculosis using a guinea-pig air-sampling model demonstrated a protective effect in a setting where relative humidity was above 70% (66). Additional considerations may be necessary in settings with high humidity (>70%), and the installation of systems with greater upper-room irradiance levels needs careful consideration. Upper-room GUV systems are not feasible for use in household settings. The American Conference of Governmental Industrial Hygienists (ACGIH) Committee on Physical Agents has established a threshold limit value for short-wavelength UV (UV-C) light exposure to avoid skin and eye injuries. Air cleansing using GUV systems requires that persons in the treated space be shielded from excessive exposure to the UV radiation. To do so, the fixtures are shielded with louvres or bafflers in order to block radiation below the horizontal plane of the fixtures. Unshielded GUV lamps should be used only in areas that are not occupied, and safety features (e.g. switching device to deactivate the lamps in case the doors are opened) should be installed to ensure that overexposure to UVGI cannot occur. Resources Although no cost or cost–effectiveness studies were analysed for this review, the Guideline Development Group recognized the variability in cost of upper-room GUV installations in different settings. The group emphasized that, in the long run, the cost of such systems may be justifiable, given the potential reduction in M. tuberculosis transmission (and the reduction in other airborne pathogens). However, the ability to justify this intervention will depend on the setting. Because upper-room GUV systems rely on effective air mixing, it is necessary to ensure adequate air movement. Also, health care authorities must ensure proper allocation of resources for proper installation, running and maintenance and overall sustainability of this intervention.