10 Experiment 4: Sunscreen and UV Protection

Reference from Stanford University

Ron’s project 5

Ultraviolet(UV) light and protection exercise

objective:  To evaluate different methods to protect from ultraviolet light (UV)

learning points:

1. study the different commercial products used for sun blocks
2. handle UV light lamp sources
3. compare effectiveness of sun blocks using simple light sensitive beads

In this project you will learn about ultraviolet light (UV), how to safely handle UV light, and how to protect yourself from it’s harmful effects.  Ultraviolet light radiation is substantially more energetic than what we refer to as visible radiation (UV light has a wavelength shorter than that for visible radiation).  In term of wavelength then it follows light this: Ultraviolet light has a wavelength between about 100 nanometers and 400 nanometers, with visible radiation in wavelengths between 400 and 780 nanometers.

Our own sun is a major source of ultraviolet radiation.  The sun emits all different kinds of electromagnetic radiation, but 99% of its rays are in the form of visible light, ultraviolet light rays, and infrared light rays (infrared radiation is also considered as heat).

Ultraviolet light rays can be divided into three different wavelength bands – UV-A, UV-B, and UV-C.  This classification is based on the amount of energy they contain and their effects on biological material. Of them all, UV-C is most energetic and most harmful; whereas UV-A is least energetic and least harmful.

It is known that UV-C rays do not reach the earth’s surface because the ozone layer blocks their penetration of our atmosphere. As UV-C light encounters the ozone molecules at very high layers of the atmosphere the energy of the radiation is enough to break apart the bond of the molecule and thereby absorb the energy. By this action no UV-C rays from the sun come into contact with living organisms on earth.  Understand that UV-C light radiation can be produced as human-produced UV-C rays which will be be a hazard in certain professions (i.e. such as welders).

In the case of UV-B type light rays which have a lower energy level and a longer wavelength than UV-C their energy is generally not sufficient to split an ozone molecule. UV-A rays do not have enough energy to break apart the bonds of the ozone, so UV-A and UV-B radiation passes through earth’s atmosphere (UV-A light is almost unfiltered). Since both UV-B and UV-A rays can be detrimental to our health, it is important for preserving health that we protect ourselves from this radiation. A most obvious way is to reduce the amount of time one spends in the sun when the sun is at its highest in the sky.

It is known that UV light from the sun has always played vital role in the environment of planet earth. Biological activity of many kinds have become to deal with the influx of UV light. Radiation at the longer UV light wavelengths of 320 nm to 400 nm, this is called UV-A, has a helpful and essential role in formation of Vitamin D by the skin.  UV-A also can have a harmful role in that it causes sunburn on human skin and cataracts in our eyes. The incoming radiation at shorter wavelengths, 290 nm to 320 nm, falls within the UV-B part of the electromagnetic spectrum.  UV-B includes light with wavelengths down to 280 nm, but little to no radiation below 290 nm reaches the Earth’s surface. UV-B can cause damage at the molecular level to deoxyribonucleic acid (DNA).

It is known that DNA readily absorbs UV-B radiation, inducing an effect which usually changes the shape of the molecule in several ways. Changes in the DNA molecule often mean that protein-building enzymes cannot correctly read the DNA code at the damaged area on the molecule. As a consequence, distorted proteins are made and  cells can die.

Exposure to UV light levels are not constant over the course of a single day, or for that matter even over the course of a year. A major factor is the position of the sun in the sky.  A second important parameter determining UV at the ground is the amount of ozone present in the stratosphere. Low ozone levels in the upper atmosphere correlates with more UV light radiation reaching the ground. However, other factors contribute to UV radiation variability, of which is the very important amount of clouds. However, clouds themselves can also lead to increased UV light levels. This takes place when the sun is not obstructed by clouds and clouds that are in the close proximity of the sun reflect additional radiation to the ground. Clouds do not necessarily protect us from the UV light coming from the sun.

The levels of UV light radiation an individual is exposed to can vary according to altitude. UV light radiation levels increase at about 4% for every 1,000 foot increase in the elevation. This increase has nothing to do with being closer to the sun but this increase is the result of a thinner atmosphere at higher elevation with a smaller number of molecules being present to absorb or scatter UV.  The physical features of the land – sand, snow, and water all will generally reflect UV rays (called albedo).

