Investigating The Efficiency of Sun Block Cream.

Jonathan Ransom

Investigating The Efficiency of Sun Block Cream

Aim of Investigation:

The aim of this investigation is to discover how efficient sun block cream is and whether the claims by the company agree with my results.

Why I chose this experiment:

I chose to do this experiment because it seemed more interesting to me. Rather than just testing a sensor I thought it would be more interesting and rewardable to use a sensor to test something else. Also sunburn can be very serious and can cause illness therefore it is important to have efficient sun cream to prevent sunburn.

What I am doing:

I am going to take a range of factors of the same brand of sun cream and measure the level of UV radiation, which is allowed through each. It would also be good to test different brands as well however I could not obtain enough different brands of sun cream to carry out an experiment like this.

What is UV radiation?

The decrease of the ozone layer in the upper atmosphere, which serves as a protective shield against ultraviolet light, has focused much attention on the harmful effects of ultraviolet light (especially ultraviolet-B light). For example ultraviolet-B (UVB) light has harmful effects on the growth of agricultural crops, on the composition and constitution of ecosystems in the oceans and also on animal and human health, particularly for the skin and eye.

Increased exposure to ultraviolet radiation, due to a decreased stratospheric ozone layer, may lead to increased effects on the skin and the eyes. There is also increasing evidence that UVB radiation interacts with immune responses to several antigens, not only in the skin, but also in other parts of the body. UV radiation is everywhere all year round. But levels are affected by several factors:

UV radiation is greatest at midday. Over 50% of daily UV radiation occurs between 10:00 AM and 2:00 PM. Thick heavy clouds absorb most UV radiation, in general, the darker the clouds, the less UV radiation. UV still passes through thin clouds and haze. You get higher UV exposure on snow, sand, water, or concrete, since these surfaces reflect the sun's rays. You also get higher UV radiation level at higher altitudes and at areas closer to the Equator.

Ultraviolet rays cause tanning and burning from the sun. These rays cannot be seen or felt, but penetrate the skin and stimulate cells containing a brownish pigment called melanin. Melanin protects the skin by absorbing and scattering ultraviolet rays. People with dark skins have high amounts of melanin, and have greater natural protection from ultraviolet rays. People with fair skins have less melanin and, therefore, burn more quickly. As ultraviolet rays stimulate melanin, it rises to the skin's surface as a tan and provides protection against future sun exposure. Two types of ultraviolet rays (UV) from the sun exist: UVA and UVB. UVB cause burning of the skin or the red associated with sunburn, skin cancer, and premature aging of skin. UVA rays stimulate tanning but are also linked to other problems such as impaired vision, skin rashes, and allergic or other reactions to drugs. Most serious and lasting damage occurs before age 18.

Skin cancer has long been associated with exposure to the sun. Three types of skin cancer exist: basal, squamous, and melanoma. Basal and squamous cancers are usually associated with long-term exposure to the sun. If identified and treated early, these skin cancers are seldom fatal. Melanoma accounts for 76% of deaths from skin cancer. Melanomas often start small, but grow. They are likely to occur in people who experience bad sunburns at infrequent intervals such as on vacations.

Ultraviolet rays also cause eye damage, regardless of skin color. The incidence of cataracts increases with sun exposure. Cataracts involve a clouding of the eye lens and result in one million surgeries each year. Corneal sunburn and growths on the surface of the eye are thought to be related to long-term sun exposure. Use of sunglasses provides some eye protection.

As the amount of melanin in skin increases, so does the natural protection from sunburn. Individuals with dark complexions, especially those with olive, brown, or black skin, can remain in the sun for longer periods before burning occurs and individuals with fair skin burn readily.

Location and Atmospheric Conditions:

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Location and Atmospheric Conditions:

The numbers of ultraviolet rays that reach the skin affect the speed and intensity of tanning or burning. When the atmosphere is thick, fewer ultraviolet rays pass through or reach the skin. At the equator and at higher altitudes, such as the mountains, possible radiation from the sun is greatest because of a clearer and less dense atmosphere to filter out ultraviolet rays. As you move away from the equator or toward sea level, burning is less intense due to thicker atmospheric conditions. The number of ultraviolet rays at the equator is four times greater than those in Alaska or the southern tip of South America.

The UVB rays, especially, scatter throughout molecules in the atmosphere and cause burning. Because sunlight seems less intense, less bright, and less warm, individuals normally take fewer precautions and increase the potential for bad sunburn. Tanning and burning can occur on hazy days when the sun does not appear to be shining brightly.

Ultraviolet rays not only create problems on hazy days but also can burn the skin through clothing or while sitting in areas shaded from direct sunlight. Ultraviolet rays bounce off bright surfaces, such as sand, and can burn individuals sitting under beach umbrellas. Sky radiation on hazy days can hit the skin at angles and burn individuals not in direct sunlight.

Sun Screens and Sun Blocks:

Sunscreens and sun blocks are suntan lotions that contain one or more protective chemicals that absorb and scatter ultraviolet rays. These have a numerical rating system to indicate the specific amount of protection. The numbers, known as Sun Protection Factors (SPF), are listed on the product label. The higher the SPF number, the greater the protection. Sunscreen products, properly selected and used, allow the wearer to extend time in the sun without burning. Only opaque products, such as those containing zinc oxide or titanium dioxide totally block out ultraviolet rays. Often sun blocks are packaged or promoted especially for protection of lips, noses and ears.

