DECEMBER 2006

Technology Review
UPPER ROOM
ULTRAVIOLET
GERMICIDAL
IRRADIATION
ELIMINATOR
(UVGI)
AN UPDATE

HEALTH TECHNOLOGY ASSESSMENT UNIT

MEDICAL DEVELOPMENT DIVISION
MINISTRY OF HEALTH
029/06

i
Author:
Dr. Sheamini Sivasampu
Principal Assistant Director
Health Technology Assessment Unit
Ministry of Health, Malaysia

Reviewed by:
Datin Dr. Rugayah Bakri
Head
Health Technology Assessment Unit
Ministry of Health, Malaysia

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 TABLE OF CONTENT

1 BACKGROUND 1
2 INTRODUCTION 1
3 TECHNICAL FEATURES 1
4 OBJECTIVE 2
5 METHODOLOGY 2
6 RESULT AND DISCUSSION 3
6.1 Safety 3
6.2 Effectiveness 3
6.3 Cost Implications 4
7 CONCLUSION 4
8 RECOMMENDATIONS 4
9 REFERENCES 5
Appendix 1 –Literature Search 8
Evidence Table 9

iii
1. BACKGROUND
The Deputy Director General (Medical) from the Ministry of Health has asked for an
update to the existing technology review by the Health Technology Assessment Unit on
UVGI Virus Zone Eliminator (2004).

2. INTRODUCTION
The control of indoor air quality (IAQ) plays an important role in the prevention of
infection in hospitals to protect both hospital staff and patients, especially
immunosuppressed and immunocompromised patients, who are highly susceptible
to the adverse effects of various airborne chemicals and microbes. Improper control of
hospital IAQ may cause hospital-acquired (nosocomial) infections and occupational
diseases (Leung & Chan 2006).

More recently the bioterrorism threat and the appearance of new pathogens with the
potential for airborne spread, such as severe acute respiratory syndrome (SARS), have
stimulated installations of upper-room irradiation systems (First et al 2005). Ultraviolet
Germicidal Irradiation (UVGI) systems for air disinfection are coming into increasing use
for indoor air quality and disease control (Bahnfleth et al 2005).

3. TECHNICAL FEATURES

Currently there are eleven distinct system of UVGI application available ranging from in-
duct air disinfection for HVAC application, to upper air single-room systems, to surgical
site surface disinfection refer to Fig 1. While there are certain common issues in the
application of such systems, such as lamp performance and safety, the information and
methods needed to design each type are quite different. The most common types of
systems (in no particular order) are in-duct and cooling coil disinfection systems in
HVAC applications, standalone recirculating units such as may be found in hospital
isolation rooms, and upper air systems (Bahnfleth et al 2005).

It has been noted that the efficiency of disinfection depends on both the UV radiation
field intensity and the residence time of microbes is exposed to the radiation field.
Therefore, the mechanical ventilation system should maintain an air speed not exceeding
the limit recommended by the UVGI system manufacturer for sufficient residence time
(Leung & Chan 2006).

At present, the most well-developed areas of standards related to the application of UVGI
are the rating of lamps (IESNA 2000; CIE 2003), electrical safety (certifiable by any of
several laboratories), and safe human exposure limits (NIOSH 1972; ACGIH 1991;
AIHA 2001; IRPA 1985; NEHC 1992).

Currently planned list of guidelines and standards for upper room UVGI and their status
as of May 30, 2005 are as follows (Bahnfleth et al 2005):
ƒ IUVA-G01A: General Guideline for UVGI Air and Surface Disinfection Systems
(draft under internal IUVA review)

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ƒ IUVA-G02A: Guideline for Design and Installation of UVGI Air Disinfection
Systems in New Building Construction (draft under internal IUVA review)
ƒ IUVA-S04A: Standard for the Testing and Commissioning of Upper Room UVGI
Systems (first draft pending)
ƒ IUVA-S05A: Standard for the Testing of UVGI Surface Disinfection Systems (first
draft pending)
ƒ IUVA-S06A: Standard for Laboratory Testing of UVGI Air and Surface Rate
Constants (draft under internal IUVA review)
ƒ IUVA-S08A: Standard for Epidemiological Testing of Air Treatment Systems (first
draft pending)

Figure 1. UVGI installation configurations: (A) Upper-room

configuration and (B) Air-duct configuration.(Leung & Chan 2006)

4. OBJECTIVE

The aim of this paper is to update on the existing evidence with regards to upper room
UVGI eliminator looking at aspects of safety, effectiveness and cost implications.

