submitted to Applied Optics,
Sept 3, 2001
A T-SHIRT UV-RADIATION EXPERIMENT IN TIBET
Norsang Gelsor (1), Fred Sigernes (2),
Yngvar Gjessing (3), Ladislav Kocbach (4)
(1) Department of Math. & Physics, University
of Tibet, Tibet, China;
(2) University Courses on Svalbard, Longyearbyen,
(3) Institute of Geophysics, University
of Bergen, Norway;
(4) Institute of Physics, University
of Bergen, Norway
An experiment has been conducted in Lhasa, the capital of Tibet, at an
altitude of 3650 m above sea level, to test the protective capability of
T-shirts from solar UV radiation. The experiment was conducted on June
9th, 2000, with clear sky conditions using 6 different types of T-shirts,
which were selected randomly according to color, fabrics and condition.
The global solar UV irradiance and the corresponding transmission properties
of the T-shirts were obtained by two identical instruments. The effective
erythemal dose rate ratios between covered and uncovered instruments were
measured to be in the range from 0.4 up to 6 %, depending mainly on fabrics
and thickness of the shirts. Estimates of the corresponding daily Tibetan
erythemal UV dose show that 3 of the shirts exceed the 1 MED dose (Minimal
Erythemal Dose) level during summer. Thin cotton shirts have daily doses
in the range 0.2 to 1.2 MED. The highest daily dose was found to be above
2.5 MED for the mixed fabricated shirt labeled 6% polyester and 94% cotton.
Thick and closely woven T-shirts of 100 % cotton have the best protective
capability with daily doses less than 0.35 MED.010.4950, 170.1870
During the last centuries, man's actions may have disturbed the ozone balance
in the middle atmosphere. The main reason for the ozone depletion in the
stratosphere is believed to be connected to the increase in anthropogenic
compounds such as chlorine or bromine (cf. Brasseur and Solomon)
. These constituents are believed to affect the chemical and photochemical
processes connected to the natural production and loss of ozone. One important
feature of the stratospheric ozone layer is that it has the ability to
absorb UV radiation emitted by the Sun. This feature makes the ozone layer
a molecular shield which protects the Earth against UV radiation. As the
ozone layer gets thinner, UV radiation at the surface of the Earth increases.
If the ozone content decreases by 10 % during the spring and summer, the
annual UV dose increases by about 12 %. Human risks connected to a prolonged
or repeated exposure to the UV are sunburns, skin cancers, premature skin
aging and eye damage. The most common effect is
sunburn, which indicates and warns us that our skin has been over exposed
to ultraviolet radiation (especially UVA, wavelengths from 315 - 400nm).
The more serious scenario is skin cancer related to an extreme over exposure
to UVB (290 - 315 nm). Protecting the human body from the sun's harmful
UV radiation is becoming more and more necessary for people who live in
high altitude and low ozone concentration regions. The most common and
simplest way to protect from UV radiation is to wear clothes.
Regions like the Tibetan Plateau, the largest and highest plateau in
the word (average altitude of about 4000 m a. s. l.) has a much higher
erythemal UV dose rate at ground level compared to most other regions.
According to previous measurements done by Ci Ren et al,
Lhasa's dose rate can reach up to 500 mW/m2 in the mid summer at solar
noon. Figure 1. shows the yearly variation of UV radiation
from July 1996 to December 1997. In summer, the dose rates are above the
half maximum (approximately 250 mW/m2 ) in a period of 5 hours centered
around local noon.
As a support, the ozone climatology measurements by the Total Ozone
Mapping Satellite (TOMS) show that a region with low ozone concentration
appears over the Tibetan plateau in summer,.
Wheter the low ozone concentration over Tibet is a natural dynamic effect
caused by the high mountain range, the increased release of anthropogenic
CFC gases or by the sun itself, is not yet understood.
