Sunscreen Decoded: The Meaning of SPF


All my life, I have had a love/hate relationship with sunscreen. Ok, so it was more like a hate/hate relationship, but who’s keeping track? So many things about it made me turn away again and again. The smell, the white-cast it left on my skin, the fact that it would keep me “pale” (for a Filipino) all made me not want to use sunscreen. After many tanning session and slight burns from my time outside for the variety of outdoor jobs I have worked, I was terrible at applying sunscreen, let alone reapplying it throughout the day (*tsk tsk*). Only recently have I understood its value and importance.



To fully appreciate the power of sunscreen, we need to understand what we are actually protecting ourselves from.  Sunscreens are meant to protect us from UV radiation from the sun. This is a type of radiation that has a shorter wavelength than visible light and can be divided into 3 categories: UVA, UVB and UVC. You don’t hear UVC mentioned as much since it is absorbed by the atmosphere before it can reach us on the Earth’s surface  and thus isn’t much of a problem. The other two categories are the ones we need to worry about. These two types are capable of directly damaging the skin cells’ DNA, increasing one’s risk of skin cancer, as well as penetrating the skin down to the connective tissue causing wrinkles and premature aging.


With our skin being the damsel in distress in the fight against UV rays, we need some rescuing. And as our guardians, sunscreens have been modified and improved upon to provide maximum protection. Both organic (carbon-based) and inorganic chemicals can offer protection. Two inorganic compounds commonly found in sunscreens are titanium dioxide (TiO2) and zinc oxide (ZnO) which provide a physical and chemical barrier as they absorb UV rays. They also leave a really…”stylish” (aka unattractive, if you ask me) white layer on your skin. Today, these inorganic chemicals are used in combination with organic ones to provide a wider range of protection. Bonds in the organic compounds are able to absorb the UV rays, deterring them from reaching your skin. Below is a great info graphic from another wonderful chemistry blog, Compound Interest showing the specific UVA and UVB blockers you might find in sunscreens. Chemistry-of-Sunscreen-724x1024These active  ingredients in sunscreens are the powerhouses supplying most of the protection. However, even the strongest heroes sometimes need backup. Despite their strength during your first application, some of these organic compounds will eventually start to breakdown after prolonged exposure to the UV rays. This is why it is so important to reapply sunscreen at least every 2 hours.


Understanding how well a sunscreen can protect you can be the key to choosing the proper ‘guardian’ for your skin. First, let’s talk about the SPF and what it means. SPF, or Sun Protection Factor, is a measure of how well a sunscreen can protect your skin from UVB radiation. The number acts as a multiplier for the amount of time you can spend in the sun without turning red. Theoretically, if it takes 10 minutes for your skin to burn without any protection, a sunscreen with SPF 30 should allow you to stay in the sun 30 times longer (about 5 hours) without getting burned.

Another way to view it is by percentage of UV that the sunscreen can protect you from. Sunscreen with SPF 30 blocks about 97% of all incoming UVB rays. The percentage, however, is not linear as the SPF increases. For example, SPF 50 is considerably higher than 30, but block about 98% of UVB rays, an increase of only 1%. This may seem like a very small amount, but for those with very fair skin or a higher risk of skin cancer, those extra percentages can make a difference.

So what about those UVA rays we spoke about? There is a whole separate system for those! UVA Protection Factor (UVA-PF) uses the symbol below to let you know how well the sunscreen can protect you from UVA.

PA Sunscreen UVA Logo

UVA-PF results are often labeled in three levels, PA+ being the lowest level of protection and PA+++ being the highest level of protection.

I hope this helps decode the mystery of sunscreens for you a bit and helps you have a little more love for them and what they do. Do you have a similar story of being a recently turned fan of sunscreen? Do you have a favorite sunscreen that you wear daily? If so please comment below and don’t forget to subscribe to get emails about my newest content!

Until next time, keep a scientific eye on the world and a happy thought in your head!

❤ Julie



  1. For me I always buy SPF 50 for my face to prevent more aging black spots on my face , but now I understood that it’s only 1% more than the SPF 30 which is almost the same protection . Now I understood the consequences of the SPF numbers . Since sometimes it’s very hard to find the SPF 50 it would be ok also with the SPF 30 . Thank you for the information .


  2. I had a hate/hate relationship as well because they always felt too heavy for my sensitive skin and clogged my pores. There were 3 occasions where I became really sunburnt – the last was unplanned because the company (I was an extra in a commercial) never told I’d be spending the afternoon outside on a hot summer Sydney day. These days, I’m using a skincare range that suits my skin a lot better so I’m used to adding it to my last step each day.


  3. I have pretty fair skin so I don’t know if it causes any white haze but Banana Boat sport of 45 to 50 has been my go to sunblock for years. I think I settled on it after trying other ones that ended up sweating off or ran into my eyes. There is a little bit of an oily feel but not too bad. I think that almost has to be the case if you want a sweat/water ressistant product. I’ve also heard some good things about Neutrogena on occasion.


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