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5. METHODOLOGY

Electronic search was carried out using various databases as included in Appendix 1. As
this is an update, limits were applied and articles were retrieved from 2005-2006. The
keywords used in the search include: ultraviolet germicidal irradiation; germicidal
ultraviolet irradiation; UVGI; air disinfection; standards; guidelines; air sterilizer;
hospital; indoor air quality; AND management.

Any article (including reviews) considered potentially relevant was included, and the
bibliographies of publications were examined for additional relevant studies.

6. RESULT AND DISCUSSION

6.1 Safety

The issue of direct exposure to UV light resulting in kerato-conjunctivitis (so-called

welder’s eye), and skin cancer was addressed in our previous technology review report
(2004). However recent studies revealed that the risk of kerato-conjunctivitis can be
prevented by installing the fixtures within ventilation systems (duct irradiation), or by
using wall- or ceiling-mounted fixtures with baffles to block rays directed downward so
that only the air in the upper room is irradiated (upper room irradiation) (Tuberculosis
and Chest Service Public Health Services Branch, 2006).

First et al (2006) also demonstrated (based on a limited number of observations) that by

limiting the maximum lower room irradiance at every point to less than the continuous 8-
hour time-weighted average threshold limit value the observed doses were one-third to a
factor lower than the doses calculated from maximum eye-level irradiances
measurements in the occupants' spaces.

6.2 Effectiveness

ASHRAE [9], AIA [11], and CDC [12] recommend that UVGI be used to supplement
the essential engineering control methods, including mechanical ventilation, filtration,
and differential pressure control and that UVGI cannot be used as a substitute to the
above measures(Leung & Chan (2006); Tuberculosis and Chest Service Public Health
Services Branch, (2006); NICE (2006).

Types of Airborne Organisms

(i) Bacteria

In an experimental study by Xu et al (2005), increasing the irradiance level of the UVGI

lamps increased the effectiveness of inactivating Mycobacterium parafortuitum. However
the relationship was linear up to a certain level. A further increase in the irradiance above

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this high level resulted in little increase in the inactivation of the airborne TB-like
bacteria.

Whereas another experimental study demonstrated that the choice of suspending medium
influenced both size and UVGI susceptibility of S. marcescens (Lai et al 2004).

These studies once again concurred with our previous report that the efficacy of upper
room UVGI is effected by room ventilation rate, UV radiance levels and distributions,
airflow patterns and relative humidity.

Mycobacteria tuberculosis
There are no recent studies that address the control of tuberculosis using UVGI
application.

Most authorities believe the most important part of TB control measure is to get the
infectious patient into the isolation room as soon as possible. Air changes per hour
(ACH) of 6 to12 under negative pressure is pivotal for infection control in an isolation
room. The other measures such as HEPA filter or UV irradiation are no replacement for a
properly maintained respiratory isolation rooms, although they may serve as
supplementary measures in selected settings (Tuberculosis and Chest Service Public
Health Services Branch (2006); NICE 2006; Leung & Chan 2006).

More recent studies have once again demonstrated that germicidal effect of UV
irradiation varies with relative humidity and intensity of the UV light (Xu et al 2005;
Tuberculosis and Chest Service Public Health Services Branch, 2006). It has been
recommended that for optimal efficacy of UV irradiation, the relative humidity should be
maintained below 60% (Tuberculosis and Chest Service Public Health Services Branch,
2006). High relative humidity above 75 percent lowers the effectiveness of UVGI to
inactivate the TB-like bacteria (NIOSH Update 2003).

Another concern raised in the Hong Kong Tuberculosis Manual is as the lamp gets old or
covered with dust, the residence time for air sterilization increases therefore affecting the
effectiveness of the device (Tuberculosis and Chest Service Public Health Services
Branch 2006).

(ii)Fungal
Two studies conducted by Green et al (2004, 2005) noted that UVGI application was less
effective against fungal spores.

6.3 Cost Implications

There are no available studies on the cost implications of upper room UVGI. However
there was statement in the Tuberculosis and Chest Service Public Health Services Branch
(2006) that “UV light are attractive because the fixtures are relatively cheap, and the
maintenance and energy costs are low”.

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7. CONCLUSION

The safety of UVGI is still a concern and UVGI should be installed and maintained
properly to prevent both short term and long term complications.

Most recent epidemiological studies on the health benefits of air treatment using upper
room UVGI are still few in number and limited in quality.

