AUSTIN (KXAN) — This time of year, you hear us telling you all the time not to forget to apply sunscreen. But how much should we be using, and what does SPF mean?

Meteorologist Sean Kelly spoke with Dr. Aleksandra Zajac, who is a medical skin expert.

Zajac said the sun produces three types of radiation: UVA, UVB and UVC. UVC is the most dangerous, but never makes it down to earth. UVA is responsible for skin aging and skin cancers, while UVB causes sunburn.

“This is the type of the sunlight that goes through the cloud, through the windows, through the car windows, basically you have to be consciously protected,” he said.

Zajac also explained SPF mostly refers to UVB protection. The SPF number on sunscreen refers to the amount of sun exposure you can take before getting burned.

For example, if it normally takes you 10 minutes to get burned, if you wore an SPF of 60, it would then take you 10 hours to get burned.

Zajac explained how scientists determine a sunscreen’s SPF.

“They take some volunteers, and they mark the piece of your skin, and then they expose you to sunlight without any protection,” he said.

Doctors then test how long this small patch of skin takes to burn.

“Then they apply the sunscreen in a very proper controlled value,” he continued.

It’s tested again to determine the SPF. However, that “very proper controlled value” is important.

“Those are two milligrams per one square centimeter of skin,” Zajac said.

So basically for every inch of your body, you need to put a glob of sunscreen the size of a quarter, meaning you’re likely not using enough.

Zajac said don’t panic, because “the sunscreen is not kind of a shield, it’s just a part of your protection.”

Clothing, sunglasses and even a baseball cap could provide the added protection you need the next time you head outdoors, as well as avoiding being outside when the UV index is highest. For us in Central Texas for this time of the year, it’s between 10 a.m. and 2 p.m.