UV radiation, SPF and health dangers of chemical sunscreens
UV radiation is part of the electromagnetic (light) spectrum that reaches the earth from the sun. It has wavelengths shorter than visible light, making it invisible to the naked eye. These wavelengths are classified as UVA, UVB, or UVC.
UVA rays account for up to 95% of the UV radiation reaching the Earth's surface. They are present during all daylight hours throughout the year, and can penetrate clouds and glass
UVA penetrates the skin more deeply than UVB and plays a major part in premature skin aging and wrinkling (photoaging) and contributes to development of skin cancers.
UVB, the chief cause of skin reddening and sunburn, plays a key role in the development of skin cancer and a contributory role in tanning and photoaging. Its intensity varies by season, location, and time of day.
UVC is absorbed by the ozone layer and does not reach the earth.
SPF and UV radiation
SPF indicates how long it will take for UVB rays to redden skin when using a sunscreen, compared to how long skin would take to redden without the product. For instance, someone using a sunscreen with an SPF of 15 will take 15 times longer to redden than without the sunscreen. An SPF 15 sunscreen screens 93 percent of the sun's UVB rays; SPF 30 protects against 97 percent; and SPF 50, 98 percent. The Skin Cancer Foundation maintains that SPFs of 15 or higher are necessary for adequate protection.
Since both UVA and UVB are harmful, you need protection from both kinds of rays. There are currently 17 active ingredients approved by the FDA for use in sunscreens. These filters fall into two broad categories: chemical and physical. Most UV filters are chemical: They form a thin, protective film on the surface of the skin and absorb the UV radiation before it penetrates the skin. The physical sunscreens are insoluble particles that reflect UV away from the skin.
Health dangers of most of the common sunscreens:
The most common sunscreens on the market contain combination of chemical filters like: oxybenzone, avobenzone, octisalate, octocrylene, homosalate and octinoxate. 
Chemical filters may cause skin reactions including acne, burning, blisters, dryness, itching, rash, redness etc. Physical sunscreens -zinc oxide and titanium dioxide don't cause skin allergy. 
Chemical filters like oxybenzone absorb into the body and can be detected in blood, urine and mothers’ milk, indicating that the developing fetus and newborns may be exposed to these substances 
Chemical filters disrupt hormone system, altering reproductive and thyroid hormones. Oxybenzone and benzophenones linked to poorer sperm quality and to endometriosis in women 
Chemical sunscreen use don't prevent melanoma and basal cell carcinoma and might even increase their occurrence. Commonly used chemical sunscreens block only UVB, but not UVA, which makes up 90 to 95% of ultraviolet energy. UVB protection prevents erythema and sunburn and may encourage prolonged exposure to UV radiation and thus may actually increase the risk of melanoma development (laboratory data suggest that melanoma and basal cell carcinoma are initiated or promoted by solar radiation other than UVB) , 
Chemical sunscreens use promotes photoageing : wrinkles, pigmentation etc. As mentioned, chemical filters block only UVB and are virtually transparent to UVA, which is the major cause of premature skin aging and wrinkling 
Inactive ingredients in common sunscreens typically make up half to nearly all of a sunscreen product and some are a cause for concern:
MI preservative is a skin sensitizer and allergen 
Parabens use in sunscreens increases UV induced skin damage, increases cell death and oxidative stress. 
Mineral sunscreens use physical filters like zinc oxide and/or titanium dioxide rate better than chemical sunscreens for safety in the EWG database. 
Zinc oxide is EWG’s first choice for sun protection. It is stable in sunlight and can provide greater protection from UVA rays than titanium oxide or any other sunscreen chemical approved in the U.S.