The use of UVGI in the upper part of rooms may be effective in killing or inactivating M. tuberculosis generated by infected persons.
19.1 Mechanism of action
UV lamps are installed into fixtures suspended from a ceiling or mounted on a wall. Fixtures are shielded with louvers or bafflers in order to block radiation below the horizontal plane of the fixtures. UV lights create in the upper portion of the room a germicidal zone where the bacilli are killed (Figure 1). Patients in the lower portion of the room are not exposed to UVGI lights. Good air mixing is needed to transport the air (and thereby the bacilli) to the upper portion of the room. Disinfection is achieved through the rapid dilution of contaminated lower room air with clean irradiated upper room air.
From the WHO, Implementing the WHO Policy on TB Infection Control in Health-Care Facilities, Congregate Settings and Households
The lamps should irradiate the entire surface of the upper part of the room (Figure 2), in order to disinfect the largest possible volume of air mixed at a low speed between the upper and lower part of the room.
UVGI Upper-room Irradiation
From Guidelines for the Utilization of Ultraviolet Germicidal Irradiation technology in controlling transmission of tuberculosis in health care facilities in South Africa9
Several factors influence the efficiency of UVGI systems:
– Ventilation rate: in controlled environment, at rates up to 6 air change per hour (ACH), UVGI systems increase the effect of air cleaning to > 12 ACH10. But when ventilation rates are increased above 6 ACH, UVGI system effectiveness could be reduced because the time for bacteria irradiation is shorter11,12.
– Effective mixing within the room may be provided by natural convection currents or fans, preferably, ceiling ones. Low velocity ceiling fans boosted UVGI system’s effectiveness up to 33% when ACH was below 613,14,15.
– Installation: the height of the room should be minimum 2.5 m and UVGI fixtures should be installed at the minimum height of 2.1 m. As a thumb rule, a 30W lamp should be sufficient for 18 m2 of surface19,20, but room shape and type of fixture should be taken into consideration when calculating the needs. For instance, wall-mounted lamps would have a smaller germicidal area than ceiling-mounted ones. Lamps should be on whenever there is a risk of TB transmission. For example, in rooms with hospitalized patients, the lamps should be turned on 24 hours a day.
– Maintenance: see below.
Dust-covered and/or old UVGI lamps are less effective, hence the need for a careful maintenance, including regular cleaning:
– Lamps and fixture surfaces should be wiped at least monthly (more often if necessary) with a cloth dampened with 70% alcohol. Do not use water and soap or any detergent. The cleaning should be performed when lamps and fixtures are cool.
– Measurement of UVGI level must be done at installation and at least once a year. A UV light meter programmed to detect UV light on a wavelength of 254 nm is needed. Measurements should be performed at eye level in the occupied zone (~ 1.6 m) and in upper irradiated portion of the room, at a distance of 1.2 m from the fixture in all possible directions (imitating a circle with measurements done while moving in circumference spaced of 1 m). Ideally, all upper room measurements should be around 30 μW/cm2 to 50 μW/cm2. Persons doing these measurements should wear protective equipment (UVprotective glasses, clothing made of tightly woven fabric, soft cotton gloves) and cover exposed skin with opaque creams with solar-protection factors > 15.
– UV lamps last between 5 000 and 10 000 hours of continue use (7 to 14 months). Check manufacturer’s information. After this period, UV lamps rapidly lose effectiveness and need to be changed.
UV lamps contain mercury and quartz and are considered as hazardous waste. Disposal is extremely difficult in many countries; this should be considered before implementing them. If adequate disposal of the lamps by specialized enterprises is not possible in the country, neither their repatriation; UV lamps should be disposed of by encapsulation (sealed in a metal 200 litre drum filled with concrete and then buried away from water sources).
Reflecting surfaces in the irradiation area of UV lamps must be avoided (i.e. oil painted ceilings, etc.).
At certain wavelengths (including UV-C) UV exposure may be harmful. Skin exposure can produce sunburn (erythema). Exposure of the eyes can produce conjunctivitis (feeling of sand in the eyes, tearing) and/or keratitis (intense pain, sensitivity to light). These symptoms typically commence 6 to 12 hours after exposure.
Despite the fact that these are reversible conditions, health care workers should immediately report them to the IC officer. This could mean that UV irradiation is higher than previously thought at lower room level (lamp poorly positioned? Reflecting surface?).
The USA National Institute for Occupational Safety and Health (NIOSH) states that safe exposure limits are set below those found to initiate eye irritation, the body surface most susceptible to UV. Next table shows the permissible exposure times for given effective irradiances at 254 nm wavelength.
|Permissible exposure time*||Effective irradiance|
* The occupational exposure limit for UV-C at 254 nm is 6,000 μJ/cm2. This can be also calculated with the following formula: Dose (in μJ/cm2) = Time (in seconds) * Irradiance (in μW/cm).
Exposures exceeding this limit would require the use of personal protection equipment to protect the skin and eyes.
In order to avoid overexposure of UVGI, education of health care workers should include basic information on UVGI systems and their potential harmful effects of if overexposure occurs.