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w jymfo Environmental and Atmospheric Ju*i994
Protection Agency Administration
x°/EPA Experimental UV Index
TECHKICAL APPENDICES TO THE EXPERIMENTAL W INDEX FACT8HEET
Starting in the summer of 1994, the National Weather Service
(NWS) and the U.S. Environmental Protection Agency (EPA) are
offering a new product, the Experimental UV Index. It is being
issued daily on an experimental basis for use by meteorologists
as part of their weather report. The Index is a next day
forecast of the amount of ultraviolet radiation that will reach
the earth's surface during the peak hour of sunlight around noon.
The Index includes the effects of cloud cover on the anticipated
UV intensity for the next day.
The U.S. Environmental Protection Agency (EPA),.
collaborating with the Centers for Disease Control and Prevention
(CDC), -has initiated a public information campaign to coincide
with the release-of the Index. This campaign is aimed at raising
public awareness of the health risks of sun exposure. The
challenge ir great because attitudes about the attractiveness of
a "healthy tan" are so firmly established.
This publication, Technical Appendices to the Experimental
UV Index Factsheet, is intended to act as a supplement to another
publication, The Experimental UV Index Factsheet: Explaining the
Index to the Public. The primary audiences for the factsheet are
meteorologists, public health officials, physicians, and other
professionals who are likely to be called upon to answer public
inquiries about the Index or who can make use of the Index to
increase public awareness about the hazards of overexposure to UV
rays.
f - ,
These appendices provide more in-depth information about two
methodologies: the calculations for the Index and the
calculations for the minutes to sunburn for the skin phototypes
described in the factsheet.
It is anticipated that professionals in the meteorological
community will be interested in Appendix A (the explanation of
the UV Index). Professionals in the medical community will be
more interested in Appendix B (the explanation of MEDs) and
Appendix C (the explanation of the minutes to burn) provided by
EPA. Although every effort has been made to provide a basic
explanation of each set of calculations, it may be that members
of either group will find that the explanation of the material
outside their area of expertise is not wholly clear. A number of
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sources of information are given at the end of this document and
more are cited in the UV factsheet cited above.
APPENDIX A: HOW THE INDEX IB CALCULATED
' The forecast methodology relies on the relationship between
solar angle, total column ozone, cloud cover, elevation, and UV
radiation.
First, projected next-day ozone values are calculated. To
do this, measurements of total column ozone for yesterday taken
by NOAA satellites are used as inputs. This data is modified
based on projected changes in various meteorological fields
(i.e., circulation and temperature at specified altitudes) for
tomorroi£. The resulting forecast is for tomorrow's ozone values.
Second, calculations of the peak amount of ultraviolet
radiation reaching the earth's surface under clear sky conditions
are made. Using these projected ozone values as an input, a
radiative transfer model calculates the amount of . ultraviolet
radiation that could reach the earth's surface at solar noon.
(Local solar noon is at 12:00 Standard Time or 1:00 Daylight
Savings Time . ) This information yields the maximum amount of
exposure possible at a location.
Third, a weighting function is applied to match the
performance characteristics of the surface-based observing
systems. This value is used in later calculations as a dose
rate.
Fourth, the results are then integrated over the solar noon
hour to provide a maximum solar noon clear sky exposure. (Solar
noon hour extends from one half hour before solar noon to one
half hour after solar noon.) The results from this calculation,
which are expressed in hectoJoules per square meter (hectoJ/m2) ,
typically fall into a range between 0-15.
Fifth, the noon time dosage is then further modified by a
factor to account for the presence of clouds in the forecast.
This factor, which ranges from 1.0 to 0.31, is determined by an
equation which uses the forecasted probabilities of clear skies,
scattered clouds, and broken clouds as inputs. The equation is
derived from statistical comparisons of the forecasted
probabilities and the ratio of the observed and computed clear
sky UV levels for multiple sites at different geographic/ climatic
locations within the United States. The final number is then
disseminated as the forecasted Experimental UV Index.
To make this Index a useful public information tool, the
Environmental Protection Agency has developed exposure level
categories (e.g., minimal, low moderate, high, and very high).
