Eugenie asks… Reading “Does 2+2 not equal 4?” (about wearing SPF 20 and another 20: it will help ensure to get to 20), I have been wondering what is the protection if the SPF are not the same?? If I wear a moisturizer SPF 30 and then a foundation SPF 15 what does it make?
The Right Brain responds:
Here are the guesses that Eugenie put forth in her question:
- SPF 15 because it’s the last applied?
- SPF 30 because it’s the first applied or because it’s the higher SPF?
- Between SPF 15 and 30 because it’s a mix?
- SPF 45 because it’s like an addition? (She didn’t really think it works like this but it’s an option)
- Greater than SPF 30 because it increases the SPF
What do you think the answer is?
Sunscreen math
Think of it this way: If you took 1 ounce of SPF 15 lotion and mixed it with 1 ounce SPF 30 lotion, you’d be creating 2 ounces of SPF 22.5 lotion. (assuming that there was no interaction between the ingredients, which is not necessarily a good assumption.) So, if you applied them at the same time you could end up with less than 30.
If you applied the SPF 30 first , let it soak into your skin, and then later applied the SPF 15, you may be helping maintain the 30 but that’s the best you can hope for.
Image credit: http://3.bp.blogspot.com
{ 4 comments… read them below or add one }
what of if you mixed your spf 30 lotion with just a regular moisturizer? would that reduce the spf ?
@Chl9ja: Yes it would.
crap, lol. thanks all the same. Happy new year
Good to know how this works. It’s much harder than I realized to make sure I have adequate protection, especially living where it’s basically summer all the time.