question: How sunscreen actually protects our skin from getting cancer or being burned by sunlight?
comment: As far as I am aware, there are two types of sunscreen: inorganic (metal based) and organic (carbon based). Inorganic based sunscreen reflects/scatters UV light, much like a UV mirror. Organic based sunscreen absorbs the UV light so our skin doesn't have to. These absorb the UV light, break down, and release just a tiny amount of heat. Sunscreens can use a hybrid of these two types!
passages: Commercial preparations are available that block UV light, known as sunscreens or sunblocks. They have a sun protection factor (SPF) rating, based on the sunblock's ability to suppress sunburn: The higher the SPF rating, the lower the amount of direct DNA damage. The stated protection factors are correct only if 2 mg of sunscreen is applied per square cm of exposed skin. This translates into about 28 mL (1 oz) to cover the whole body of an adult male, which is much more than many people use in practice. Sunscreens function as chemicals such as oxybenzone and dioxybenzone that absorb UV radiation (chemical sunscreens) or opaque materials such as zinc oxide or titanium oxide to physically block UV radiation (physical sunscreens). Chemical and mineral sunscreens vary in the wavelengths of UV radiation blocked. Broad-spectrum sunscreens contain filters that protect against UVA radiation as well as UVB. Although UVA radiation does not primarily cause sunburn, it does contribute to skin aging and an increased risk of skin cancer. 
   Zinc oxide and titanium oxide are often used in sun screen to provide broad protection from UVA and UVB ranges.
  
  Eating certain foods may decrease the risk of sunburns but this is much less than the protection provided by sunscreen.
  
  A meta-analysis of skin cancer prevention in high risk individuals found evidence that topical application of T4N5 liposome lotion reduced the rate of appearance of basal cell carcinomas in people with xeroderma pigmentosum, and that acitretin taken by mouth may have a skin protective benefit in people following kidney transplant.
  
  Section::::Treatment.
   Section::::Measurements of protection.:UVA protection.:Persistent pigment darkening.
  
  The persistent pigment darkening (PPD) method is a method of measuring UVA protection, similar to the SPF method of measuring sunburn protection. Originally developed in Japan, it is the preferred method used by manufacturers such as L'Oréal.
   For total protection against damage from the sun, the skin needs to be protected from UVA, UVB, and also IRA (infrared-A light). Roughly 35% of solar energy is IRA.. However, note that there is continuing debate within the dermatology community over the impact of sun-sourced IRA: Some sources indicate that early morning IRA exposure may be protective against further sun exposure by increasing cell proliferation and initiating anti-inflammatory cascades; these effects are not observed for artificial sources of intense IRA.
  
  Section::::Active ingredients.
   Sunblock—Sunblock is opaque and stronger than sunscreen, since it is able to block most of the UVA/UVB rays and radiation from the sun, and does not need to be reapplied several times in a day. Titanium dioxide and zinc oxide are two of the important ingredients in sunblock.
   Water-resistant sunscreens were introduced in 1977, and recent development efforts have focused on overcoming later concerns by making sunscreen protection both longer-lasting and broader-spectrum, as well as more appealing to use.
  
  Section::::Research.
  
  New products are in development such as sunscreens based on bioadhesive nanoparticles. These function by encapsulating commercially used UV filters, while being not only adherent to the skin but also non-penetrant. This strategy inhibits primary UV-induced damage as well as secondary free radicals.
   Section::::Health effects.:Benefits.
  
  Sunscreen use can help prevent melanoma and squamous cell carcinoma, two types of skin cancer. There is little evidence that it is effective in preventing basal cell carcinoma.
   Since the photosensitivity results from light in the visible spectrum, most sunscreens are of little use (with the exception of non-nano zinc oxide which provides uniform protection between 290-400 nm and some protection up to 700 nm). Sun protective clothing can also be very helpful, although clothing with UPF values are only rated based on their UV protection (up to 400 nm) and not on their protection from the visible spectrum. Some sun protective clothing manufacturers use zinc oxide in their fabrics, such as Coolibar's ZnO Suntect line, which will offer protection from visible light.
   Increasing the permeability of skin allows nanoparticles to penetrate and target cancer cells. Nanoparticles along with multi-modal imaging techniques have been used as a way to diagnose cancer non-invasively. Skin with high permeability allowed quantum dots with an antibody attached to the surface for active targeting to successfully penetrate and identify cancerous tumors in mice. Tumor targeting is beneficial because the particles can be excited using fluorescence microscopy and emit light energy and heat that will destroy cancer cells.
  
