Spectroscopic Analysis of Ultraviolet Lamps for Disinfectionof Air in Hospitals

Syed Hussain Shah

Received: 12 February 2009 /Accepted: 25 July 2009 /Published online: 19 November 2009# Springer Science + Business Media B.V. 2009

Abstract In order to have feasibility study ofultraviolet lamps for air disinfection in hospitals, anumber of lamps were studied spectroscopically. Itwas required to understand if these lamps reallyemitted 253.7 nm. Was the radiation intense enoughto disinfect the surrounding air in the hospital? And toknow about the percent transmission of the UV raysthrough the glass of the lamps. The intensity and UVdose were calculated at various distances in air fromthe lamps. While putting 1-min exposure time in thereciprocity law, the intensity/distance and dose/distance graphs were drawn for various lamps. Fromthe trend line of the graph the distance at which thelamp emitted 10 μW/cm2 was calculated. In this studythe 1-min exposure time was considered, using theRiley’s designed upper room air disinfection data for90% kill rate. In the light of the results these lampswere considered fit to be installed either in the airducts or in the upper room at certain distances for airdisinfection, and suggestions were given for theimprovement of the system proposed.

Keywords Ultraviolet germicidal irradiation .

Deoxyribonucleic acid . Ultraviolet dose .

Ultraviolet lamps . Quartz materials .

The reciprocity law

1 Introduction

Ultraviolet (UV) radiation is defined as that portion ofthe electromagnetic spectrum which is lying in betweenX rays and visible light and occupies from at least 100 to400 nm (Matthes and Ingolstaedter 2004).

Ultraviolet radiation has numerous applications indaily life. It is used in a wide variety of medical andindustrial processes and for cosmetic purposes. Theseinclude photo curing of inks and plastics, photo-resistant processes, solar simulation, cosmetic tanning,fade testing, dermatology, and dentistry (Matthes andIngolstaedter 2004). UV rays have various commercialuses as well, including sterilization and disinfection,disinfection of drinking as well as waste water (Matthesand Ingolstaedter 2004), sterilization of air for indoorair quality, disease control, and biodefense applications.

2 Mechanism of Ultraviolet Disinfection

In the double-stranded helical structure of the deox-yribonucleic acid (DNA), both the strands are linkedtogether by a sequence of four nitrogenous bases(adenine, cytosine, guanine, and thymine) which

Water Air Soil Pollut: Focus (2009) 9:529–537DOI 10.1007/s11267-009-9231-0

S. H. Shah (*)College of E&ME,National University of Sciences and Technology (NUST),Peshawar Road,Rawalpindi, Pakistane-mail: syed_hussain_shah@hotmail.com

constitutes the genetic code. These bases grouptogether to form “base pairs” (adenine with thymineand cytosine with guanine) held together by hydrogenbonds. If two thymine bases are located adjacent toeach other, absorption of an UV photon (253.7 nm)by one of the thymine leads to formation of a chemicalbond between the two thymine (called a thyminedimer; Fig. 1). This disrupts the DNA structure, and incase enough number of thymine dimers is produced,the DNA cannot participate in cell mitosis (Bolton2004).

3 Ultraviolet Lamps

A typical ultraviolet lamp system includes an irradiatorto produce high-intensity UV light, a power supply toprovide electrical power to the irradiator, and aninterconnecting high-voltage cable. UV lamps are eitherlow pressure or high/medium pressure mercury lamps.In general, low pressure lamps (2×10−3–2×10−5PSI;Clark 2006) have filaments on each lamp end. Theycarry the arc current after ignition and in most casesthey are preheated before ignition to maximize thelamp life. These lamps emit the characteristic wave-length of mercury of 253.7 nm in the UV-C wavelengthband (Clark 2006).

Medium/high pressure lamps (2–200 PSI) alsocalled high-intensity discharge tube, have tungstenelectrodes that are not preheated as in low-pressurelamps. Ignition voltages of several kilovolts arerequired to start the lamps (Clark 2006). Medium-pressure lamps have broader emission lines and less

relative output at 254 nm. Low-pressure UV lampsare line sources of light, emitting only a very narrowline of energy, with no emission between the lines, whenatoms are raised to an excited state (Clark 2006).

