Part F: A Closer Look at....
Modeling the Effects of Ultraviolet Radiation
Understanding and predicting the effects of UV on organisms requires
insights from biology, physics, and chemistry. Biology and the physical sciences
meet in biophysics. Scientists understand both the physical and biological
principles well, so we can construct models of UV interactions and use them to
make predictions. Some of these predictions can be tested using the experimental
procedures described in earlier sections.
Elements of a Model of UV Action
We must consider the interactions among three factors to generate a model
of UV action.
1) Start with the intensity (number of photons/sec) and the photon
energy spectrum of the source of the UV radiation.
2) Consider how the blocking
power and energy dependence (attenuation spectrum) of all the materials
through which the radiation passes will alter the energy spectrum that reaches the
organism.
3) Look at the relative biological response of the organism. The
relative response for a particular effect plotted against photon energy is known as
the action spectrum for that effect. All of these quantities can be measured and
their combined effects calculated. To simplify these calculations we developed a
computer program called UVRISK.
We will illustrate the steps in setting up a model with an example. Suppose
you want to explore the risk of UV damage to DNA, from exposure to sunlight. In
real life this could correspond to the risk of skin cancer. (See Table 1 for
definitions of terms and the symbols and equations used for a mathematical
description.)
1. Select the spectrum for the source of UV. In this case, we will use the
spectrum of UV from the sun over the range of photon energies from
3 eV (a wavelength of about 400 nm) to 5 eV (about 250 nm).
2. Select the attenuation spectrum for each of the UV blocking agents
that the radiation will pass through. Since the sunlight passes
through the atmosphere, we will select ozone. We can add to that
other absorbers that we want to model, such as sunscreen, window
glass, or the plastic lid of a Petri dish.
3. Calculate the spectrum of the radiation after it has passed through
these materials. For our ozone example, we will calculate the solar
UV that reaches the surface of the earth. We must consider the ozone
concentration and the distance the radiation travels through the
atmosphere, which depends on the angle of the sun.
4. Select the action spectrum for the biological effect. We have selected
DNA effects, which include any biological effect that is a
consequence of damage to DNA.
5. Calculate the biological risk spectrum by multiplying the action
coefficient by the intensity of the net radiation for each energy. For
the DNA effect we have chosen, we find a sharp peak around 4.1 eV
(300 nm) in the UV-B region. This peak is where the action
spectrum, which increases with increasing photon energy, overlaps
the surface UV spectrum, which decreases with increasing photon
energy.
6. Calculate the total relative risk by adding up the risk for each energy.
You can use this value to characterize the risk of a particular
exposure, and then explore the effect of different factors on that risk.
For example, the ozone concentration, sun angle, or amount of UV
absorber (glass, plastic, sunscreen) will all change the relative risk.
7. Change the value of one variable at a time to evaluate its importance.
For example, you can show that the angle of the sun plays an
important role. When the sun is lower in the sky, the amount of UV
reaching the surface, and therefore the risk, is reduced.
Table 1: Definition of important terms as they are used in this discussion:
| Term
| Definition
| Symbol or Equation |
| Incident
intensity
| The intensity, usually in
photons/m2/sec, of radiation of a
particular energy perpendicular to the
absorber(s)
| I0 |
| Transmission
coefficient
| The fraction of the incident radiation
of a particular energy that passes
through an absorber
| T |
| Net
transmission
coefficient
| The fraction of the incident radiation
of a particular energy that passes
through a series of absorbers
| Tnet |
| Attenuation
coefficient
| The fraction of the incident radiation
of a particular energy that does not
pass through an absorber
| A = 1-T |
| Net attenuation
coefficient
| The fraction of the incident radiation
of a particular energy that does not
pass through a series of absorbers
| Anet = 1-Tn |
| Net intensity
| The intensity of radiation of a
particular energy that passes through
an absorber
| Inet = I0 T |
| Action
coefficient
| The relative probability, per incident
photon of a particular energy, that
some specific biological effect will
occur
| P |
| Relative risk
| The relative probability per second
for a given intensity of radiation of a
particular energy that some specific
biological effect will occur
| R = Inet P |
Using UVRISK
Start the program by typing UV at the DOS prompt. The screen that
appears will look like Figure 1. The screen is divided into several types of
windows. Along the left side of the screen are three windows for selecting
the UV source, one or more UV blocking agents, and a biological response.
The major area on the right side of the screen contains the display window.
Below this window is the key to the graph color codes, and above it is the
function key menu.
