The Fourth of July weekend is here and whether you're planning a barbecue at a park, the beach or in your own backyard, don't forget the sunblock. Sunblock is a major factor in protecting you from getting skin cancer from sun exposure. According to The Skin Cancer Foundation, "more than 90 percent of all skin cancers are caused by sun exposure and sunscreens are a key weapon in the arsenal against the disease."

So, here are some of the answers to what people want to know about sunblock.

What makes the sun harmful?

There are two types of ultraviolet (UV) radiation that the sun emits. They are UVA and UVB rays. UVB rays are the rays that cause sunburn. UVA rays penetrate the skin more deeply and cause long-term damage, such as wrinkling, leathering, sagging, and other effects of aging.

What is the UV Index? The UV Index provides a forecast for the risk of overexposure to the sun. Knowing the UV Index gives you an idea about the dangers of overexposure to the sun when you are working or playing outside. It is calculated on a daily basis by the National Weather Service and the Environmental Protection Agency. The measurement is based on the clouds and local conditions that will affect the amount of UV rays to hit the ground.

It ranges from zero to 10+. Zero implies a low risk of overexposure to the UV rays of the sun and 10+ is a very high risk of overexposure. For the average person, a UV Index of 3 to 5 is a moderate risk of overexposure to the sun

How can we protect ourselves from the harmful effects of sun?

o Limit your exposure to direct sun, i.e., spend time in the shade. o Wear protective clothing if you'll be in the sun for along periods of time, especially, a wide brim hat. o Whether you're in the direct sun or in the shade, use sunblock with a SPF of 15 or higher.

What is SPF? SPF is an acronym for Sun Protection Factor. It is laboratory measurement of a sunscreen's ability to filter the UVB rays to prevent sunburn. The higher the SPF, the more protection it provides against the sun. In other words, if you burn in eight minutes and you use a sunblock with a SPF of 10, it will take you 80 minutes to burn. If you use SPF 15, it will take 120 minutes for you to burn.

In reality, the protection provided depends on several factors: The person's skin type, the amount applied and the frequency of application, activities that are engaged in while the product is on, and amount of sunscreen that is absorbed into the skin.

How should you apply sunblock?

Frequently and liberally! Despite the fact that it adds hours on to the time it takes for a person to burn, it is best to apply it at least every two hours. Apply it more frequently, if you are swimming or sweating. It should also be applied liberally -- one ounce per use. Therefore, if you buy an 8 ounce bottle, it should only last for 8 uses.

Which is the best number to get?

Anything above SPF 15 is best. SPF 15 will filter out 92 percent of the UVB rays, SPF 30 will filter out 97 percent of the rays and SPF 50 will filter out about 98 percent.

Whatever number you get, remember to apply it frequently and liberally!

What is the best kind to get?

It does not have to the most expensive one on the shelf to be the most effective. You should look for ones that filter the UVA and UVB rays. Look for ones that are waterproof or sweatproof. Needless to say, take waterproof and sweatproof with a grain of salt. If you go swimming for more than a quick dip and you're sweating more than a droplet on your forehead, it's not enough to apply it once. If you go swimming, apply it again when you come out of the water. If you're sweating, apply it more frequently.

Are there clothes that have SPF?

A regular white T-shirt has an SPF of 3. There are clothes that are made with zinc oxide and can provide an SPF of 30. Tighter knit clothing also provides some protection. Always try to wear a wide-brim hat to give more protection too.

Am I protected from the UV rays in the shade? The shade does provide some protection but the UV rays of the sun can reflect off the water, sand, concrete, and snow (not usually a problem in the summer!) and then penetrate the skin. So sitting in the shade does provide good protection, but you still need to apply sunscreen.

And remember, whether you're walking on the beach or just sitting on the porch reading a book, it's always a good idea to have a bottle of sunblock close by. It's a key factor in reducing your risk of developing skin cancer.

If you have questions about protecting yourself from the harmful rays of the sun or skin cancer - email Dr. Manny at Drmanny@foxnews.com.

Dr. Cynara Coomer is an assistant professor of surgery specializing in breast health and breast cancer surgery at Mount Sinai Medical Center in New York City. She is a FOX News Health contributor providing medical expertise on a variety of topics in cancer research with a focus on women's health, breast diseases and tips for healthy breasts at any age.