  Section::::Permeability.:Sunblock and sunscreen.
  
  Sunblock and sunscreen are different important skin-care products though both offer full protection from the sun.
   Sunscreen (also commonly known as "sun screen", "sunblock", "suntan lotion", "sunburn cream", "sun cream" or "block out") is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet (UV) radiation on the skin exposed to sunlight and thus helps protect against sunburn. Depending on the mode of action, sunscreens can be classified into physical sunscreens (i.e., those that reflect the sunlight) or chemical sunscreens (i.e., those that absorb the UV light).
   Medical organizations such as the American Cancer Society recommend the use of sunscreen because it aids in the prevention of squamous cell carcinomas. The routine use of sunscreens may also reduce the risk of melanoma. However, many sunscreens do not block UVA (UVA) radiation, yet protection from UVA has been shown to be important for the prevention of skin cancer.
   Sunscreening agents are used in some hair care products such as shampoos, conditioners and styling agents to protect against protein degradation and color loss. Currently, benzophenone-4 and ethylhexyl methoxycinnamate are the two sunscreens most commonly used in hair products. The common sunscreens used on skin are rarely used for hair products due to their texture and weight effects.
  
  The following are the FDA allowable active ingredients in sunscreens:
  
  Zinc oxide was approved as a UV filter by the EU in 2016.
   Section::::Human health-related effects.:Harmful effects.:Skin damage.:Sunscreen safety debate.
  
  Medical organizations recommend that patients protect themselves from UV radiation by using sunscreen. Five sunscreen ingredients have been shown to protect mice against skin tumors. However, some sunscreen chemicals produce potentially harmful substances if they are illuminated while in contact with living cells. The amount of sunscreen that penetrates into the lower layers of the skin may be large enough to cause damage.
   To provide a better indication of their ability to protect against skin cancer and other diseases associated with UVA radiation (such as phytophotodermatitis), the use of broad-spectrum (UVA/UVB) sunscreens has been recommended. The use of the term "Broad Spectrum" on the label sunscreen products is regulated by the U.S. Food and Drug Administration.
   Excessive UV radiation can cause sunburns, photoaging, and skin cancer. UV classifications include UVA (320-400 nm), UVB (290-320 nm) and UVC (200-280 nm). UV-absorbing compounds are used as active ingredients in sunscreen and other personal care products, such as lipstick, shampoo, hair spray, body wash, toilet soap, and insect repellent.Chemical filters provide protection against UV radiation by absorbing, reflecting, or scattering it. Reflection and scattering are accomplished by physical UV filters, which are inorganic UV filters such as titanium dioxide (TiO) and zinc oxide (ZnO). Absorption, mainly of UVB, is done by organic UV filters, which are known as chemical UV filters.
   The SPF is an imperfect measure of skin damage because invisible damage and skin aging are also caused by ultraviolet type A (UVA, wavelengths 315–400 or 320–400 nm), which does not primarily cause reddening or pain. Conventional sunscreen blocks very little UVA radiation relative to the nominal SPF; broad-spectrum sunscreens are designed to protect against both UVB and UVA. According to a 2004 study, UVA also causes DNA damage to cells deep within the skin, increasing the risk of malignant melanomas. Even some products labeled "broad-spectrum UVA/UVB protection" have not always provided good protection against UVA rays. Titanium dioxide probably gives good protection, but does not completely cover the UVA spectrum, as early 2000s research suggests that zinc oxide is superior to titanium dioxide at wavelengths 340–380 nm.
   Sunscreen
  
  Sunscreen, also known as sunblock, is a lotion, spray, gel, foam (such as an expanded foam lotion or whipped lotion), stick or other topical product that absorbs or reflects some of the sun's ultraviolet (UV) radiation and thus helps protect against sunburn. Diligent use of sunscreen can also slow or temporarily prevent the development of wrinkles, moles and sagging skin.
  