8. RECOMMENDATIONS

To ensure hospital indoor air quality and disease control, it is recommended priority lies
in having good ventilation design. UVGI systems are only effective as supplementary
measures for deactivating bacteria in selected settings.

Further research on the effectiveness of upper room UVGI are required.

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9. REFERENCES

ACGIH. 2002. TLVs and BEIs. In Proc. American Conference of ACGIH (1991)
Threshold Limit Values and Biological Exposure Indices for 1991-1992., American
Conference of Governmental Industrial Hygienists, Cincinnati, OH.

AIHA. 2001. Nonionizing Radiation Guide Series, Ultraviolet Radiation, American

Industrial Hygiene Association, Akron, OH.

ASHRAE. 1999. ANSI/ASHRAE Standard 52.2 -1999 Method of Testing General

Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size, American
Society of Heating, Ventilating, and Air Conditioning Engineers, Atlanta, GA.

Bahnfleth WP, Kowalski WJ, Freihaut J (2005). Standard And Guideline Requirements
For UVGI Air Treatment Systems Proceedings: Indoor Air 2005. 3464

CIE. 2003. Ultraviolet Air Disinfection. CIE 155:2003, International Commission on

Illumination, Vienna, Austria.

First MW, Weker RA, Yasui S, Nardell EA (2005). Monitoring human exposures to
upper-room germicidal ultraviolet irradiation. J Occup Environ Hyg. May;2(5):285-292.

Green CF, Davidson CS, Scarpino PV, Gibbs SG. (2005) Ultraviolet germicidal
irradiation disinfection of Stachybotrys chartarum. Can J Microbiol. Sep; 51(9):801-4.

Green CF, Scarpino PV, Jensen P, Jensen NJ, Gibbs SG (2004). Disinfection of selected
Aspergillus spp. using ultraviolet germicidal irradiation. Can J Microbiol. Mar;50(3):221-
4.

Griffiths WD, Bennett A, Speight S, Parks S (2005). Determining the performance of a

commercial air purification system for reducing airborne contamination using model
micro-organisms: a new test methodology. J Hosp Infect. 2005 Nov; 61(3):242-7. Epub
Jul 11

IESNA. 2000. Lighting Handbook 9th Edition IESNA HB-9- 2000, Illumination
Engineering Society of North America, New York, NY.

IRPA. 1985. International Radiation Protection Association Guidelines on Limits of

Exposure to Ultraviolet Radiation of Wavelengths Between 180 nm and 400 nm
(Incoherent Optical Radiation)." Health Phys. 49, 331-340.

Ko G, First MW, Burge HA. Influence of relative humidity on particle size and UV
sensitivity of Serratia marcescens and Mycobacterium bovis BCG aerosols. Tuberc Lung
Disease 2000;80:217-28.

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Lai KM, Burge HA, First MW (2004). Size and UV germicidal irradiation susceptibility
of Serratia marcescens when aerosolized from different suspending media. Appl Environ
Microbiol. Apr;70(4):2021-

National Institute for Health and Clinical Excellence (2006). Tuberculosis – Clinical
diagnosis and management of tuberculosis and measures for its control and prevention.
March. Clinical Guideline 33. Royal College of Physicians

Tuberculosis and Chest Service Public Health Services Branch (2006). Tuberculosis
Manual. Hong Kong: Department of Health

Xu P, Kujundzic E, Peccia J, Schafer MP, Moss G, Hernandez M, Miller SL (2005).

Impact of environmental factors on efficacy of upper-room air ultraviolet germicidal
irradiation for inactivating airborne mycobacteria. Environ Sci Technol. Dec
15;39(24):9656-9664

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Appendix 1

LITERATURE SEARCH

An electronic search of the HTA agency websites, Cochrane Database of Systematic

Reviews via OVID, ACP Journal, Cochrane Controlled Trial Register and other EBM
sites were initially carried out. Subsequently PUBMED database was searched and limits
were applied (2004 to 2006). Various relevant guidelines on tuberculosis were also
retrieved. The keywords used in the search either singularly or in combination were
“UVGI; air disinfection AND ultraviolet; UVGI zone eliminator; UVGI lamps; upper
room UVGI; safety; effectiveness; efficacy and cost.