Figure 1: Diurnal and daily variation of the erythemal
UV dose rates in Lhasa during the period from June, 1996 to December, 1997
from Ci Ren et al
Click on the figure to view full size
A new site for ground based monitoring of UV radiation has been established
at the University of Tibet, Lhasa, 1999. The site has been supplied with
both broadband filter instruments and an imaging spectrometer (Sigernes
al. A yearly calibration have been carried
out on these instruments to secure the quality
of the data and support the already existing instrument that was installed
at the Meteorological Bureau in Lhasa, 1996. The aim is to monitor the
global UV irradiance at ground level and the stratospheric ozone content
to seek out if there is any detectable climatic trend. However, as an interesting
thing and somewhat urgent for people who live and visit Tibet, a 'T-shirt
UV experiment' was designed to provide in situ information on the
protective property of T-shirts when exposed to high doses of UV radiation.
Two identical multi-channel filter NILU-UV Irradiance Meters were used
in this study. The experiment was conducted at the roof of the Department
of Mathematics and Physics, Tibet University, two hours after local noon
(14:32-15:48 LT) June 9th, 2000. The sky condition was partly cloudy with
clear sky in the zenith. The instruments are produced by the Norwegian
Institute for Air Research. Each instrument has
six filters, five in UV and one in the visible part of the spectrum. The
center wavelengths are 305, 312, 320, 340 and 380 nm, representing the
spectral regions classified as UVB (290 - 315 nm) and UVA (315 - 400 nm)
of the solar spectrum. The Full Widths at Half Maximum (FWHM) are close
to 10 nm. The sixth filter covers the whole visible region from 400 to
Figure 2: Experimental setup for the two NILU-UVs mounted
at the roof of the Institute of Mathematics and Physics, Tibet University.
Click on the figure to view full size
Teflon diffusors are used as front optics to the filters to obtain the
global radiation. Both instruments were calibrated prior to the experiment.
The experimental setup is shown in figure 2.
The experimental procedure was first to check that both instruments
measured the same intensities. Secondly, one of the instruments was covered
by a T-shirt for about 10 minutes. The transmission Twas then calculated
as the ratio between the intensity Sof the covered and the intensity
of the uncovered instrument:
the center wavelength. The procedure was repeated for each T-shirt. Six
T-shirts were selected randomly according to color, fabric, woven state,
condition and number of volunteer owners. Table 1. shows the specific characteristics
of each T-shirt.
Table 1: A list of the characteristics of the six T-shirts
used in the experiment together with sample period. The shirts are in the
Qualitative row labeled by color. White is thin and light colored cotton
shirts. Light gray is thick dark closely woven cotton shirts, while dark
gray represents the only shirt which is not marked 100% cotton (medium
||White; thin; (new)
||Light gray w/ dark pattern; thin
||Black; thick; closely-woven
||Light gray; normal thickness
||94% cotton; 6% polyester
||Brown; tightly-woven; thin
||Dark blue; thick; closely woven; (new)
The biological effective dose rate of a NILU-UV covered by a T-shirt
is calculated as a discrete sum
is the measured intensity at center wavelength .
Note that the set of weights or dose coefficients are
calculated for each instrument by a procedure outlined by Dahlback.
Results and discussion
The aim of this study is to provide information on the transmission characteristics
of a typical T-shirt in order to retrieve the corresponding effective UV
dose that people are exposed to at the Tibetan Plateau. The dose rates
visualized in figure 1. are extremely high. According
to the US National Oceanic and Atmospheric Administration (NOAA), the UV
Index range from 0 to 15 or 16 (in the tropics at high elevations under
clear skies). The UV index is divided into minimal (0 - 3), low (3 - 4),
moderate (5 -6), high (7 - 9), and very high ( > 10) levels. These levels
are defined according to the time it takes to risk sun burns. At the Tibetan
Plateau, the UV index is greater than 10 around solar noon from April to
October, which indicate that the length of time to skin damage may be less
than 10 minutes without protection. The maximum UV index in mid summer
is measured to be as high as 20. This is to the authors' knowledge the
highest index levels measured in areas populated by humans.
