SUN SCREENS

Sunscreen: It is a substance that alter or inhibit UV radiation effect on skin by absorption and / or reflection

Characteristics of an ideal sunscreen
• A combination of physical and chemical agents
• Broad spectrum
• Cosmetically elegant
• Substantive
• Non irritant
• Hypoallergenic
• Non comedogenic
• Economic


The common indications for the use of sunscreens in dermatology are in the prevention and management of
1. Sunburn
2. Freckling, discoloration
3. Photoaging
4. Skin cancer
5. Phototoxic/ photoallergic reactions
6. Photosensitivity diseases
o Polymorphous light eruption (290-365 nm)
o Solar urticaria (290-515 nm)
o Chronic actinic dermatitis (290 nm-visible)
o Persistent light reaction (290-400 nm)
o Lupus erythematosus (290-330 nm)
o Xeroderma pigmentosum (290-340 nm)
o Albinism
7. Photoaggravated dermatoses
8. Post-inflammatory hyperpigmentation (post-procedure)

Sunscreens can be broken down into two types: physical and chemical.
Physical sunscreens contain extremely fine particles of minerals such as zinc oxide, titanium dioxide and iron oxide that stay on the skin's surface, creating a barrier that reflects the sun's rays.
Pros of physical sunscreen include its ability to keep skin cool, the protection is natural, and it's unlikely to cause skin irritation because it doesn't get absorbed. The con: potentially heavy residue it can leave behind. However,
modern refining techniques are helping to prevent this.
Chemical sunscreens contain synthetic compounds like parsol, benzophenones and paba as active ingredients, and they absorb ultraviolet radiation that causes sunburn, damages DNA and can lead to skin cancer.
Pros include its ability to defend the deeper layers of skin-including collagen fibers and other tissue-against the aging effects of UVA rays; and it prevents penetration by the UVB rays responsible for tanning and sunburn. Cons include irritation and allergic reactions in some people, as well as the tendency to break down quickly and generate free radicals.


Classification of Sunscreens
The last FDA sunscreen monograph was issued in 1999, with a list of 16 approved sunscreen agents. It recommends that sunscreens be classified as organic and inorganic, replacing the previously used terms "chemical" and "physical", respectively. There are three commonly used nomenclatures for sunscreen agents in the world. These are the International Nomenclature Cosmetic Ingredient (INCI) name, US adopted name (USAN), and trade name. Taking avobenzone (USAN) as an example, the INCI name for avobenzone is butylmethoxydibenzoylmethane, while Parsol 1789 is one of its many trade names.


A. Organic Sunscreens
Organic UV filters are active ingredients that absorb UV radiation within a particular range of wavelengths, depending on their chemical structure. Once the UV filter absorbs energy, it moves from a low-energy ground state to a high-energy excited state. From this excited state, any of the following three processes may occur, depending on the ability of the filter to process the energy it has absorbed:


• Photostable filter: This type of filter dissipates its absorbed energy to the environment as heat energy, and returns to the ground state. It is subsequently fully capable of absorbing UV energy again.
• Photounstable filter: The filter undergoes a change in its chemical structure, or is degraded after absorbing UV energy. It is not capable of absorbing UV energy again.
• Photoreactive filter: In its excited state, the filter interacts with surrounding molecules, including other ingredients of the sunscreen, oxygen, and skin proteins and lipids. This leads to the production of reactive species, which may have unwanted biological effects.


Organic sunscreens are further divided into UVB and UVA filters:
1. UVB filters
a. PABA derivatives - Padimate O
b. Cinnamates - Octinoxate, Cinoxate
c. Salicylates - Octisalate, Homosalate, Trolamine salicylate
d. Octocrylene
e. Ensulizole


2. UVA filters
a. Benzophenones (UVB and UVA2 absorbers) - Oxybenzone, Sulisobenzone, Dioxybenzone
b. Avobenzone or Parsol 1789 (UVA1 absorber)
c. Meradimate (UVA2 absorber)


Note: Although avobenzone is the only sunscreen agent that has its absorption peak in the UVA1 spectrum (357 nm), making it a very effective UVA filter, it has the disadvantage of being photounstable. This is overcome, to some extent, by adding agents that photostabilize avobenzone, to the final sunscreen product. These agents include other UVA filters such as oxybenzone, UVB filters such as enzacamene (not yet FDA approved), salicylates, octocrylene, broad spectrum filters such as bemotrizinol (not yet FDA approved), and inorganic filters such as titanium dioxide and zinc oxide.


3. Newer generation broad spectrum (UVA + UVB) filters - Ecamsule (Mexoryl SX), Silatriazole (Mexoryl XL), Bemotrizinol (Tinosorb S), Bisoctrizole (Tinosorb M)

Ecamsule is primarily a UVA filter, the patent for which is held by L'Orιal (sunscreens containing ecamsule are exclusive to L'Orιal and its brands). Tinosorb M is the first of a new class of UV filters that combine the properties of both UV conventional filters (organic and inorganic) - it scatters, reflects and absorbs UV light. Apart from Ecamsule, these filters are not yet US FDA approved, but are being used in other countries, such as the European Union and Canada.

B.INORGANIC SUNSCREENS
1. Zinc oxide
2. Titanium dioxide
3. Others - iron oxide, red veterinary petrolatum, kaolin, calamine, ichthammol, talc

Inorganic agents function by reflecting, scattering or absorbing UV radiation. Their opaque nature and "whitening effect" are an inherent disadvantage, which may be minimized by the use of micronized or ultrafine particles.