Number of search: 8
Number of relevant abstracts: 7
Number of full text articles obtained: 2
Guidelines: 3

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 EVIDENCE TABLE: _ULTRAVIOLET GERMICIDAL IRRADIATION
Study design, sample size &
No Author, Title, Journal, Year, Volume Outcome & Characteristic Grade Comment
Follow up
EFFECTIVENESS ULTRAVIOLET GERMICIDAL IRRADIATION
1 Leung M, Chan AH. (2006) Cross Sectional study Airborne pathogens, such as multidrug-resistant Level 8
Mycobacterium tuberculosis
Control and management of hospital literature review and conducted bacteria, Legionella bacteria, and measles viruses,
indoor air quality. comprehensive IAQ can be killed by UVGI [12,21]. However, UVGI is
assessments in nine hospitals in less effective against fungal spores. The efficiency
Hong Kong of disinfection depends on both the UV radiation
Med Sci Monit. Mar;12(3):SR17-23. field intensity and the residence time of microbes
Epub 2006 Feb 23. The control and mitigation exposed to the radiation field.
measures cover mechanical
ventilation, filtration, Therefore, the mechanical ventilation system
differential pressure control, should maintain an air speed not
directional airflow control, exceeding the limit recommended by the UVGI
local exhaust ventilation, and system manufacturer for sufficient residence time.
ultraviolet
germicidal irradiation (UVGI) ASHRAE [9], AIA [11], and CDC [12]
disinfection. Their applications recommend that UVGI be used to supplement the
in critical environments, such essential engineering control methods, including
as operating theatres, isolation mechanical ventilation, fi ltration, and differential
rooms, and other typical units, pressure control, but UVGI cannot be used as a
such as outpatient departments substitute for any of these methods.
and laboratories, are also
considered

UV radiation of wavelength
220–300 nm can penetrate cell
walls and inactivate tiny
airborne droplet nuclei by
disrupting
their reproductive
mechanisms]..

BACTERIAL
2. Xu P, Kujundzic E, Peccia J, Schafer Cross sectional evaluated the Performance of the UVGI system degraded Level 8
MP, Moss G, Hernandez M, Miller SL efficacy of an upper-room air significantly when the relative humidity was
(2005) (UVGI) system for inactivating increased from 50% to 75-90% RH, the horizontal
airborne bacteria, which UV fluence rate distribution was skewed to one side

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 Study design, sample size &
No Author, Title, Journal, Year, Volume Outcome & Characteristic Grade Comment
Follow up
irradiates compared to being evenly dispersed, and the room
Impact of environmental factors on the upper part of a room while air temperature was stratified from hot at the ceiling
efficacy of upper-room air ultraviolet minimizing radiation exposure to cold at the floor.
germicidal irradiation for inactivating to persons in the lower part of
airborne mycobacteria. the room. The inactivation rate increased linearly with
effective UV fluence rate up to 5 microW cm(-2);
Environ Sci Technol. Dec A full-scale test room (87 m3), an increase in the fluence rate above this level did
15;39(24):9656-64. fitted with a UVGI not yield a proportional increase in inactivation rate.
system consisting of 9 louvered
wall and ceiling fixtures (504
W all lamps
operating) was operated at 24
and 34 degrees C, between 25
and 90% relative humidity, and
at three ventilation rates.
Mycobacterium parafortuitum
cells were
aerosolized into the room such
that their numbers and
physiologic state were
comparable both with and
without the UVGI system
operating.

3 Lai KM, Burge HA, First MW Experimental systems S.marcescens suspended in water-only medium was
(2004) the most susceptible to UVGI, followed by those in
It is generally recognized that serum-only medium. The count median diameters
Size and UV germicidal irradiation data from different laboratories (CMDs) for culturable particles from water-only
susceptibility of Serratia marcescens might vary significantly due to and serum-only media were 0.88 and 0.95 micro m,
when differences in systems and respectively, with the measurements based on their
aerosolized from different suspending experimental conditions aerodynamic behavior.
media.
In this study, the effect of the The bacteria suspended in phosphate buffer,
Appl Environ Microbiol. composition of the suspending synthetic saliva, and phosphate-buffered saline had
Apr;70(4):2021-7. medium on the size and UVGI similar UVGI susceptibility and CMD at 1.0, 1.4,
. susceptibility At low humidity and 1.5 micro m, respectively. At high humidity
(36%),. (68%) the CMD of the particles increased by 6 to
16%, and at the same time UVGI susceptibility
decreased, with the magnitude of decrease related to

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 Study design, sample size &
No Author, Title, Journal, Year, Volume Outcome & Characteristic Grade Comment
Follow up
the type of suspending medium.
In conclusion, the choice of suspending medium
influenced both size and UVGI
susceptibility of S. marcescens. These data are
valuable for making comparisons
and deciding on the use of an appropriate medium
for various applications.