Table 2: Transmission coefficients for six T-shirts as
a function of center wavelengths 305, 312, 320, 340 and 380 nm. Each with
a bandwidth of approximately 10 nm. The visible transmission factor covers
the spectral region from 400 - 700 nm. The corresponding biological effective
dose rates are calculated for each T-shirt. The biological effective ratio
between the doses are also shown.
||CIE dose rate
||CIE dose rate
||[ % ]
||ratio [ % ]
Table 2. shows the transmission factors calculated by equation (1) for
each T-shirt used in the experiment. As expected, the transmission factors
in the visible wavelength region seem to follow the color and thickness
of the shirts. Thick dark colored T-shirts transmit less than thin light
colored ones. The transmission in UV is in general much lower and more
complex than in the visible. The state of the material becomes of equal
importance. The thick dark colored and closely woven T-shirts 3 and 6 have
a mean transmission of 0.3 and 0.6%, while the thin and light colored shirts
1, 2, and 5 are correspondingly up to approximately 10% more transparent.
All of the shirts are labeled 100% cotton, except for T-shirt 4 which consist
of 6% polyester and 94% cotton. This shirt stands out to have the overall
highest transmission, especially in UVB. Note that the transmission curve
for all shirts increase with wavelength in UVB. In UVA, a local maximum
transmission occurs at approximately 320 nm. T-shirt 2 is the only exception.
It has an increasing transmission with a small local maximum shifted down
to 312 nm. This effect is believed to be associated with the shirt's dark
gray pattern. A combined effect of the transmission from light and dark
gray areas of the shirt causes the local maximum at 320 nm to disappear.
The biological effective dose rates during the experiment dropped from
237 to 112 mW/m2. This effect is clearly consistent with Figure
1 both according to the UV dose level and the time of year. During
calibration of the instruments the dose rate was as high as 420 mW/m2.
The corresponding dose rates for the T-shirts in Table 2 were 10.4, 10.6,
1.3, 23.1, 5.2 and 2.5 mW/m2, respectively. Table 2 also shows the dose
rate ratio between covered and uncovered instrument. This ratio represents
the effective biological transmission rate of the shirts. For simplicity
we assume this transmission rate to be constant as a function of solar
zenith angle in order to retrieve an estimate of the daily doses wearing
T-shirts. The monthly mean daily erythemal UV doses of figure
1 was calculated by Ci Ren et al to
vary between 2.2 to 7.6 kJ/ m2 . A maximum daily dose of 9.18 kJ/m2was
obtained in July 1996. Table 3 shows the corresponding daily doses for
T-shirts used in the reported experiment.
Table 3: The daily UV dose using six T-shirts as a function
of typical daily erythemal UV doses measured in Lhasa, Tibet.
|[ kJ/m2 ]
The doses are given in units of MED (Minimal Erythemal Dose). 1 MED
equals 210 J/ m2and is defined as the minimum UV dose level for perceptible
reddening of previously unexposed human skin. It should be well known that
human individuals are not equally sensitive to UV radiation due to different
pigmentation to a certain degree related to ethnic origin. Human skin is
often classified into four main groups according to the skinís ability
to protect itself,.
The value of 1 MED for skin types I, II, III and IV are approximately 200,
250, 350 and 450 J/ m2, respectively. The 1 MED value for Tibetans are
to the authors not know. Nevertheless, according to the International Radiation
Protection Association, the daily doses of the
ultraviolet radiation should not exceed the limit of 1 MED. From Table
3. it is clear that thick dark and closely woven T-shirts (100% cotton)
protect human skin from solar UV even under extreme dose levels. The light
colored and thin cotton shirts do also protect us, but not in the extreme
cases. T-shirt No. 4 stands out to have the worst protective characteristics
with daily doses above 2.5 MED. Compared to color and thickness of the
100% cotton shirts, the result must be due to the mixture of polyester
From the above in situ experiment we recommend people that visit
Tibet during summer, as a minimum wear protective thick closely woven T-shirts
labeled 100% cotton, especially individuals of skin type I and II. Thin
or even medium thick T-shirts made of polyester and cotton should not be
used. Further studies are need to retrieve the Minimal Erythemal Dose levels
for the local population. A larger ensemble and a qualitative description
of the shirts should also be included to improve future T-shirt experiments
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About this document ...
A T-shirt UV-radiation experiment in Tibet: submitted to Applied Optics,
Sept 3, 2001
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