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In addition, EPA has issued a publication with illustrative
public health messages for each of the five categories. (See
publication, Bulletin of Sample Public Heath Messages to
Accompany Experimental UV Index.) In particular, these messages
are being made available for use by broadcast.meteorologists, if
they so choose. It is hoped that wide distribution of these
messages will help the public better interpret the Index and will
encourage people to take preventive steps to reduce the risks of
overexposure.
APPENDIX B: EXPLANATION OF MINIMAL ERYTHEMAL DOSES (MBDs)
A MED is defined as "the smallest amount of sunlight
exposure necessary to induce a barely perceptible redness of the
skin within 24 hours after sun exposure." [Perry Robins, Sun
Sense.] It is a widely used measure of skin damage from the sun.
MEDs are a useful tool in predicting an individual's response to
the energy dosage represented by each of the Index values.
Medical researchers study MEDs under controlled conditions
in laboratories or other research facilities. The amount of
energy it takes to produce a MED varies for many reasons,
including the laboratory equipment, the experimental conditions,
varying degrees of pigmentation among subjects, and differences
in thickness of skin and amount of previous UV exposure on parts
of a single individual's body. As a result, definitions of MEDs
can vary among researchers or health practitioners.
As this discussion would indicate, the dosages of solar
energy needed to produce reddening or erythema (the MED) can vary
for a single person. Even greater variability in MEDs is seen
among people with different skin phototypes.
To make use of the MEDs in public information efforts, it is
necessary to use some set of skin types. For purposes of the
educational efforts associated with the UV Index, EPA is using a
set of four skin phototype categories and a set of minimal
erythema1 doses associated with each. Since there are gradual
variations of coloration in human beings, the ranges of energy
required to produce minimal redness overlap. Information on the
skin phototypes and MEDs used by EPA is given in the figures
below.
EPA recognizes that there are other characterizations of
skin types with fewer and greater numbers of skin type
categories. There are also different calculations about the
energy doses needed to produce one MED for the various skin
types.
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Figure 1; Description of Skin Phototypes
SKIN PHOTOTYPES
SKIN COLOR IN
UNEXPOSED AREA
TANNING HISTORY
Never Tans/
Always Burns
pale or milky white;
alabaster
develops red sunburn;
painful swelling;
skin peels
Sometimes Tans/
Usually Burns
very light brown;
sometimes freckles
usually burns;
pinkish or red
coloring appears; can
gradually develop
light brown tan
Usually Tans/
Sometimes Burns
light tan, brown, or
olive; distinctly
pigmented
rarely burns; shows
moderately rapid
tanning response
Always Tans/
Rarely Burns
brown, dark brown,
or black
rarely burns; shows
very rapid tanning
response
Figure 2: Skin Phototypes and KEOs
SKIN PHOTOTYPES
.Vever Tans /Always Burns
Sometimes Tans/Usually Burns
Usually Tans /Sometimes Burns
Always Tans /Rarely Burns
ENERGY (MINIMAL
ERYTHEMAL DOSE) REQUIRED
TO PRODUCE REDDENING
10 - 30 mJ/cm2
30 - 50 mJ/cm2
40 - 75 mJ/cm2
50 - 120 mJ/cm2
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Figure 3: Skin Phototypes and Erythemal Doses Graphed
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APPENDIX C: HOW THE MINUTES TO SUNBURN ARE CALCULATED
People are likely to ask how to use the Index number to make
decisions about limiting their exposure to the sun. To help
provide guidance about how to -answer this question, "Figure 3:
Range of Minutes to Burn for Different Index Values" includes
minutes to sunburn for the most and least susceptible skin types.
The explanation of how these numbers have been calculated
follows.
First, the Index value is converted to milliJoules per
square centimeter. (The Index is the amount of UV energy in
hectoJoules per square meter, which is then rounded off to the
nearest whole number.)
Second, the number of MEDs in the solar noon hour is
calculated. As explained in Appendix B above, a MED is the
minimal ultraviolet radiation energy required to produce just
perceptible reddening of previously unexposed skin twenty-four
hours after exposure. MEDs vary for different skin phototypes.
See "Figure 2: Skin Phototypes and MEDs" for the energy units
for minimal reddening used in these calculations. To calculate
the number of MEDs at solar noon, the number of mi Hi Joules per
square centimeter computed in step one above is divided by the
energy required to produce one MED.