UV lamps may either be “hot cathode” or “coldcathode” depending upon the manufacturing andfunctioning of the electrode inside the tube. Hotcathode lamps contain tungsten filament, which areusually, coiled-coil or triple-coiled, coated withalkaline earth oxides. Hot cathode fluorescent lampsrequire large amounts of electric current that is pushedthrough a thin tungsten wire filament, superheating it(Clark 2006). Cold cathode electrodes do not rely onadditional means of thermionic emission besides thatcreated by the electrical discharge through the tube.Instead, they are fitted with a firm solid cylindricalelectrode at each end of the lamp.

Germicidal lamps are similar to fluorescent andmercury-vapor lamps used for general lighting.However, germicidal lamps lack phosphor coatingson the bulb wall, and they use special glass capable oftransmitting the short-wavelength radiation that wouldotherwise be absorbed by conventional lamp glass(Philips et al. 2003).

Quartz is a good UV transmitting material (Philipset al. 2003). Quartz transmits 180 nm in ozonegenerating lamp and only to 253.7 nm for disinfectionof water and air, while other glasses are opaque to it.Teflon plastic also allows UV rays to pass through.

4 Indoor Air Quality

Indoor air can be a number of times more pollutedthan outdoor air (Kizer 1990). Exposure to indoorpollutants is a key contributor to a number of diseases(Kizer 1990). The air in a single room can containhundreds of thousands of infectious bacteria, viruses,fungal spores, and contaminants, which can only beseen with a microscope. In addition to other methods,airborne infections can be prevented by air disinfectionwith UV rays. In case of air disinfection with UV rays,UV lamps can be either suspended in ceiling fixtures ormay be installed in air circulation ducts. The firstmethod is called (Kizer 1990) over head or upper aircirculation, while in the second method a UV lamp isplaced inside the ventilation system ducts. In theoverhead air circulation, the fixtures are shielded onthe bottom so that the radiation is directed only up

Fig. 1 Mechanism of ultraviolet disinfection (wikimedia.org/DNA–UV mutation, NASA, 2006)

530 Water Air Soil Pollut: Focus (2009) 9:529–537

towards the ceiling and out to the sides. The height ofthe fixtures should be at least 7 ft above the floor sothat people will not bump into them or look directly atthe bare tubes. In the air circulation ducts, the airentering or leaving a room can be disinfected with UVlamps placed inside the ventilation system ducts(Fig. 2). This method could be used in clinics fromwhere air is recirculated to other parts of the buildings,but overhead disinfection cannot be done because theceiling is too low (Kizer 1990). For air disinfection tobe effective, the microorganisms must receive asufficient dose of UV radiation.

This can be achieved by using lamps of the correctwavelength and intensity are important, and byexposing the bacteria for sufficiently long periods oftime gives good disinfection results.

5 Materials and Methods

The following sequence was followed during theexperimental work:

5.1 Labeling of the UV Lamps

A number of ultraviolet lamps were collected andlabeled alphabetically from “A” to “O” simplifyingthe analysis work.

5.2 Measurement of the Input Power of the Lamps

The input power of UV lamps was recorded in thepresence of ballast set in the circuit. The current

through the circuit was measured and multiplied bythe input voltage (220 V) to find input power in watts.

5.3 Spectroscopic Analysis of Ultraviolet lamps

This portion of the study is divided into the followingtwo phases:

1. The qualitative analysis

In order to confirm the emittance of 253.7 nm froma given lamp, spectra of 11 ultraviolet lamps weretaken with the help of “Spectrophotometer, HitachiF-220–800 nm”. The spectra show relative emittancealong the Y-axis and wavelength along the X-axis.

2. The quantitative analysis

Percent transmittance of 253.7 nm through theglass of UV lamps

In order to study the percent UV transmittancethrough the glass of UV lamps “Perkins Elmer UV/Vis/NIR spectrometer Lambda 190–3,200 nm” was used.

a. Ultraviolet energy dose emitting from the lamps.

In the quantitative phase, the emittance power of253.7 nm of 10 UV lamps was recorded at variousdistances from a given UV lamp in air. The intensity wascalculated by dividing the recorded values with the areaof the sensor (03 films 016). Due to nonavailability ofradiometer, UV wattmeter recorded linear emittance.

Specifications of the UV sensor are:Percent transmittance of 253.7 nm=11.54Area of the UV sensor

A ¼ pr2¼ 1:1309 cm2

¼ mw=1:13 cm2

¼ 0:884mw= cm2

¼ 884 mw= cm2

Let us consider 0.884 m W/cm2 as a unit:Intensity “I” of the lamp “L”, when the sensor was

placed just above it

I ¼ 2:314� 0:884mw= cm2

I ¼ 2045:58mw= cm2

According to the reciprocity law, dose of UV raysis equal to the product of the intensity and contacttime (Kizer 1990).

