Many have thought, "hey, why aren't all sunscreens just SPF 100​"? It's a valid question, let's look at why that is.

The meaning behind SPF (sun protection factor) is a bit convoluted and misunderstood. SPF doesn’t necessarily refer to how well a sunscreen protects you, but for how long. Yes, it's a bit of tricky semantics.

SPF 30 means that  it will take 30 times longer for your skin to burn as as it would take without sunscreen.

 
Consider the example below:

Let’s say that your unprotected skin will start to get sunburned in 10 minutes (depends on the individual)
 

SPF 30 X 10 minutes = 300 minutes
SPF 40 X 10 minutes = 400 minutes
SPF 50 X 10 minutes = 500 minutes

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Now you're probably thinking, "that's great, but what sunscreen stays on for 5 hours (300 minutes) or more?" Therein lies the problem. 

 

To put it into the words of the Skin Cancer Foundation, “...there are problems with the SPF model: First, no sunscreen, regardless of strength, should be expected to stay effective longer than two hours without reapplication.”

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What is arguably more important than the SPF number is whether or not the product is broad spectrum (protects from UVA and UVB effectively), stays on, and that it is reapplied when necessary. 

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The current scientific literature supports the fact that high-concentration zinc oxide formulas offer the most thorough protection while maintaining optimum safety for people and the environment.

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Finally, big name-brand sunscreens (probably the ones you're thinking of) have been proven by independent labs to wildly exaggerate/lie about the actual SPF rating of some of their products.