Third, the time required for sunburn is calculated by
dividing 60 minutes (number of minutes in the solar noon hour) by
the number of MEDs. In this way, a range of minutes to burn for
each skin type and each number on the UV Index scale can be
produced.
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Figure 4: Two calculations of minutes to burn for a sample day
under different sky conditions.
Minutes to burnfor sample dav described abovewith prediction
for clouds (UV Index of 51;
UV Index (UV energy expressed in
hectoJoules/m2 and rounded off to nearest
whole number: 5
Convert to miHiJoules/cm2: 50 mJoules/cm2
(multiply by 10)
Energy for minimal erythema for Never Tans
skin type (taken from attached figure): 10 mJoules/cm2
Number of MEDs in one hour: 5 MEDs
(50 mJ/cm2 divided by 10 mJ/cm2)
Minutes to sunburn for Never Tans skin type at
UV Index of 5: 12 minutes
(divide 60 minutes by 5 MEDs)
Minutes to burn for sample dav if prediction had been for no
clouds (UV Index of 9):
UV radiation dosage (UV index which has been
rounded off from hectoJoules/m2): 9
Convert to milliJoules/cm2: 90 mJoules/cm2
(multiply by 10)
Energy for minimal erythema for Never Tans
skin type: 10 mJoules/cm2
Number of MEDs in one hour: 9 MEDs
(90 Mj/cm2 divided by 10 mJ/cm2)
Minutes to sunburn for Never Tans skin type at
UV Index of 9: 6 minutes
(divide 60 minutes by 9 MEDs)
There is a large difference between ranges of minutes to
burn under the sky with no clouds scenario and the sky with
clouds scenario. These calculations demonstrate the fact that an
important part of the public information effort accompanying the
release of the UV Index must be to educate people on the fact
that the relationship between the UV Index forecast (which
includes cloud cover) and the potential for much greater UV
exposure if cloud cover is not present.
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Figure 5; Range of Minutes to Burn for Different Index Values
EXPOSURE
CATEGORIES/
INDEX VALUES
Minimal 0-2
Low 3
4
Moderate 5
6
High 7
8
9
very High 10
11
12
13
14
15
MINUTES TO BURN FOR
"NEVER TANS"
SKIN PHOTOTYPE
(most susceptible)
30 minutes
20 minutes
15 minutes
12 minutes
10 minutes
8.5 minutes
7.5 minutes
7 minutes
6 minutes
5.5 minutes
5 minutes
< 5 minutes
4 minutes
< 4 minutes
MINUTES TO BURN FOR
"RARELY BURNS"
SKIN PHOTOTYPE
(least susceptible)
>120 minutes
90 minutes
75 minutes
60 minutes
50 minutes
40 minutes
35 minutes
33 minutes
30 minutes
27 minutes
25 minutes
23 minutes
21 minutes
20 minutes
Also refer to Figure 6: Range of Minutes to Burn Range
Between Most and Least susceptible Skin Phototypes." It is a
more visual presentation of the range of time to burn intervals
for the most and least susceptible skin types for the range of
possible UV exposures presented in Figure 5 above.
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Figure 6: Minutes to Burn: Range Between Most Sensitive and
Least Sensitive
X3ONI AH
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OTHER RESOURCES FOR MORE INFORMATION
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The following federal agencies have information on the UV
Index or related issues, including skin cancer and ozone
depletion. Public inquiry telephone numbers are provided where
available.
NOAA/National Weather Service
National Meteorological Center
Washington, DC 20233
301-713-0622
(for information on Experimental UV Index)
Be Sun Wise! Program
U.S. EPA
401 H Street SW (6205J)
Washington DC 20460
(for information on Index and health messages)
EPA Stratospheric Ozone Information Hotline
1-800-296-1996
Centers for Disease Control and Prevention
National Center for Chronic Disease Prevention and Health
Promotion
Division of Cancer Prevention and Control
4770 Buford Highway NE, Mailstop K-57
Atlanta, GA 30341-3724
Cancer Information Service
National cancer Institute
1-800-4-CANCER
(for information on skin cancer)
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