  Depending on the mode of action, sunscreens can be classified into physical sunscreens (i.e., those that reflect the sunlight) or chemical sunscreens (i.e., those that absorb the UV light).
   Sunscreen is effective and thus recommended for preventing melanoma and squamous cell carcinoma. There is little evidence that it is effective in preventing basal cell carcinoma. Typical use of sunscreen does not usually result in vitamin D deficiency, but extensive usage may.
  
  Section::::Prevention.:Sunscreen.:Recommendations.
   In an experiment by Hanson et al. that was published in 2006, the amount of harmful reactive oxygen species (ROS) was measured in untreated and in sunscreen treated skin. In the first 20 minutes, the film of sunscreen had a protective effect and the number of ROS species was smaller. After 60 minutes, however, the amount of absorbed sunscreen was so high that the amount of ROS was higher in the sunscreen-treated skin than in the untreated skin. The study indicates that sunscreen must be reapplied within 2 hours in order to prevent UV light from penetrating to sunscreen-infused live skin cells.
   Other ingredients approved within the EU and other parts of the world, that have not been included in the current FDA Monograph:
  
  Many of the ingredients awaiting approval by the FDA were relatively new, and developed to absorb UVA. The 2014 Sunscreen Innovation Act was passed to accelerate the FDA approval process.
  
  Section::::Application.
   Ingested medications may cause systemic photosensitivity and topically applied medications, cosmetics and essential oils may lead to local (or perhaps systemic) photosensitivity. Para-aminobenzoic acid (PABA), found in some sunscreens, can also cause photosensitivity.
  
  Upon exposure to light, notably light containing ultraviolet radiation, discolouration of the skin (whether as inflammation, lightening or darkening) or rashes may result. In extreme cases, blistering may also occur.
  
  Section::::Uses.
   UVA is blocked less than UVB by many sunscreens, but is blocked to some degree by clothing. UVA is known both to cause DNA damage and to be carcinogenic. However, it operates not by inducing direct DNA damage, but by producing reactive oxygen species which damage DNA indirectly. UVA (see above) induces a cosmetic tan but little extra melanin protection against sun damage, sun burn, or cancer. 
  
  Section::::Tanning process.:UVB.
  
  Ultraviolet B (UVB) radiation is in the wavelength range 280 to 320 nm. Much of this band is blocked by the Earth's ozone layer, but some penetrates. UVB:
   BULLET::::- Avoid sun exposure during noontime hours between 10:00 AM and 2:00 PM when UV light is most powerful
  
  BULLET::::- Minimize all time in the sun, since UV exposure occurs even in the winter and on cloudy days
  
  BULLET::::- Using sun protection
  
  BULLET::::- Applying sunscreens with SPF ratings 30 or greater that also block both UVA and UVB light, at least every 2 hours and after swimming or sweating
  
  BULLET::::- Applying sunscreen at least 15 minutes before going outside, as this allows time for the sunscreen to be absorbed appropriately by the skin
   Sunscreen labeling standards have been evolving in the United States since the FDA first adopted the SPF calculation in 1978. The FDA issued a comprehensive set of rules in June 2011, taking effect in 2012–2013, designed to help consumers identify and select suitable sunscreen products offering protection from sunburn, early skin aging, and skin cancer:
  
  BULLET::::- To be classified as "broad spectrum", sunscreen products must provide protection against both UVA and UVB, with specific tests required for both.
   The photochemical properties of melanin make it an excellent photoprotectant. However, sunscreen chemicals cannot dissipate the energy of the excited state as efficiently as melanin and therefore, if sunscreen ingredients penetrate into the lower layers of the skin, the amount of reactive oxygen species may be increased. The amount of sunscreen that penetrates through the stratum corneum may or may not be large enough to cause damage.
  