We’ve all been there. A fun day yesterday out on the boat with friends laughing, swimming, drinking. Today, sunburn.

As you soak in a cold bath wishing you could reach your back to apply aloe, you scowl, wondering just what, exactly, does sunscreen really do? You specifically remember applying your sunscreen yesterday after lunch. Right around the time you noticed you were starting to get a little pink and decided you had enough sun and needed your protection.

This is why you never bother to apply sunscreen. It just doesn’t work.

Or, does it?

If you applied it once yesterday, why are you burnt to a crisp today?

What Is Ultraviolet (UV) Radiation?

According to the American Cancer Society, exposure to UV radiation is a major risk factor for most skin cancers. While UV rays make up only a very small portion of the sun’s rays, they are the main cause of the sun’s damaging effects on the skin.

There are three main types of UV rays: UVA, UVB, and UVC. Since UVC can’t get through our atmosphere, we’re mainly concerned with UVA and UVB rays.

UVA rays age skin cells and can damage their DNA. They penetrate deeper than UVB and cause the premature wrinkling, age spots, and can heighten the risk for some skin cancers.

UVB have more energy than UVA and can damage DNA directly. They’re the main cause of sunburn and several types of skin cancer. (Think B– for burning).

Opting for a broad-spectrum sunscreen will protect against both UVB and UVA rays.

How Does Sunscreen Work?

Sunscreens come in a variety of formats: sprays, lotions, gels, or waxes. They’re made up of a mixture of chemicals: Organic (carbon-based) and Inorganic.

Inorganic chemicals like zinc oxide or titanium dioxide act as a physical sunblock. They can reflect or scatter the light away from the skin. Because companies are making these particles much smaller now, you no longer see the white noses on the beach any more.

Sunscreens often contain organic chemicals like avobenzone and oxybenzone. They absorb the UV radiation so that your skin doesn’t have to. As the chemical bonds absorb the radiation, the components of the sunscreen slowly break down and release the heat.

Inorganic (usually man-made) chemicals will either reflect or scatter the light away from the skin. Organic (carbon-based) ones can absorb UV rays so that your skin doesn’t have to.

What is SPF?

SPF stands for Sun Protection Factor and refers to how well the sunscreen protects against UVB radiation, keeping you from burning. Most organizations recommend using sunscreen with an SPF between 15 and 50. Anything higher than 50 has not been proven to be more effective. According to the Mayo Clinic, a sunscreen with an SPF of 15 protects against about 93 percent of UVB rays, and one with an SPF of 30 protects against 97 percent of rays.

While some UV radiation still gets through, the SPF number refers to roughly how long it will take for a person’s skin to turn red. An SPF of 15 will prevent your skin from getting red for approximately 15 times longer than usual (so, if you usually start to burn in 10 minutes, SPF 15 will prevent burning for about 150 minutes, or 2.5 hours), according to the American Academy of Dermatology.

Why and When Do You Need Sunscreen?

You may consider applying sunscreen only during the summer when you’re outside playing enough. The fact is, it’s not. Whenever you’re outside, you’re pummeled by invisible rays from the sun. These rays are the cause of your tan and painful sunburn, but their effects are much more sinister. UV radiation can damage the DNA in your skin cells, causing genetic mutations that can lead to skin cancer. They are also the leading cause of early signs of aging like fine lines, wrinkles, and sunspots. The damage that you receive from the sun has a cumulative effect, meaning it builds up over time until it shows up physically as “old skin.”

By protecting your skin, you’re not only helping to prevent skin cancers from growing, but you’re ensuring that you’re doing everything you can to prevent those premature wrinkles from forming.

In addition to when you’re outside, you should also protect yourself when you’re inside. UV rays have a way of making their way in through your windows while you’re sitting and working at your desk and through your car windows while you’re driving.

It’s a good habit to apply sunscreen to your face as part of your morning moisturizing ritual, and then again before you go outside. Your daily moisturizer should automatically contain an SPF to give you added protection throughout your day.

If you’re heading outside to play in the sun, apply your sunscreen 30 minutes before heading outside so that it has a chance to soak in and take effect. While you’re outside, you should reapply it every two hours, and more often if you’re swimming or sweating. That’s because most people don’t use enough sunscreen, plus it tends to rub or wash off. Check here for the CDC’s guide to applying and using sunscreen.