A slight delay in launching the sunscreen report to answer an important question. I’m sure you are wondering…
How do you know your homemade sunscreen is effective if there are no testing and you don’t know the SPF?
To determine an SPF rating for UVB protection (and hence, sunburn), the FDA requirement (1999 monograph) is to perform an in vivo test (clinical test on human subjects). An even 2 mg/cm2 is applied on the back of a human volunteer; a series of five different dosage of UV radiation is emitted to five subsite on the test site; after a specified amount of time, an examiner visually inspects the test site to evaluate the UV dosage that produced skin redness or MED – minimal erythema dose defined as the “quantity of erythema-effective energy (expressed as Joules per square meter) required to produce the first perceptible, redness reaction with clearly defined borders”. The ratio of the MED between the protected and unprotected skin is used to calculate the SPF.
Accurately determining SPF is a known problem. For example, “…studies suggest that, in practice, consumers may apply amounts of sunscreen below the density of 2 mg/cm2, which is the amount of product required for the SPF determination …that consumers may apply as little as 0.5 to 1.0 mg/cm2.” The FDA’s response is to keep the 2 mg/cm2 but modify the application directions on the sunscreen product label “to reduce the likelihood of underapplication.” The FDA also “agrees that SPF values do not reflect exact levels of sunburn protection that consumers receive under actual use conditions. The required SPF test is a clinical test conducted with strict control over factors such as product application density. However, under actual use conditions, these factors are not controlled and vary greatly.” Furthermore, the current test method uses a standardized sunscreen SPF 4 as the control formulation. The likelihood of inaccuracies increase with increasing SPF values being tested due to its deviation from the controlled SPF 4 standard. “Better assurance of an accurate SPF value is obtained by using a standard that is closer in SPF value to the sunscreen product being tested.” The FDA has revised the SPF testing in the proposed 2007 monograph to require sunscreens with SPF above 15 to use a standardized sunscreen SPF 15 as the control formulation instead of the SPF 4.
Though the FDA has proposed changes that would improve SPF testing, it still does not change the fact the testing is done under a controlled environment that hardly mirrors real world situation. Also, the examiner that evaluates the MED is a human prone to human errors and variance and most likely not the same person evaluating for all the sunscreen brands. Nonetheless, SPF numbers are still useful when comparing sunscreen products especially from the same brand (assuming the testing methods for different SPF values have little variance). A higher SPF product does give more sunburn protection. “Although SPF values do not convey actual levels of sunburn protection, when comparing multiple sunscreen products, SPF values enable consumers to determine which products provide the most sunburn protection. For example, FDA believes most consumers would correctly identify an SPF 20 product as providing more sunburn protection than an SPF 10 product.”
It is a common misconception that SPF is only related to the time of solar exposure – that if it normally takes one hour for your unprotected skin to burn, using an SPF of 10 means you get 10 hours of protection before sunburn occurs. SPF measures the amount of solar energy required to produce a sunburn; time is just one factor. Other factors affecting solar energy intensity are time of the day, seasons, weather conditions, and geographic locations. Many of these factors are also interrelated resulting in a very uneven distribution of solar radiation throughout the world. Your skin type and different parts of your body also determine your susceptibility to sunburn. Using an SPF 20 vs. an SPF 10 sunscreen does not mean you are twice as protected, that you can be exposed to twice the solar energy amount, or that you can stay in the sun twice as long. Yes, SPF 20 does give more protection than SPF 10. But by how much is not so straightforward – SPF rating is not a linear relationship.
Note that elsewhere in the world, SPF testing and regulation follow other guidelines. The FDA’s SPF testing method varies from Europe’s Colipa method in the details but the basic principle remains the same. Click here for a presentation at the 2001 SCC Florida Chapter Sunscreen Symposium discussing some of the differences.. Basically, an SPF on a US sunscreen determined by the FDA has a different meaning than the SPF on a European sunscreen. Disagreements on SPF measurement are ongoing.
SPF rating is a guide to prevent sunburn. Sunburn (skin damage due to UVB rays) is easy to detect. If you pay attention, you can also feel when your skin is getting hot. Instead of relying on the ever-disputed, error-proned SPF numbers as a guide, a more practical approach is to practice sunscreen effectiveness awareness on yourself: choose a sunscreen – commercially made or homemade, chemical or physical, and get to know it well. Pay attention to your use (amount, frequency, evenness of application) taking external factors into consideration (i.e. environment, time of day, type of activity). If you got sunburned or missed a spot, then adjust your sunscreen use accordingly for the next time. In essence, you are performing your own in vivo testing under real world conditions. Though this approach may seem vague and difficult to put in practice at first, sunscreen (and sun) awareness is habit forming. In the long run, you can better tailor your personalized sunburn protection needs than by relying on various agencies’ and sunscreen manufacturers’ attempts to determine sunscreen needs for you.
For comparison, consider yourself as an average everyday athlete, one that is not training for the Olympics or an ironman triathlon. During the course of a day’s physical activity, you are not constantly monitoring your fluid and caloric intake to the detail. If you feel dehydrated, you drink more liquid. If you lack energy, you eat something. You don’t even think much about it anymore because you have made a habit of self-awareness of your own body’s needs. There are no regulations that classify a person into the type of athlete they are and based on that, prescribe them a numeric amount of fluid or calories they ought to consume. Energy bars and sports drinks do not have regulated fuzzy numbers and directions printed on their labels. That would just be silly.
So why not just explain sunburn awareness to people ? I don’t know. Oh by the way…sun products are a billion dollar industry. For me, I choose sunburn protection through sun-awareness and my $0.25/oz homemade sunscreen.
Up next, UVA testing and protection.