FUNGAL
4. Green CF, Davidson CS, Scarpino PV, UVGI dose necessary to The UVGI dose necessary to inactivate 90% of the Level 8
Gibbs SG. inactivate fungal spores on an S. chartarum was greater than
2005 agar surface and the efficacy of the maximum dose of 144 mJ/cm2 evaluated in this
UVGI were determined for study. While UVGI has been used to inactivate
Ultraviolet germicidal irradiation cultures of Stachybotrys several strains of culturable fungal spores, S.
disinfection of Stachybotrys chartarum. chartarum chartarum was not susceptible to an appropriate
dose of UVGI. The results of this study may not
Can J Microbiol. Sep; 51(9):801-4. This study employed a UVGI correlate directly to the effect of UVGI on airborne
testing unit consisting of four fungal spores.
chambers with a 9-W, Phillips,
low pressure, However, this indicates that current technology may
mercury UVGI lamp in each not be efficacious as a supplement to ventilation
chamber. The testing unit's unless it can provide higher doses of UVGI to kill
apertures were adjusted to spores, such as S. chartarum, traveling through the
provide 50, 100, 150, and 200 irradiated zone.
microW/cm2 of uniform flux to
the Petri dish
surfaces, resulting in a total
UVGI surface dose ranging
from 12 to 144 mJ/cm2.

5. Green CF, Scarpino PV, Jensen P, The efficacy of UVGI and the The UVGI dose necessary to inactivate Lev el 9
Jensen NJ, Gibbs SG. UVGI 90% of the A. flavus and A. fumigatus was 35 and
dose necessary to inactivate 54 mJ/cm2, respectively.
Disinfection of selected Aspergillus fungal spores on an agar UVGI can be used to inactivate culturable fungal
spp. using ultraviolet germicidal surface for cultures of spores. Aspergillus flavus was more susceptible
irradiation. Aspergillus flavus and than A. fumigatus to UVGI.
Aspergillus fumigatus were
Can J Microbiol. 2004 Mar;50(3):221- determined These results may not be directly correlated to the

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 Study design, sample size &
No Author, Title, Journal, Year, Volume Outcome & Characteristic Grade Comment
Follow up
4. effect of UVGI on airborne fungal spores, but they
A four-chambered UVGI indicate that current technology may not be
testing unit with a 9-W, efficacious as a supplement to ventilation unless it
Phillips, low pressure, can provide higher doses of UVGI to kill spores
mercury UVGI lamp in each traveling through the irradiated zone.
chamber was used in this study.
An aperture was adjusted to
provide 50, 100, 150, and 200
micro W/cm2 of uniform flux
to the surfaces of the Petri dish

6. Griffiths WD, Bennett A, Speight S, The performance of a duct- A three UV lamp system was effective against Level 9
Parks S (2005) mounted air disinfection airborne phages, removing an average of 97.34% of
system, designed to reduce the aerosolized challenge. With the UV component
Determining the performance of a airborne pathogens in the switched off, the average efficiency dropped to
commercial air purification system for hospital environment, was 61.46%.This demonstrates that the chemical-coated
reducing airborne contamination using determined using a new testing filter component plays a more significant role than
model micro-organisms: a new test methodology. The methodology the UV radiation in destroying phages. When six
methodology. places the equipment in a test UV lamps were used, the system was able to
duct, a microbial aerosol is remove mycobacteria with an efficiency exceeding
J Hosp Infect. Nov; 61(3):242-7. Epub generated and then sampled 99.99%. This test methodology can be used to
2005 Jul 11. simultaneously before and after assess manufacturers' claims of efficacy of
the test system. equipment against airborne micro-organisms in the
hospital environment.
This allows a percentage
efficiency value to be
calculated. The air disinfection
system is a novel chemical-
coated filter and ultraviolet
(UV) radiation air purification
system, operating at a flow rate
of 500 m(3)/h, against aerosols
of MS2 phage and
Mycobacterium vaccae
(surrogates of viral and
mycobactericidal pathogens).
7. Tuberculosis and Chest Service Public UV light has been Studies have shown that the germicidal effect of Level 9
Health Services Branch (2006). recommended for institutional UV irradiation varies with relative humidity and
TB control, because of its intensity of the UV light:120 1. Increase in relative

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 Study design, sample size &
No Author, Title, Journal, Year, Volume Outcome & Characteristic Grade Comment
Follow up
Tuberculosis Manual. efficacy in eradicating humidity reduces the efficacy of UV irradiation. It
airborne pathogens in has been recommended that for optimal efficacy of
Hong Kong: Department of Health experimental studies. With a UV irradiation, the relative humidity should be
room of 200 ft3 and 10 ft maintained below 60%.
ceiling, installing a 30 W UV 2. As the lamp gets old or covered with dust, the
lamp was described as residence time for air sterilisation increases.
equivalent to adding 20
ACH.38 UV light is attractive Due to these potential complications, low
because the fixtures are penetration power and slow onset of action of UV,
relatively cheap, and the the year round high humidity and unproven efficacy
maintenance and energy costs in practice, UV light is not so widely used locally.
are low.