Dose ¼ intensity� exposure time ð1Þ

Service door with

inspection window

Fig. 2 UV lamps located inside ventilation system duct(Kizer 1990)

Water Air Soil Pollut: Focus (2009) 9:529–537 531

Considering “Riley’s” designed model upper roomfor UV disinfection of air where both air mixing as wellas UV disinfection have been considered, in this modelthe exposure time for 90% kill rate was put 1 min.

Substituting for exposure time in Eq. 1, we get thedose of lamp “L” as 122,735 μw s/cm2. The dose ofradiation from all other the lamps at various distancefrom the lamp were calculated in the same way. Doseof the lamps was plotted vs distance. From the trendline equation of the graphs we find the distance fromlamp where intensity drops to 10 μw s/cm2.

6 Results and Discussions

6.1 Input Power of the Lamps

The input power of all the lamps recorded in thepresence of ballast set is shown in Table 1. Most ofthe lamps required an input of 52.8 W while some ofthem needed 35.2 W. It was practically seen duringthe quantitative analysis that lamps with high inputpower emitted intense beam of UV rays while thosetaking low input power emitted weak beam of UV rays.

6.2 Qualitative Analysis of the Emitting Rays

The spectroscopic analysis revealed that there weretwo kinds of low pressure UV lamps in the lot collected.

Figure 3 of lamp “B” shows strong peak at 253.7 nmin addition to a fraction from visible portion. Thislamp is purely low-pressure UV lamp havingnegligibly small emission of the visible light. Spectraof most of the other lamps show peaks at 253.7 nm(UV-C), in addition to very small kinks at 311.8 nm(UV-B) and 364.2 nm (UV-A) and sufficient emission inthe visible range beyond 400 nm. These lamps givesufficient glow as a mixture of green and blue light dueto the presence of visible wavelengths beyond 400 nm.Figure 4 represents the overall trend of most of theselamps. This qualitative study indicates that majority ofthese lamps are low-pressure mercury lamps (Clark

Table 1 Input power of ultraviolet lamps (with ballast set in circuit)

Lamp Arc length (ft) Current (A) Voltage (V) Apparent power (W) Power (W) with 0.8 power factor

A 1.00 0.3 220 66 52.8

B 0.72 0.2 220 44 35.2

C 0.53 0.3 220 66 52.8

D 0.53 0.3 220 66 52.8

E 0.53 0.3 220 66 52.8

F 0.53 0.3 220 44 52.8

G 0.72 0.2 220 44 35.2

H 0.53 0.2 220 44 35.2

J 0.72 0.3 220 66 52.8

K 1.36 0.3 220 66 52.8

L 0.66 0.3 220 66 52.8

M 0.53 0.2 220 44 35.2

N 0.53 0.2 220 44 35.2

O 0.66 0.3 220 66 52.8

Fig. 3 Low pressure UV lamp “B”

532 Water Air Soil Pollut: Focus (2009) 9:529–537

2006), emitting UV-visible light. Spectrum of anincandescent lamp is also shown below indicating thewavelength distribution beyond 400 nm (Fig. 5). Thisstudy revealed that all of the lamps were germicidal innature which could be employed for disinfection,provided the intensity level does not drop from thethreshold level required.

6.3 Transmittance Through the Glass of Lamps

In this study the percent transmittance spectra for theglass of 8 UV lamps was recorded as shown in Table 2.The study shows that transmittance of 253.7 nmthrough the glass of the lamps ranges from 43.6% to68 %. Figure 6 for lamp “L” has a representativesketch for all the lamps.

A silicate glass (water drinking bowl) was alsostudied, as shown in Fig. 7, indicating 0% transmittance

of 253.7 nm. The analysis work shows that glass of allthe lamps is quartz material of different qualities. Asquartz is above 80% transmitting to germicidalultraviolet rays (Engineering Bulletin, OSRAMSULVANIA 2006), but the results obtained are lessthan the expected values. The analyzed lamps wereclean but these lamps were not “brand new” itemsbecause some of them were used in UV water reactorfor disinfection purposes. Due to this reason, theymight have fractionally lost the transmissivity due toscaling on their surfaces.

