… Basically I have a question about the last question you got. The person who asked mentioned that we’re required to use 1/2 teaspoon of sunscreen for the face alone and now I’m a little confused because I’ve thought that we “only” need 1/4 of a teaspoon? Or about 1.25 ml? Is that wrong? I’m sorry for asking, I’m just always worried about not using enough product to get the proper protection.
Great question, thanks for asking @naevery 🙂
It’s not right, and it’s not wrong…it depends!
Let me explain 🙂
The density of sunscreen used in SPF testing is always 2 mg/cm2
That means for every square centimeter or skin, 2 mg of sunscreen is applied.
This is easier to do when testing, because it’s done on the back where you can draw a 30 cm2 square and apply 60 mg to it.
Figuring out the area of the face is trickier.
The ¼ teaspoon or ½ teaspoon or 1.25 ml are based on two estimates.
The first estimate is the average area of a human face, and the second estimate is the average density of a sunscreen. Remember that ml is a measurement of volume, it provides no information about weight.
Water at 4°C has density of close to 1. 1 ml of water will weigh about 1 mg. However, oil for example has a lower density, 1 ml of oil won’t weigh 1 mg, it might weigh 0.8 mg (depending on the type of oil).
That makes things more complicated, because you can’t assume that 60 ml of sunscreen will weigh 60 mg, it might weigh 65 mg or 55 mg.
So the ¼ teaspoon, or ½ teaspoon are just estimates for an estimated human face, and an estimated sunscreen density.
These researchers for example used a beer bottle cap to measure out sunscreen.
So in order to know how much sunscreen, exactly, to apply you’ll need two measurements. The density of your sunscreen, which you can take by measuring out, say, 10 ml of sunscreen then weighing it. You’ll probably want a jewelry scale that has 0.001 g accuracy, and to measure a few times and average your measurements!
Finding out the area of your face is more difficult.
There are studies where people are told to apply sunscreen, and how much they’ve applied is weighed. It usually ranges between 0.5-1.5 mg/cm2, when they’re unprompted about applying 2mg/cm2. Their recommendation is to have people apply their sunscreen twice, instead of worrying about ¼, ½ teaspoon.
It’s up to you to how you decide to apply your sunscreen. Whether it be weighing it, measuring out the volume, or applying it twice, it’s better to err on the side on more for sun protection!
For Reference: ml = milliliter, mg = milligram, cm = centimeter, g = gram, and 1 mg = 0.001 g
Hope that helps!