8. NICE (2006) Evidence based guideline Studies were searched for that focussed on Level 9 Still unclear of how the
Search methodology given. measures directed at patients with infectious TB to went on to base their
Tuberculosis Looked at combination with prevent transmission to other patients or contacts. It recommendations based on
Clinical diagnosis and management of single formulation regimens. was expected that these measures might include two RCT
tuberculosis, and measures for its mask wearing by the patient, isolation in a single
prevention and control room, negative pressure rooms, germicidal
ultraviolet radiation or air disinfectant at sites of
March 2006. Clinical Guideline 33 transmission.

Given the unexpected data on negative pressure

facilities from the review of current service (see
9.3.2), and similar findings in other surveys, the
recommendations spell out the three categories of
infection control, and require simple steps to clarify
which rooms meet the agreed standards. There can
be conflicting guidance on whether staff should
wear masks. It was agreed that masks are only
required for MDR TB or during close contact in
cough-inducing procedures, for example
bronchoscopy and sputum induction. Patients are
reassured by effective infection control measures,
but are also often worried unnecessarily by masks
or gowns, especially if these steps are not explained
to them. The only role for patients wearing masks
was within the first two weeks of treatment (when

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 Study design, sample size &
No Author, Title, Journal, Year, Volume Outcome & Characteristic Grade Comment
Follow up
the patient remains infectious) and when they are
outside their single room, for example going for an
X-ray (as they may come into contact with other,
susceptible, patients).

The recommendations below deal with three levels

of isolation for infection control in hospital
settings:
● negative pressure rooms, which have air pressure
continuously or automatically measured, as defined
by NHS Estates105
● single rooms that are not negative pressure but
are vented to the outside of the building
● beds on a ward, for which no particular
engineering standards are required.

SAFETY
7. First MW, Weker RA, Yasui S, Nardell The objective is to flood the Air exchanges between the upper and lower room Level 8
EA. (2005) entire volume of a room above result in air disinfection of the occupied space.
6.5 ft with high intensity Designers of these systems have adopted the
Monitoring human exposures to upper- ultraviolet germicidal practice of limiting the maximum lower room
room germicidal ultraviolet irradiation. irradiation, while minimizing irradiance at every point to less than the continuous
unintentional irradiance below 8-hour time-weighted average threshold limit value,
J Occup Environ Hyg. May;2(5):285- 6.5 ft to avoid eye and skin severely limiting the irradiation intensity in the
92. irritation. upper room and thereby reducing one of the two
major factors
The method employed was to determining germicidal effectiveness, the other
have subjects wear a small being room air mixing. The hypothesis of this study
photometer that recorded total is that eye and skin exposure will be well below the
ultraviolet dose over the period recommended safe dose even when maximum eye-
of exposure while subjects went level irradiance levels in the room exceed the 8-
about their normal routine, and hour continuous exposure threshold limit.
comparing this value with a
hypothetical dose calculated The results of the study, based on a limited number
from the highest measured eye- of observations, confirmed the hypothesis.
level irradiance. Observed doses were one-third to a factor of a
hundred or more lower than the doses calculated

14
 Study design, sample size &
No Author, Title, Journal, Year, Volume Outcome & Characteristic Grade Comment
Follow up
from maximum eye-level irradiances measurements
in the occupants' spaces.

2. Tuberculosis and Chest Service Public Consensus opinion Direct exposure to UV light can result in kerato- Level 9
Health Services Branch (2006). conjunctivitis (so-called welder’s eye), and
prolonged direct exposure is associated with skin
Tuberculosis Manual. cancer. The risk of kerato-conjunctivitis is easily
prevented by installing the fixtures within
ventilation systems (duct irradiation), or by using
Hong Kong: Department of Health
wall- or ceiling-mounted fixtures with baffles to
block rays directed downward so so that only the air
in the upper room is irradiated (upper room
irradiation).

15