Need suggestions for testing spf in lotion.

[samanthalevy]

My (4th grade) daughter wants to test the spf in sun block using hotdogs, rather than the uv beads or photo paper . All of the other science projects I have come across have not done it this way....it's her idea- she say's the hotdog is like our skin. I've seen projects that cook hotdogs in the sun. But will this work to protect it from "cooking". I'm concerned the lotion will not react in the same way with the hotdog as our skin...because it can't really absorb into it? Any thoughts would be helpful.

[ChrisG]

Sunscreens work by absorbing ultraviolet radiation and turning that energy into heat, so I do not expect that putting sunscreen on a hotdog would slow or prevent solar cooking. However, sunblocks such as titanium dioxide work by reflecting UV (and visible) wavelengths of light, so covering a hotdog in an opaque sunblock should slow the cooking (or heating) of the hotdog.

It might be interesting to see if a hotdog heats more quickly (less reflected UV energy) under sunlight when sunscreen is added. In terms of practical applications, you could ask the question of whether sunscreen could potentially contribute to heat-stroke.

[samanthalevy]

thanks for the idea. My hypothesis was lotion would lower body temperature and reduce risk of heat stroke. The higher the SPF the more protection and the lower the temp. I,ve done it twice and both times after the first hour the 45 Spf(the highest used) was 10 degrees hotter than the others. I can't figure out why this happens. After that I test them for 3 more hours.The temp of the 45spf becomes the lowest by about 20 degrees! Do you know why it's hotter after the first hour only? My mom says maybe the heat gets caught inside. It can't sweat-the lotion is very thick? The hot dog(is this one or two words?) with nothing got reddish and sweat alot more and got hottest in the end 105 degrees (in 80 degree weather- how?). heat stroke is 104 or higher!

[ChrisG]

Very interesting results! How did you do the experiment, exactly? Did you place the hot dogs on the ground, or on a cookie sheet or something else? Did you enclose them to reduce the breeze? Were they all exposed to identical conditions of sunlight, heat, wind, and other factors?

[samanthalevy]

I used a 10x12 (2-3 inch deep pan) wrapped with aluminum foil. I took a start temperature all were within a degree of each other. I stuck the hot dogs into dowels and rested the dowels on each side so that they were not touching the bottom and so that the sun could reflect onto all sides of the hotdog. The pan was placed in the sun up on the second
floor balcony with four hotdogs marked on the dowel and an the pan which was which. I checked them hourly with my mom using a meat therometer(checked in the same place each time on each hotdog) and put on more lotion. Thermometer is good my dad is a chef and showed us hot to use it.I did the experiment a third time on a cloudy overcast day(i live in florida so it was still warm 75 but not humid like the sunny days) and this time all of the hotdogs stayed almost the same temperature. They all increase on average the same.Some quicker than others by 4-6 degrees.Not like the sunny day differences. I started at 10 and finished at 2 each time. On the cloudy day the one without lotion still turned reddish and sweat like the other days but they all had a temperature of 75 degrees. I am not sure how to explain this. My hypothesis was right after the first hour on sunny days but not cloudy days. Would this be my variable?

[ChrisG]

It sounds like you did a very good job on your experiment, and you got some very interesting results. It's ok if you can't explain your results with 100% certainty. What you can do is to discuss the various possible explanations of why you got the results that you did.

Some things to consider in your discussion are:
1) The amount of sunlight exposure on each hot dog. Were there any changes in shadows or light reflected off the pan during the course of the experiment?
2) Evaporation of water from the surface of the hot dog. Evaporating water will greatly cool the surface from which it evaporates. Oily substances don't evaporate as much, so adding sunscreen can affect the evaporation from the surface of the hotdog. Do you data and observations suggest that the hot dogs may have had different amounts of evaporation? Do your data and observations suggest that the rate of evaporation may have changed over the course of the experiment? If you have time, you could rerun the experiment and try to account for the total evaporation by weighing each hotdog at the start and end of each hour (before and after you reapply the lotion). If you see that one hot dog is losing more mass during the hour, that one may be losing more water to evaporation. You'd need a sensitive scale to be able to do this. Also, you could try to control the evaporation by using two or more sunscreens with the same inactive ingredients, but with different SPF's. If you don't have time to do these additional experiments, you could propose them as possible future experiments to follow up on your current work.
3) Anything else you can think of that might have affected your results.

Regarding your question about your variable - the way I understand it, your variable would be whether or not the sunscreen is applied (or the SPF of the sunscreen if you used more than one). The experiment in cloudy weather might be considered a control, in which all factors remain the same except the amount of sunlight - (I don't know whether the humidity and the wind on that couldy day were actually comparable to the conditions on the day of the sunny experiment). A control would be helpful in this way: if you knew that the wind, humidity, temperature etc were all the same on the two days, and you didn't see any heating differences between sunscreen and non-sunscreen hot dogs on the cloudy day, then you could propose that differences in heating seen on the sunny day resulted from the sunlight, and not from evaporation. If the humidity, wind, and temperature were not similar, then you can treat the cloudy day experiment as a separate trial of the same hypothesis.