Selection windows:
You can move among these windows using TAB
and SHIFT-TAB. Within each window is an input box and a help area. Use
the UP-ARROW and DOWN-ARROW keys to scroll through the available
spectra in each category. A description of each selection is displayed in the
help area, and a graph of the associated spectrum is plotted in the display
window. You can also scroll through the spectra with the LEFT-ARROW
and RIGHT-ARROW keys, leaving the graphs of previously viewed spectra
displayed. You can use this feature to compare different spectra. To select
a spectrum to be included in the model, press the ENTER key while the
name of the spectrum is in the input box. To move to the next selection
window, press the TAB key. SHIFT-TAB moves to the previous selection
window.
Sources:
If you select Sunlight as the UV source, you will be
prompted to enter the date and time-of-day (the current date and time are the
defaults) and your location. If you enter a city and state, the program will
try to look up the latitude and longitude. It knows more than 9000 places in
the US, so try several nearby towns if the first try fails. If you wish to enter
the latitude and longitude directly press the ENTER key. The program will
use this information to calculate the current position of the sun, which it will
report as the zenith angle (0 when the sun is directly overhead) and the air
mass, which is the relative thickness of the atmosphere through which the
sunlight must pass (1 when the sun is directly overhead). If you select one
of the artificial sources, the program uses an intensity appropriate to the
laboratory experiments described elsewhere.
Blocking agents:
If you select Ozone as the blocking agent
(appropriate when you have selected Sunlight as the source) you will be
prompted to enter the amount of ozone as column abundance in Dobson
Units (DU). The default value is 300 DU, the generally accepted global
average. Other blocking agents are applied as described in the help area.
You may select up to 8 blocking agents (or the same one repeatedly) and
their effects will be combined to calculate the net attenuation. When you
have selected all the blocking agents you wish, press the TAB key. The
program will then calculate and display the net spectrum, which is the
source spectrum after it has been filtered through the selected blocking
agents.
Biological Effect:
Most UV effects on cells, whether yeast and other
microorganisms or human skin cells, are the result of damage to DNA.
Selecting DNA Effects will allow you to model any biological consequence
that depends on DNA damage. Sunburn and cataract formation are
examples of biological effects that are not simply the result of DNA
damage. When you select the biological effect and its action spectrum, then
the program will calculate and display as a spectrum the relative risk of that
effect occurring. It will also total the risk over all wavelengths and display
it as a single number, the relative risk.
To Learn More...
See Video tape section: Global Ozone
Figure 1: UVRISK screen layout
Table 2: Key Functions for UVRISK program
| Key
| Function |
| TAB or SHIFT-TAB
| Move among the selection windows |
| UP-Arrow or DOWN-Arrow
| Browse through spectra in active input box,
displaying each spectrum in turn |
| LEFT- or RIGHT-Arrow
| Browse through spectra in active input box,
displaying each spectrum without clearing the
previous one |
| ENTER
| Select the spectrum displayed in the active input
box or an input value such as time, date, etc. |
| ESC
| Go back to previous option in the active selection
window |
| BACKSPACE
| Delete the most recent character typed into an
input box |
| F3
| Exit from the program |
| F4
| Go into EDIT mode |
Table 3: Library of spectra in UVRISK program
UV Sources:
- Sunlight
- 300 watt quartz-halogen security light available from hardware and
discount stores; spectra are given with the glass cover in place and also
removed
- 15 watt germicidal lamp--intense source of UV-C at 4.9 eV (254 nm)
- 15 watt UV-B fluorescent lamp
- 15 watt fluorescent black light lamp--UV-A
- 15 watt cool white fluorescent lamp--emits only a trace of UV-B and
UV-C
UV Blockers:
- Ozone for 300 Dobson units (global average) with sun directly overhead
- Window glass, nominal 3/32" from hardware shore
- Plastic acrylic safety glazing, 3/32" from hardware store
- Plastic pop bottle--polyethyleneterephthalate (PETE)
- PABA--one of the original sunscreen ingredients no longer in general
use
- Padimate-O--sunscreen ingredient that blocks UV-B
- Benzophenone-3--sunscreen ingredient that blocks both UV-A and UV-B
- Parsol--sunscreen ingredient that blocks UV-A
- Polystyrene plastic Petri dish cover
- Pyrex glass Petri dish cover
- No. 1 glass microscope cover slip, Fisher brand
- No. 2 glass microscope cover slip, Fisher brand
- Glass microscope slide, Fisher brand
Action Spectra:
- DNA effects--corresponds to the UV absorption spectrum for DNA
- Cataract--damage to the lens of the eye that reduces transmission of light
- Erythema--reddening of the skin (sunburn)
Last Updated May
15th 1996
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