The closer an individual is to the equator, the more ultraviolet rays one is exposed to. This is due to the condition that the sun is usually higher at the sky at lower latitudes. The ozone layer is thinner at the equator as it is over the United States or Europe, with this contributing to more exposure to UV light.

Here is the energy levels of light radiation from the sun

Type                                     Wavelength             Energy

Ultraviolet       UV                  400 – 100 nm         3.10 – 12.4 eV

Ultraviolet A    UVA                400 – 315 nm         3.10 – 3.94 eV not absorbed by ozone

Ultraviolet B UVB                 315 – 280 nm         3.94 – 4.43 eV most absorbed by ozone

Ultraviolet C UVC                 280 – 100 nm         4.43 – 12.4 eV all absorbed by ozone

Ultraviolet light C and short-wave ultraviolet light B is blocked by the atmospheric ozone layer.

Action and destiny of various wavelength of light from the sun.

OZONE DESTRUCTION

Transmission of the incident solar radiation through glass is a function of the type and thickness of the glass, the angle of incidence, and the specific wavelength bands of radiation. Ordinary glass of the soda-lime-silica type (window or plate glass) can transmit more than 90% of the incident radiation in the UV-A and visible regions of the spectrum. Increased thickness of glass diminishes transmittance. Ordinary glass is opaque (blocks) to radiation in the UV-B and UV-C regions; Pyrex glass (borosilicate type) is opaque to radiation in the UV-B band but reaches a maximum transmission level at 340 nm and greater wavelength. The coefficient of transparency for borosilicate glass, 1.0 cm in thickness, is 0.08 at 310 nm, rises sharply to 0.65 at 330 nm, and attains a peak level of 0.95-0.99 from 360 to 500 nm (Weast, R.C. 1972. Handbook of chemistry and physics. Chemical Rubber, Cleveland, OH, USA. p. E-192.). The transmission properties of Pyrex are exceeded only by quartz (Weast 1972).

Safety

Always have your goggles on while working in the lab.  Avoid subjecting your skin to

exposure to the UV light emitting from the lamps.

PROCEDURE

1. You will work with glass tubes, metal test tube holders rack, UV light lamps, UV light sensitive beads, and various “sun blocks” to evaluate protection from actual UV light.
2. Obtain a metal test tube holder rack and glass tubes to hold the UV light sensitive beads.
3. Place 4 or 5 UV light sensitive beads into the number of glass tubes designated by the Instructor (equal to the number of sun blocks you are testing plus one more).
4. Remember to keep one glass tube uncovered with sun block as a control for evaluating the effects of UV light with “no protection”.
5. On the remaining tubes holding the beads, then coat the OUTSIDE of the tube with the sun block material that you are testing. Do this for all sun blocks.

6. Remember to always keep your goggles on so to block any UV light exposure to yourself. Always point the UV lamp away from yourself and your partner.
7. After you have positioned the uncovered “control” tube alongside the other covered and protected tubes into the tube rack, place the rack into an available hood.

UV lamp help by hand and with emission toward rack of prepared tubes.

8. Position the UV light lamp so that all tubes get exposure to the light at the same time. Now turn on the UV light lamp and keep track of the time it takes to turn the sensitive beads inside the tubes to turn color.  Record the time for EACH set of beads to turn color and to which sun block they refer to.
9. Once all the tubes have colored beads, then turn off the lamp and remove the rack with beads in tubes so that they will return to colorless.
10. Repeat steps 8 and 9 above as many times as designated, recording the time for each tube beads to turn color.
11. When finished, return the light sensitive beads to their origin and wash the tubes in soap and water to remove the sun block—return to their origin.
12. Place your time data for each sun block in a table form indicating the type of sun block and time to turn color. Be sure to include the “control” having no sun block.

Answer the following questions:

1. Why is UV-C light and UV-A light harmful to humans?

Why do we not worry about exposure to UV-C light?

2. Which sun block performed the best?

Which sun block performed the worst?

Which sun block performed at the Intermediate level?

List the average times recorded for each sub block in a table (example below)?

3. Why did you perform multiple trials for the sun blocks?
4. Explain why you included a “control” with your test samples?