6.4 The “Riley’s” Room

Before we design a model room for the use of theseUV lamps, we should have a glance on the “Riley’s”designed model upper room where UVair disinfectiontakes into account both air mixing and upper room

Fig. 4 Low pressure UV lamp “L”

Fig. 5 The wavelength distribution beyond 400 nm of anordinary incandescent lamp

Table 2 Percent transmittance through the glass of the lamps

UV lamp Range of wavelengthtransmitted (nm)

% transmittance

A 190–400 52

F 200–400 50

J 193–400 68

K 200–400 48

L 217–400 68.2

M 200–400 43.6

N 200–400 47.4

O 200–400 47.4

Fig. 6 Percent relative transmittance of 253.7 nm through theglass of lamp “L”

Water Air Soil Pollut: Focus (2009) 9:529–537 533

UV inactivation of organisms (Philips et al. 2003).The room height is assumed to be 8 ft, the lower 4 ftof air circulates vertically through the upper 4 ft. It isfurther assumed that a minimum number of 10 lowerroom air volumes pass through the upper roomultraviolet germicidal irradiation (UVGI) exposurezone per hour (Philips et al. 2003) as shown in Fig. 8.Where UV dose for 90% kill rate with an exposuretime of 1 min is 10 μW/cm2. The UV fixture isadjusted at sufficient height so that the occupants ofthe room are safe from UV rays.

6.5 Power, Intensity, and Dose of UV Lamps in Air

Results of the power, intensity, and dose emittingfrom the lamps are given in a comprehensive Table 3given below. Lamps “A” and “K” of Philips company,lamps “L” and “O” with So-Safe label, and lamp “F”(fabricators unknown) were comparatively intense

lamps, while lamps labeled as “Sankyo Denki” wereless intense lamps. The real manufacturer of the“Sankyo Denki” lamps were unknown. The intensityof UV radiation in air drops with the square of thedistance from the source (Kizer 1990).The trend ofthe graph was in good agreement with the law, whenintensity and dose were plotted vs. distance from thesource as shown in Figs. 9 and 10. Using the trendline equations of the dose/distance and intensity/distance graphs, we find the distance from the lampswhere the UV dose drops to 10 μW/cm2. Forexample, if one of the “Sankyo Denki” lamps, labeledas “M” from the table is selected, its trend lineequation is y ¼ 92017 e�0:1146�. From this equationwe find the distance from the lamp as 80 cm or 32 in.(quarter to 3 ft). In the same way, we can finddistances or ranges of UV rays for all other the lampsfrom the data and the graphs (Kizer 1990).

6.6 Air Cleaning with Ultraviolet Raysand Other Methods

Air changing per hour, or air mixing and/or ultravioletgermicidal irradiation, are methods normally consideredfor cleansing of air of a hospital room. The effectivenessof UVGI can be compared to ventilation in terms ofequivalent air changes. As an example, if a roomnormally ventilates by six air changes per hour,adding an upper room UVGI system might achievethe air cleaning equivalent of approximately anadditional 10 to 20 air changes per hour (Kizer1990). This effect is being amplified further byincreasing air mixing either by the use of fans or by

Fig. 7 Percent relative transmittance through pyrex glass

Fig. 8 Riley’s designed model upper room (Philip et al. 2003)

534 Water Air Soil Pollut: Focus (2009) 9:529–537

Tab

le3

Pow

er,intensity,anddo

seof

radiationfrom

ultravioletlamps

atadistance

"X"in

air

X(cm)

Lam

p"A

"Lam

p"B

"Lam

p"F"

Lam

p"G

"Lam

p"J"

mW

μw/cm

2μw

s/cm

2mW

μw/cm

2μw

s/cm

2mW

μw/cm

2μw

s/cm

2mW

μw/cm

2μw

s/cm

2mW

μw/cm

2μw

s/cm

2

02.9

2,56

3.6

153,81

6.0

1.8

1,59

1.2

95,472

.02.1

1,85

6.4

111,38

4.0

1.8

1,59

1.2

95,472

.01.9

1,67

9.6

100,77

6.0

12.1

1,85

6.4

111,38

4.0

1.5

1,32

6.0

79,560

.01.7

1,50

2.8

90,168

.01.4

1,23

7.6

74,256

.01.5

1,32

6.0

79,560

.02.5

1.9

1,67

9.6

100,77

6.0

0.9

795.2

47,736

.01.2

1,06

0.8

63,648

.00.9

795.6

47,712

.01.01

892.84

53,570

.45

1.3

1,14

9.2

68,952

.00.7

618.8

37,128

0.8

707.2

42,432

.00.7

618.8

37,128

.00.6

530.4

31,824

.010

0.8

707.2

42,432

.00.4

353.6

21,216

.00.5

442

26,520

.00.4

353.6

21,216

.00.4

353.6

21,216

.015

0.5

442.0

26,520

.00.3

265.2

15,750

.00.3

265.2

15,912

.00.3

265.2

15,912

.00.3

265.2

15,912

.020

0.3

265.2

15,212

.0–

Fused

–0.2

176.8

10,608

.0–

Fused

––

Fused

–

X(cm)

