Ultraviolet radiation is that portion of the electromagnetic spectrum with a narrow band of radiation from 200-400 nm. Spectrum – UVC (200-290 nm), UVB (290- 320 nm) and UV A (320-400nm –UV A 2 =320-340; UVA 1 = 340-400). UVA rays constitute 90-95% of the ultraviolet light reaching the earth and it is not absorbed by the ozone layer. UV A light penetrates deep into the skin and is involved in sun tanning, UV A tends to suppress the immune function and is implicated in photoaging of the skin by inducing changes in the extracellular matrix decreased collagen synthesis, increased degradation of collagen, infiltration of inflammatory cells and release of ROS from neutrophils etc. UVB rays – partially absorbed by the ozone layer, are the primary cause of sunburn and reach upto papillary dermis only. It is also responsible for photoaging, photo carcinogenesis and are implicated in cataract formation. UVC rays are almost totally absorbed by the ozone layer. However with the depletion of the ozone layer because of environmental pollution UVC rays are beginning to contribute to sunburn and premature ageing of the skin.

Sunscreens- divided into chemical absorbers and physical blockers. Chemical sunscreens absorb high-energy ultraviolet rays and release the energy later as heat. No significant photo degradation occurs except avobenzones.

Physical blockers or non chemical sunscreens reflect or scatter UVR. They contain inert minerals such as titanium dioxide or zinc oxide. (new nanosized particles may help by absorption also)

1. Ultraviolet B Blockers –

Para-aminobenzoic acid- one of the first chemical sunscreens, padimate 0 or octyl dimethyl PABA is associated with greater compatibility, Padimate 0 is the most potent UV-B absorber.

Cinnamates- Octyl methoxy cinnamate ( OCTINOXATE) is the most frequently used sunscreen ingredient.

Octyl salicylate – Octisalate or octyl salicylate is used to augment the UV-B protection in a sunscreen.

Octocrylene – Octocrylene used in combination with other sunscreen ingredients, such as avobenzone, add to the overall stability.

Phenyl benzimidazole sulfonic acid – (Ensulizole) is water soluble ,used in products formulated to feel lighter and less oily.

2. Ultraviolet A Blockers – Benzophenone

Although benzophenones are primarily UV-B absorbers, oxybenzone absorbs well through UV-A2. Oxybenzone is considered a broad-spectrum absorber.

Anthranilate – absorb mainly in the UV-A2 portion, less widely used.

Avobenzone – (ParsoI1789) a large portion of the UV-A range, including UVAl

Mexoryl SX- ( L’oreal) – (ecamsule) photo stable, block UV -A range (320- to 340-nm); it is water soluble and less water resistant.

Helioplex (Neutrgena)- Oxybenzone + abobenzone stabilised with 2, 6, diethylexylnaphalate

Bisethylhexyloxyphenol methoxy phenyl triazene – This broadband sunscreen filter lends photostability to avobenzone containing sunscreens.

Physical blockers – inert minerals such as titanium dioxide or zinc oxide; particles scatter and reflect the solar radiation. Ultrafine titanium dioxide, provides broad-spectrum protection against UV, it is invisible when applied in cosmetic formulations. Chemically stable and do not cause photo-allergic or contact dermatitis and do not break down over time and are far less liable to cause skin irritation. However protection against UVA 1 is superior for zinc oxide (340- 380nm), more broad spectrum. (Good for porphyrias)

Sunscreen vehicles: Most effective UV absorbers are oils – heavy, greasy feel, polymeric film formers and silicone oils – less greasy; ‘ultrasheer’ products utilise silica; gels- rely on phenyl benzimidazole sulfonic acid or trolamine salicylate

Systemic photo protection – Systemic agents, eliminate substantivity , include PABA, indomethacin, retinol, steroids, psoralen, antimalarials and antioxidants like vitamin A, vitamin C, vitamin E and beta-carotene. Antioxidants are less potent than sunscreens in preventing sunburn.

Indications & Contraindications of Sunscreens –

Indications:

Protection from UVR to prevent the following-

– Sunburn

– Skin or lip damage, freckling, skin discoloration

– Skin aging

– Skin cancer

– Phototoxic or photo allergic drug reactions

– Photosensitivity diseases

– Photo aggravated dermatoses.

Contraindications

– Known sensitivity to any sunscreen ingredient or vehicle ingredient

– Infants < 6 months

– As sole component of an overall program of photoprotection

Side effects of sunscreens –

– Upto 19% of users can develop adverse reaction, to both active and vehicle ingredient equally.

– Majority are irritant in nature, < 10 % allergic, and most patch test negative.

– Subjective irritation- immediate stinging or burning , without visible erythema, most frequently in the eye area, even if applied away from, by migration through sweating; even several hrs after application, falsely implicated as allergy.

