The Environmental Working Group (EWG) released its annual guide to sunscreens Monday explaining how sunscreens work and why all products do not help protect your skin from UV rays. In the report, researchers gave low scores to products that had SPF over 50 and contained chemicals like Oxybenzone and Retinyl palmitate.

The report claims that 73 percent of the 880 sunscreens tested by the group show the products don't work as well as advertised, or contain "worrisome" ingredients.

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“People are still confused about sunscreen and how it works and what the drawbacks may be,” Sonya Lunder, a senior analyst at EWG said in the report, which rates almost 1500 products based on their ingredients. "Sunscreens are really mismarketed, and as a result, people who depend on them think they are far more powerful than they really are."

The advocacy group compiled a list of best and worst-rated products after examining SPF protection, chemical ingredients, overall safety and effectiveness of several sunscreens.

Sunscreen is made of both organic (carbon-based) and inorganic (non-carbon-based) materials. Organic materials act chemically and absorb UV rays. Inorganic ones include zinc oxide and titanium oxide, and they physically block UV rays from scorching your skin.

Dermatologists recommend using sunscreen to block the sun's ultraviolet rays. Both the types of UV rays — UVA and UVB — can cause skin cancer. Most sunscreens sold today promise protection from both.

To get the complete list of EWG's sunscreen ranking list, click here.

SPF and Sunscreen Ratings: A sunscreen’s Sun Protection Factor (SPF) refers to its ability to shield from the sun’s ultraviolet rays. The SPF number is the level of protection a sunscreen provides against the rays. In general, it takes about 10 to 20 minutes for a person’s skin to start burning if he/she is not using a sunscreen.

"If you're standing on the equator at high noon, it would usually take your skin one minute without sunscreen to become red and irritated. With a sunscreen having SPF 15,  you could stand in that same sun exposure for 15 minutes," Dr. Dawn Davis, a dermatologist at the Mayo Clinic who was not involved in the new reports told CNN.

For many, the numbers can get confusing. "SPF is not a consumer-friendly number," Florida dermatologist James Spencer told WebMD. "It is logical for someone to think that an SPF of 30 is twice as good as an SPF of 15 and so on. But that is not how it works."

A sunscreen with an SPF 15 blocks about 94 percent of the sun’s harmful rays. SPF 30 products block about 97 percent of such rays and SPF 45 sunscreen protects from 98 percent of such rays.

Lesser known facts about sunscreens:

Below are five facts that many people using sunscreens are less aware of, according to EWG.

1. There’s no proof that sunscreens prevent most skin cancer.

2. The common sunscreen additive vitamin A may speed development of skin cancer.

3. European sunscreens provide better UVA protection.

4. Sunscreen doesn’t protect skin from all types of sun damage.

5. Some sunscreen ingredients disrupt hormones and cause skin allergies.