Sensors: Loggers: Ultraviolet Any EASYSENSE Science at Work Logging time: SnapShot mode Teacher s notes 18 How good is my suntan cream? Read Most students will have heard about the dangers of ultraviolet on the skin and the need to wear good protective sun cream in bright sunshine. They may also have picked up on information that undermines the knowledge e.g. that SPF is indication of the time you can spend in the sun, that constant application offers protection. This experiment will allow the students to test a variety of sun products to see how much of the ultraviolet they block. It will also show that even high factor sun lotions are letting ultraviolet rays through. If the investigation is extended to include fabric as a sun block they may be surprise at how easily ultraviolet can penetrate a lot of summer clothing. In Australia beach wear often carries an indication of how much a fabric blocks ultraviolet. Many people have anecdotally referred to getting sun burnt from reflections when it is more likely they were burnt from ultraviolet getting through the fabric of the parasol they were sitting under. It is a good stimulus for discussion about personal responsibility in health. Other questions you may be able to answer 1. The price of sun products varies a lot, how much of the cost is what the product is worth and how much is what the company selling it knows you are willing to pay? 2. Are home products as good as the ones purchased in a shop? 3. Can you work out the SPF? 4. Does the SPF change if the sun lotion gets older? 5. Does a quick swim really wash lotion away? What you need 1. An EASYSENSE logger. 2. A Smart Q Ultraviolet sensor set to 50 W/m 2 range (fast or slow) 3. A piece of thin clear plastic marked with squares or circles. 4. A variety of products used to protect you from the sun. 5. Ultraviolet light source (bank note / security marker light). Do not use a UV source that produces no visible light (Woods light); this will almost certainly be a UV B source. You could of course use natural sunlight (make sure no glass is between the sun and the experiment). Hazard information Only use an ultraviolet source that is transmitting in the UV A band. A risk assessment should be made with reference to local guidelines. The small bank note forgery / security marker sources are usually acceptable. T18-1
Set up of the software Use SnapShot. Notes Even though an ultraviolet source maybe low grade it still produces potentially harmful levels of ultraviolet. Students should be warned of the dangers (but not to the extent it frightens them away from the practical). They should be certainly warned about the potential for damage if the lamp is looked at for long periods. The lamp should be allowed to warm up for about 5 minutes before use and reach full output. Check the range of the sensor is correct for the lamp. Alter the range if necessary. Try to use sun products that use the same carrier system for fairness, e.g. use all oils or all creams. The sudden appearance of a white cream will tend to undo the good work (the white will be claimed to be the reason why the sun is blocked in reality the white is there to help you see which areas have been creamed up ). Some people advocate the use of organic oils as a more natural solution. It is worth the time if they can be sourced to do a comparative test (you could try using fake tan). If the work is extended, many fabrics do let ultraviolet through. It may not be much but, like all radiation, it is the cumulative dose that needs to be considered. Sun glasses are also ultraviolet blockers, again the comparison between what is best for you and what looks good (and costs more) is worth making it usually gets a good discussion going and shows how health is seen as someone else s problem to fix when it has gone wrong. If you use a pair of microscope slides the sun lotion can be applied to the surface of one slide and the second slide placed on top to make a sandwich of the two slides with a lotion filling. Using a rubber band to hold the slides together will squeeze the lotion into a uniform thin layer. The glass of the slide will block some UV, but there will still be enough transmission for comparison. Use two slides with no filling as the standard. Results and analysis You will have set of bars, one for each sun product. Use Add Text to identify each bar. The height of the bar indicates how much ultraviolet light reached the sensor, a short bar will mean more ultraviolet has been blocked; a tall bar will mean more ultraviolet has been let through. T18-2
Working out SPF This is an industry agreed method of showing the relative effectiveness of the lotions sun blocking ability to block the more harmful ultraviolet B. Ultraviolet A is often described as the safe tanning ultraviolet there is dispute about the long term potential for harm even from this source. The SPF is not the same as figures quoted for length of time you can be exposed to sunlight, while often quoted they need knowledge of the dose your skin can accept with damage. For most people this is unknown. There are a lot of myths out there regarding suntan lotions; it can be worth a bit of research to use them as examples of bad science. 1. Find the amount of ultraviolet that can be detected using no filters. 2. Find the amount of ultraviolet that can be detected with the filter. 3. Calculate the Percentage of the Ultraviolet that is being blocked 4. Take the difference (in percentages) of the ultraviolet blocked to unblocked and divide this into 100. The answer is the SPF factor. Worked example 1. Ultra violet measured without filter is 35 W/m 2 2. Ultra violet measured with filter is 30 W/m 2 3. Percentage of ultraviolet blocked is (30/35) x 100 = 86% (rounded) 4. 100 86 = 14 5. 100/14 = 7.14 = SPF 7.0 More to do 1. Try the same experiment but use different oils to see if they block ultraviolet. 2. Instead of the clear plastic try sunglasses. 3. What about other materials that may block ultraviolet? Does T-shirt material protect you from the suns harmful rays? 4. Why don t you get sun burnt behind a glass window? Is all glass the same? Additional notes You may come across a unit called the minimal erythema dose (MED). An erythema is a reddening of the skin, usually as a result of dilation of subcutaneous blood vessels; it is an indicator of exposure to radiation. Sunlight is radiation, a point worth making. The MED is the dose of the radiation required to create a minimal redness of the skin. It is measured in joules (J) as it is an indicator of energy vs. skin damage. A suggested mechanism for the onset of reddening is the release of chemicals into the subcutaneous skin as a result of interaction / damage of DNA from the received ultraviolet. Sunburn is therefore a strong indicator of chemical damage inside the cells of the skin. It will not be possible to use the MED for students, it has to be individually tested and calculated. However sufficient knowledge has been built up to create a guidance table with descriptors of skin types. The downside is that people tend to work backwards, from what they wish, to find the skin type (i.e. the desire to stay in the sun for 4 hours will lead them to overestimate the tanning history descriptor). The von Luschan scale is method of describing an individual s skin colour to a set of predetermined coloured tiles. It is not a scale designed to identify ethnicity; it is a measure of the de facto skin colour of an individual. For this reason the type index is more commonly used than the word descriptor. A unified skin type index is often used; this merges types 1 and 2 and moves the boundaries slightly to give 5 skin types. T18-3