Nice work!

[samanthalevy]

I did all 3 tests on 4 hotdogs one with nothing, spf 15 spf30 spf 45.I forgot to put that in my information sent to you. they are all the same brand but the 45 has zinc oxide the others don't. So this could have in the start made the hot dog not evaporate/ sweat making it hotter but maybe as it got hotter it started to allow the hot dog to sweat and that is why it cooled off?the 45 was only 3 hotdogs (3 inches)from the none. could that make a difference? I know the none , 15 and 30 had little drippings from the hotdog where i took the temperature. the 45 never did even when it cooled off. I have read alittle more trying to find out how to explain it. I hope hot dogs were ok to use. The skin is alive and has sweat glands. The hot dog looks similar but may not react the same way with the lotion. Does this matter? It is too late to start something else I think I will just have to make it work.My variable is the different spf.My control is sunlight/clouds.So it's better to use the cloudy day or should i do one more on a sunny day like the others? Doesn't the sun light affect the heat that affect the evaporation/sweat?I don't think i have anything that can weigh them right. I have a bathroom scale and food scale.The two sunny days were the same temp/humidity but the cloudy day was windy and not humid. But from my research i learned even on cloudy days you must wear sunscreen so you don't get a sunburn but i can say you don't need the sunscreen to prevent heat stroke on the cloudy day- it's not hot and doesn't make a difference in the temperature.Right? I guess it was good to do the cloudy day Thanks for your help. I think for my next year I will do if exercise/ sweat or water will effect the outcome? My mom laughed and said we can put the hot dogs a rotissere for the exercise and in the pool for water.

[ChrisG]

OK, I misunderstood. I thought you had two treatments with spf45 and two treatments with no sunscreen. Your explanation of why the rate of heating changed for the spf 45 treatment sounds reasonable. I don't think it would make much difference if the hotdogs were near to eachother - only one received more sunlight than another due to shadows or reflection from the side of the pan. I would suggest focusing on the results from the sunny day, because those are more relevant to people's concerns of being out in the sun. Like you said, the cloudy day results are also useful.

Yes, the heating by the sun will increase the rate of evaporation, and if the hot dogs 'sweat' then the evaporation will slow down the heating. I have read that some sunscreens are designed to react with human skin, but I have not read exactly what sort of reaction takes place.

I think it was quite a good idea to use hot dogs. Congratulations on a very creative and well-done experiment!

[samanthalevy]

I was doing more research online at my mom's work. Do you think the "problem' with the spf could be explained by saying it took time for the spf to start working? I waited 30 minutes after applying the lotions before putting them in the sun- but maybe it takes longer than that. Also the zinc oxide doesn't evaporate and it makes up a part of the 45 spf so it could be the 20 degrees difference was because the zinc slowed the process making the temp higher but after a short time the other ingredients stated to evaporate and cool it off and the zinc stayed on so it won in the end keeping it cooler . Like the turtle and hare story the slower one seemed to be losing but won in the end. When I write my conclusion and paper should i use both ideas since i can't prove either one. or how would i find out how to prove it? A scale? what kind do i need and how would i find it.where?i kind of want to figure it out if i can. i am going to put in my report how you help if its ok. would i put it on my board?

[ChrisG]

Sunscreen is generally recommended to prevent heatstroke because sunscreen prevents UV damage to the sweat glands, which reduces the ability of the skin to regulate body temperature. Your experiment doesn't deal with this effect, so, in the introduction of your report, you should explain that your experiment applies only to the physical effects (and not the biological effects) of solar heating with various sunscreens. In your results and conclusions, if you write that spf 45 caused an increased rate of solar heating during the first hour, you should probably also mention that prevention of UV damage to skin sweat glands is another very important consideration.

Zinc oxide is a reflective powder, so I would not expect that zinc oxide itself would directly interfere with the process of evaporation, if that's what you meant, but I do think you are on the right track. My hunch would be that the inactive ingredients that carry the zinc oxide (and the other active ingredients) might have prevented evaporation initially. Perhaps the spf45 is more of a 'water proof' formulation? It's also conceivable that the zinc oxide somehow settled or adsorbed onto the surface of the hot dog after a while and created a more reflective barrier. To try to prove one explanation versus another would require additional experiments which control for the various factors. For example, you could run the experiment in a container with 100% relative humidity in order to minimize evaporation of water. However, some sunscreens might contain alcohol or other volatile ingredients that would still evaporate and cause some heat loss. Using a very sensitive scale to measure loss of water (or alcohol and other volatile ingredients) would help, though it sounds like such an instrument isn't available.

For your report, yes, I'd suggest that you briefly discuss the feasible, alternative explanations for the change in the rate of heating. As you can see, it's easy to get swept away with all the possible explanations, so make sure you also make a priority to clearly explain your your initial hypothesis, results, and whether you think the experimental results cause you to confirm or reject that hypothesis.

I don't think it's necessary to acknowledge my help, though it is certainly OK with me.