Screen Selection: Use the SPF Rating

You have to select a sunscreen or sun block product according to the SPF rating to achieve optimum protection for your needs. SPF is a numerical rating system to indicate the degree of protection provided by a sun care product. It is based on a multiple of the time required by the sun to produce a given effect (redness) on an individual's skin without protection. For example, if your skin would normally burn in 20 minutes with no protection, using a sun screen product with an SPF of 6 means you could spend an additional 120 minutes (or 2 hours) in the sun without burning. This is based on first exposure to the sun after a lengthy period such as winter. The table below shows how you select sun care protection using the SPF rating with consideration to your skin type and personal history, environment and amount of time in the sun:

Prediction:

I predict that if I increase the factor of sun cream then the level of UV allowed through will decrease proportionally. This is because if you double the factor level you can stay out in the sun for twice as long, which suggests that you are only coming in contact with half as much UV radiation. Therefore I predict that if I had a sun cream of factor 4 which caused a 10% decrease in UV radiation reaching the skin, then factor 8 sun cream would cause a 20% decrease. I have made a graph to show my prediction:

This shows that the factor size is proportional to the percentage decrease. Obviously if there is any errors in my experiment then the final graph will be different from this. Therefore it is important to minimize errors.

Safety precautions:

There are not many safety precautions that need to be taken into consideration in this experiment, however the few that there are, are very important.

Obviously this experiment involves the use of electricity so the general rules for safety with electricity apply. Do not carry out the experiment in areas where water is present, because water is a good conductor of electricity and it could prove dangerous. Also do not obstruct walkways with equipment as people may trip over them.

Although not as harmful as UV from the sun, the UV lamp does emit UV radiation so it is better not to look directly at it.

Apparatus list:

Electrical circuit:

Before starting my experiment I did some preliminary work to check that my circuit worked and that the UV photodiode was actually sensing a change in UV radiation levels. I did preliminary work for no sun cream and for sun cream factors 4 and 8. I got the following results:

From doing this preliminary work I have learnt a lot. Firstly the preliminary work disproves my prediction. However I am going to remain with the same prediction for the real experiment because I do not think these preliminary results are that reliable. I need to make sure I keep the same thickness of sun cream for each factor.

According these results as you double the factor the percentage decrease increases by only 1.25. I will carry out the experiment fully to discover whether this is true or not.

Method:

Results:

I calculated the percentage decrease by taking the average P.D away from the p.d for no sun cream (4.86). Then I divided that answer by 4.86 and multiplied it by 100 to give a percentage.

E.g.:

Percentage change for sun cream factor 4:

Percentage decrease = (4.86 – 4.47)/4.86 x 100%

= 0.39/4.86 x 100% = 0.0803 x 100% = 8.03%

I have done three sets of readings for each factor. I took the average of the three voltage readings, which I then used to calculate the percentage decrease, which in turn can be used to plot a graph of percentage decrease against sun cream factor.

Analysis:

From the results I have obtained, it has shown me clearly that as you increase the SPF of sun cream the level of UV, which is allowed to pass through decreases. My results have no or very small deviation between them and so I can state quite confidently that was experiment was reliable.

I predicted that as I increased the sun cream SPF the percentage decrease would increase proportionally. So if I doubled the SPF the percentage decrease would also double. My results show this to be true. For example for the sun cream factor 4 there was a % decrease of 8.03% and for twice the factor, factor 8 there was a % decrease of 16.26% which is double. This is not exactly double, but I would not expect it to be because of errors in the experiment, which I will mention later.

Evaluation:

From my graph I can see that the results that I collected are very reliable as they all fit the line and have very low deviation, showing no anomalous results.

Whilst taking my readings my measurements might have been slightly inaccurate as the thickness of sun cream I used each time was not exactly the same each time. It was difficult to get an accurate reading of the thickness by eye, as the microscope slide was underneath the UV lamp. If the thickness were slightly different to what it was supposed to be, it would affect the level of UV allowed through. However it would be so minimal that I do not need to worry about it, as it did not affect my results.

The claims on the back of the bottles of sun cream appear to agree with my results. For example on the back of the factor 4 sun cream bottle it says:

‘The factor number 4 gives you up to 4 times the skin’s natural protection against sun burn.’

And on the back of the bottle of factor 8 sun cream it says:

‘The factor number 8 gives you up to 8 times the skin’s natural protection against sun burn.’

This shows that the factor 8 allows you to stay outside in the sun for twice as long as the factor 4 allows you to. Therefore the amount of UV allowed through the factor 8 will be half of that allowed through the factor 4 and therefore for the facto 8 the percentage decrease in UV levels passing through to the skin will be double that of the factor 4.

My results for these factors were:

As expected the percentage decrease for factor 8 is roughly double that of the factor 4.

The line on my graph is a proportional line. Therefore it is easy to determine the sensitivity of the sensor.

The sensitivity is the change in % decrease/ change in factor. In other words it is the gradient of the line. The gradient and therefore the sensitivity of the sensor if about 2.

If I repeated the experiment I would attempt to ensure the thickness remains the same for each factor. I would also try further SPF values of sun cream to strengthen my results. If there were any significant factors affecting my experiment then there would be a clear indication of it on the graph.

I took three readings of potential difference for each factor and I think this is sufficient for this experiment because each of the three readings were very similar, the biggest difference being, 0.02V.

I don’t believe I can really improve on the way I performed the experiment because my results were very accurate, as I had no anomalous results.

I did not encounter any problems whilst setting up the equipment, as it was simple and uncomplicated. I did not encounter any problems with natural light affecting my results either. Putting the black paper around my UV lamp and UV photodiode easily prevented this. My results are reliable because I repeated them and there were no contradicting results. The pattern that I have discovered for my results is as I predicted in my plan. The factor of sun cream is proportional to the percentage decrease it causes in UV levels:

Factor ∝ % decrease

To extend my investigation I could test more factors of the same brand of sun cream. I could also compare different brands of sun cream to see which is the most efficient at block UV radiation.

Bibliography

AS Advancing Physics by Jon Ogborn