Type Word descriptor Tanning history Associated hair and eye colour von Luschan scale I very light, or "Celtic" Often burns, rarely tans. Tends to have freckles, red or blond hair, blue or green or grey eyes. 1-5 II light, or light-skinned European, Usually burns, sometimes tans Tends to have light or dark hair, blue, green, hazel, brown or grey eyes. 6-10 III light intermediate, or darkskinned European or "average Caucasian" Sometimes burns, usually tans. Usually has brown hair and blue, green, hazel, or brown eyes. 11-15 IV dark intermediate, also "Mediterranean" or "Olive skin" Sometimes burns, often tans. Tends to have dark brown hair and eyes. 16-20 V dark or "Brown" type Naturally blackbrown skin Often has black- brown hair and eyes. 21-28 VI very dark, or "Black" type Naturally blackbrown skin Usually has black-brown hair and eyes. 29-36 The UV index is a weighted index based on the relative harm and abundance of the 3 ultraviolet types in the environment. The weighting is necessary as the very small quantities of UV C that reach the surface produce significantly more damage per joule than the UV A. The Smart Q Ultraviolet sensor does not discriminate between the ABC bands of UV and measures the absolute levels. Another problem of environmental measurement is the cosine function, essentially this describes that the angle of incidence of the rays has an effect on the mix of bands present and the power contained in the bands. Ideally UV Index measurements should only be made when the sun is at its maximum elevation (solar mid day). The UV index is what is given in weather forecasts when they are giving risk information for sunburn / protection. Global solar UV index Approximate time for sunburn to begin in skin type 2 1 sunburn unlikely 2 1 hour 3 50 minutes 4 40 minutes 5 30 minutes 6 25 minutes 7 20 minutes 8 or more less than 20 minutes When the UV Index is 3, a fair skinned person will experience minimal skin redness after approximately one hour in the sun, when UV Index is 6 this is reduced to approximately 24 minutes, and when UV Index is 10 its after only 6 minutes in the sun. UV Index levels over 7, are common (in the UK values of 7 or above can be recorded from about a month after the spring equinox) and will result in a fair skinned person burning after 15 minutes in the sun. T18-4
How does the SPF relate to sunburn? This is an obvious question to ask and there should be a logical and reasonable answer. Unfortunately, living things don t follow the rules of physics and rules tend to become very fuzzy when applied to sentient organisms. Which, put another way, means that the rule may be there but don t discount bravado, peer pressure or misinformation to override the rules! SPF is the abbreviation for sun-protection factor. As indicated in the calculations it is a value derived from the ability to block UV (it is the percentage of UV that will get through treated skin compared to what gets through untreated skin expressed as a ratio to 100). The SPF gives a number such as 15, 30, or 50 that indicates the degree of sunburn protection provided. SPF is related to the total amount of sun exposure rather than simply the length of sun exposure, it is cumulative. It is a common mistake to assume that the duration of exposure to sunlight can be calculated simply by multiplying the SPF by the length of time it takes to suffer a burn without sunscreen. The amount of sun exposure a person receives is dependent upon more than just the length of time spent in the sun, it depends upon intensity. It also relies on correct application of the product; most people severely under apply the product and do not follow the application regime on the packaging. Key variables that will be overlooked or poorly estimated are, 1. Geography: Everyone knows as a fact that you cannot become burnt in the UK! Sadly much of the northern hemisphere lives under a zone of ozone depletion and receives more UV than many tropical areas. 2. Weather: You can t get sunburnt on a cloudy day. Nothing could be further from the truth; the lack of sun can reduce the feel of heat, and therefore increase exposure time. It also needs a very thick cloud to block UV and how thick is thick? 3. Topography: Height above sea level reduces exposure times radically. Snow skiers get good sun tans! Above 1,000 meters protection is reduced rapidly. 4. How much you put on: For an average person (adult) the amount used should be in the region of 30 ml (6 medicine spoons) applied evenly over all skin surface, remember clothing lets UV through. 5. When you put it on: BEFORE exposure to sun, never after exposure. Most manufacturers recommend at least 20 30 minutes before exposure, the lotion needs to be absorbed into the skin. It should also be reapplied every 2 hours to retain effectiveness. Note this does not mean it resets the exposure counter, it simply means that if you exposure limit is 2 hours 30 to get protection for the full time you need to reapply. 6. Underlying tan and response to previous exposure: If you have a tan the exposure time can be increased, but by how much? 7. Bravado: The bright red marks of sunburn are still the source of laughter and a mark of good weather. They should be cause of concern; it is a burn after all. Sun lotion is for wimps, I don t like the smell, it marks my clothes, etc. are very powerful anti application factors. See what the response is to compulsory application, if the students don t complain there is high likely hood the parents will! People with sensitive skin who burn quickly and must spend a lot of time outdoors should always apply a sunscreen with an SPF of 30 or more. It is possible to purchase sun exposure meters that use your admitted skin type and the SPF of any sun lotion you are using to calculate the exposure time and set an alarm. They vary considerably in effectiveness, some only use the skin type and perpetuate the myth that SPF x exposure time = tanning time. More sophisticated units will ask for SPF being used and Skin type and then use direct readings to give a closer time of exposure. T18-5