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:

UV Blockers:

Last Updated May 15th 1996

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Last updated Wednesday, 04-Dec-2002 20:16:56 UTC