Lam

p"K

"Lam

p"L

"Lam

p"M

"Lam

p“N

”Lam

p"O

"

mW

μw/cm

2μw

s/cm

2mW

μw/cm

2μw

s/cm

2mW

μw/cm

2μw

s/cm

2mW

μw/cm

2μw

s/cm

2mW

μw/cm

2μw

s/cm

2

02.9

2,56

3.6

153,81

6.0

2.3

2,03

3.2

121,99

2.0

1.8

1,59

1.2

95,472

.01.8

1,59

1.2

95,472

.02.6

2,29

8.4

139,76

4.0

12.1

1,85

6.4

111,38

4.0

1.5

1,32

6.0

79,500

.01.6

1,41

4.4

84,864

.01.4

1,23

7.6

74,256

.01.5

1,32

610

6,20

6.0

2.5

1.9

1,67

9.6

100,77

6.0

1.3

1,14

9.2

68,952

.01.2

1,06

0.8

63,648

.00.9

795.6

47,736

.01.01

892.84

53,570

.4

51.3

1,14

9.2

68,952

.00.7

618.8

37,128

.00.7

618.8

37,128

.00.7

618.8

37,128

.00.6

530.4

31,824

.0

100.7

618.8

37,128

.00.4

353.6

21,216

.00.4

353.6

21,216

.00.4

353.6

21,216

.00.4

353.6

21,216

.0

150.5

442.0

26,520

.00.3

265.2

15,912

.00.3

265.2

15,912

.00.3

265.2

15,912

.00.3

265.2

15,912

.0

200.4

353.6

21,216

.00.02

17.68

1,06

0.80

0.2

176.8

10,608

.0–

Fused

–0.29

256.36

15,381

.6

Exp

osuretim

e,1min

Water Air Soil Pollut: Focus (2009) 9:529–537 535

increasing air temperature gradients between theupper and lower room (Fig. 11).

6.7 Feasibility of the Analyzed Lampsfor Air Disinfection

It is clear from the results shown in Table 3 that all thelamps have sufficient UV dose to disinfect thesurrounding air in a considerable range. In case ofour study we may use these lamps for disinfection inhospitals in a number of ways like air-stream disinfec-tion applications, surface-disinfection applications,room air disinfection by air mixing and UV germicidalaction etc. In case of room air disinfection by airmixing and UV germicidal action the vigorous up flowof air rapidly brings infectious particles into the upperroom. The air current will get more and more exposureto more intense radiation for disinfection. The air maybe pumped up for mixing or otherwise the passivemixing is also sufficient for disinfection. Heat flowsfrom an adult human being at the equivalence of a100 W incandescent bulb (Kizer 1990).

In addition to a single patient, other parameters likethe incandescent light bulbs, the attendant of thepatient, the doctors, and nurses on duty and electricappliances like heater, refrigerator etc., producesufficient heat to keep the passive air circulationcycle maintained (Fig. 12).

6.8 Model Hospital’s Room

These lamps may be installed in a hospital’s room forair disinfection in the form of a fixture either insidethe air duct or in the form of fixtures in the upperzone of the room below the ceiling in rows andcolumns as shown in Fig. 13. In the first case, theducts may be arranged at suitable positions in theroom like close to wall or at the entrance or exit ofthe air through an exhaust fan, so that there is no humanexposure to the UV rays and the air is pumped throughthem for disinfection without any health hazard.

In the second case, the UV fixture may be arrangednear the ceiling so that they provide a kill zone to themicrobes in the air on exposure. As the arc length ofthe lamps ranges from 21 to 44 cm, we arrange asuitable number of lamps in bundles of uniformlengths. In this case either the air may rise passivelyor may be pumped up.