– contact urticaria

– allergic contact dermatitis – PABA earlier, benzophenones increasingly nowadays, fragrances, preservatives and other excipients

– photosensitivity reactions- sunscreen active ingredients have become the leading cause of photo contact allergic reactions, mainly to avobenzone. Photosensitive eczematous pts particularly predisposed (suspect in clinical exacerbation).

– acne induction and exacerbation vehicles may induce ; mainly aggravation of pre-existing acne; contact folliculitis (shortly after application) a form of irritation.

Effectiveness and efficacy of Sunscreens

1. UV B
a) Sun protection factor (SPF) – Amount of ultraviolet energy (UVB) required to produce minimal erythema on sunscreen protected skin-(2 mg/cm 2 layer) to the amount of energy required on unprotected skin.

b) Substantivity – the ability of a sunscreen to remain effective under the stress of prolonged exercise, sweating and swimming.

Sweat-resistant: protects up to 30 minutes of continuous heavy perspiration.

Water-resistant: protects up to 40 minutes of continuous water immersion and

Waterproof (very water resistant) : protects for up to 80 minutes of continuous water immersion

PABA and its esters demonstrate more resistance to sweating and/ or water immersion than do other chemical sunscreens.

2. UV A 

Protection of Sunscreen- no uniformly accepted standard method for measuring UV A protection of sunscreen. PPD (persistent pigment darkening) is most commonly used because pigmentation remains stable between 2 and 24 hours.

Broad-spectrum label should have a critical wavelength of more than 370 nm and a PPD or protection factor in UV A greater than 4.

Some more facts –

– Even on cloudy days up to 80% of UVR is transmitted to the Earth’s surface.

– Sun exposure during childhood (upto 18 yrs is about 80% of an average person’s lifetime exposure ton the sun)

– Sun protection should begin at young age and may start as early as 6 months of age

– Surfaces such as sand, snow, concrete and water can reflect up to 85% of the UV radiation

– UV A penetrate window glass, UVB do not; indoor light sources have been shown to emit UV A, UVB & sometimes UVC rays.(i.e. indoor exposure possible)

– UV A varies much less in intensity throughout the day than UVB.

Instruction to patients:

1. Depending on latitude and climate, sunscreens may be needed yearround.

2. Sunscreens most important from lOAM to 4 PM. To protect from harmful radiation during this period, stay in shade or indoors .

3. For intermittent casual daily use, an SPF 15 is sufficient. For prolonged recreational exposures, an SPF30 is desirable.

4. Sunscreens should be applied 15-30 minutes before exposure, reapply after activities such as swimming, sweating and rubbing. Common advice earlier was to apply sunscreen every 2-3 hours, (however new research shows that reapplication 15- 30 minutes after the sun exposure begins may be the best way)

5. Apply liberally. Most do not. An adequate amount of sunscreen (2 mg/ ern 2) provides greater sun protection than using an inadequate amount of a sunscreen with a higher SPF rating. The teaspoon rule: (~3ml) to each arm, to the face and the neck. On each leg, the chest and back, (~6ml); about 30 ml to cover entire body.

6. Patients should select broad-spectrum sunscreens that contain agents that effectively block both UVB and UV A preferably above SPF 30. The SPF applies for UVB rays and near UVB- i.e. UV A-2 only. A sunscreen with a SPF of 15 filters out approximately 94%, with a SPF of 30 filters out 97 % of UVB (difference in penetration of about 3 % not significant in most clinical situations).

Patients requiring UV A protection should apply sunscreens longer particularly with drug photosensitivity or various photoaggravated dermatoses.

Natural pigments such as titanium dioxide and zinc oxide are good UVB and UVA blockers. Today, certain chemicals can be added to sunscreen products to give them good UVA ray blocking abilities. Such chemicals include avobenzone and Mexoryl SX.

However, most of cosmetic products that contain sunscreen chemicals offer various SPF levels up to 15-30.

Future trends in photoprotection – in stages of investigation- The antioxidants are caffeic acid, poly podium leukotomes, zinc, polyphenolic compounds, isoflavone, N-acetylcysteine and butyrated hydroxy toluene (synthetic antioxidant). Calcitriol and citrus which are free radical scavengers. Plant oligosaccharides and genistein help in prevention of UV ind uced immunosuppression.

Sunscreens alone are insufficient for protection from UVR as they prevent sunburn from UV -B radiation and provide more limited protection from UV-A radiation. Protective clothing and sun avoidance particularly at times of peak intensity is warranted.

-Dr. Th. Nandakishore

This article is reproduced from the edition of “Souvenir” VII CUTICON Manipur 2010.