Distance vs Power of UV lamp "M"

y = 1.7349e-0.1146x

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25Distance (cm)

Po

wer

Mic

ro W

atts

Fig. 9 Power/distance of UV lamp in air

Distance vs Dose of UV lamp "M"

y = 92017e-0.1146x

0

20000

40000

60000

80000

100000

120000

0 5 10 15 20 25Distance in cm

Do

se (

mic

ro W

atts

/ Cm

2 )

Fig. 10 Dose/distance of UV lamp in air

Fig. 11 UV duct with fixtures inside (Philip et al. 2003)

Fig. 12 Perpendicular airflow through a UV duct (Philips et al.2003)

536 Water Air Soil Pollut: Focus (2009) 9:529–537

Let us select a 12×12 ft room of a hospital whichis 12 ft high (Fig. 13) for air disinfection and upper airmixing with passive air circulation. We may distributethe number of fixtures according to the room size. Ifthe lamps in the fixture have comparatively smallerarc length, then a greater number of fixtures areneeded to be installed along the vertical axis. In this casethe arc length of lamp “M” is just 0.53 ft (Table 1).

The trend line equation gives the range of this lampas 32 in. Two fixtures at a distance of 3 ft/less than3 ft from either the left/right wall is sufficient to coverthe X-axis, because one fixture will cover about 6 fton both sides, the number of fixtures along the Z-axismay also be adjusted according to the vertical space.

The other option in this case is the active aircirculation. If two fixtures are installed close to the rarewall and air is pumped up, it will enter the radiation zoneon reflection from the rare and the side walls as well asfrom the ceiling and disinfection will occur.

As the ceiling is relatively high, greater than 9 ft,such fixtures can be open type. When the ceiling islower, louvered fixtures are necessary to ensure thatpeople are not exposed to direct germicidal radiation.The ceiling in case of open fixtures should not be UVreflecting, so that it should not harm the people in theroom (Kizer 1990).

7 Conclusions

The study shows that all the lamps are useful fordisinfection of room’s air in hospitals as well as inother places of public gathering. Air may be dis-infected when lamps of uniform lengths are groupedin the form of fixtures and fixed inside the air ductsreceiving air through the exhaust or any other pumpedsystem. These lamps may also disinfect air, if anumber of fixtures are located near the ceiling in theupper 3 ft zone, distributed horizontally as well asvertically and the air current rises passively. When airis pumped up along the rare wall of the room, UVfixtures may do the job, if they are installedhorizontally in the upper zone of the room close tothe rare wall. The number and size of the fixtures alsodepends upon the size and type of the room.

Acknowledgements The author gratefully acknowledges theunforgettable cooperation, advice, and assistance received fromDr. James Bolton, Dr. Javed Iqbal, Dr. Shoukat Hameed, Mr.Gohar khan of PEPA, So-Safe, and Bin-Qutab water purifiers.

References

Bolton, J. R. (2004). Ultraviolet applications hand book. Ayr:Bolton’s Photoscience. ISBN 0-9685432-1-9, 2004.

Clark, S. H. (2006). Ultraviolet light disinfection in the use ofindividual water purification devices: Technical Informationpaper #31-006-0306, pp.01-14, March 2006. handle.dtic.mil/100.2/ADA453967.

DNA–UV Mutation Picture from A research article fromNASA (2006) (wikimedia.org/DNA_UV_mutation. Gif)http://fi.wikipedia.org/wiki/Tiedosto:UV-DNA.PNG.

Engineering Bulletin. (2006). Germicidal and Short-Wavelength UV Radiation Lamps: OSRAM SYLVANIA.10/2006. http://www.sylvania.com/content/display.scfx?id=003690188.

Kizer, K. W. (1990). Using Ultraviolet Radiation and Ventilationto Control Tuberculosis. California Indoor Air QualityLaboratory, Director Department of Health Servicespp. 1–19, 1990. http://www.americanultraviolet.com/uvc/techsheets/UVCureTB_all.pdf.

Matthes, R., & Ingolstaedter, L. (2004). Guidelines on limits ofexposure to Ultraviolet Radiation of wavelength between180 nm and 400 nm ( Incoherent optical radiation). Theintenational Commission on Non-Ionizing RadiationProtection, 171–175. www.srp-uk.org/biblio5.html.

Philips Bricknr, W., Richard, L., et al. (2003). Application ofUltraviolet Germicidal Irradiation to Cotrol Transmissionof Airborne Disease: Bioterrorism Countermeasure.Public Healthh Reports/ March–April 2003/Volume 118.www.medicalairsolutions.com/techref/UVGI%20and%20Bioterrorism%20Countermeasures.pdf.

Fig. 13 Design of UV fixture arrangement with passive oractive air circulation for disinfection in a room

Water Air Soil Pollut: Focus (2009) 9:529–537 537