Sunscreens

Complete photo-protective package

  • Shade + sunscreen + hat + photo-protective clothing + glasses

Sunscreen

  • Coating on skin surface: attenuates/filters out radiation that would otherwise reach the epidermis & dermis
  • Efficacy determined by SPF

SPF

  • Dose of ultraviolet radiation (UVR) required to produce 1 minimal erythema dose (MED) on protected skin (2mg/cm2 of product) divided by the UVR to produce 1 MED on unprotected skin (higher Sun-protection factor (SPF) provides better ultraviolet A rays coverage)
  • SPF 15: 93,3% protection; SPF 30: 96,7%; SPF 50: 98%

Water resistant

  • SPF maintained after 40-80 min of water immersion 

Broad spectrum

  • ultraviolet A + ultraviolet B protection as measured by in-vitro critical wavelength (370 nm: wavelength at which 90% of rays >290 nm absorbed by sunscreen)

FDA key concepts

  • Safety, efficacy & labeling

Chemical & physical blockers

  • Chemical: Soluble organic compound, absorb ultraviolet radiation producing excitation to higher state & conversion into heat upon reverting to ground state 
  • Physical: Insoluble inorganic compound, reflect or scatter ultraviolet radiation photons

Note: Most sunscreens include a combination of 2 or more compounds so that broad-spectrum protection could be achieved

Chemical

UVB protection 

  • Padimate O (octyl dimethyl PABA): Ester derivative Para-aminobenzoic acid “PABA” (PABA not used due to staining & allergic contact dermatitis), most potent FDA-approved ultraviolet B absorber
  • Cinnamates: Types: octinoxate (octyl methoxycinnamate) or cinaxate. Next most potent ultraviolet B (UVB) absorber, #1 sunscreen ingredient
  • Ensulizole: Water-soluble, can be used in lighter products/less oily feel
  • Stabilizers (mechanism of action: used w/other agents to prevent photodegradation)
    • Salicylates (octisalate, homosalate, trolamine salicylate): stabilizer + weak UVB absorber
    • Octocrylene

Ultraviolet A (UVA) protection

  • Oxybenzone (benzophenone 3): Broad-Spectrum: UVB + UVA2 > UVA1, Benzophenone 4 (UVA1) & 8 (UVB > UVA2)
  • Meradimate: UVA 2 & weakly UVB; less effective & not often used
  • Avobenzone: Great UVA1 coverage, but photolabile & degrade other sunscreen ingredients
  • Others:
    • Ecamsule (Mexoryl SX) & Tinosorb S: only UVA (2 & 1)
    • Mexoryl XL & Tinosorb M: UVA & UVB
Physical (UVB & UBA protection)
  • Titanium dioxide: Broad spectrum (Nano sized particles ↓ effective in UVA & visible spectrum range)
  • Zinc oxide: Better than titanium dioxide for UVA coverage (Nano sized particles ↓ effective in visible spectrum)

Other agents (not active screens)

  • Dihydroxyacetone: Chemical sunless tanner, minimal protection of 3-4, but extends into visible spectrum
  • Iron oxide: visible spectrum

FDA

  • Sunburn, skin/lip damage, freckling, skin discoloration, skin aging, skin cancer

Off-label

  • Phototoxic & photoallergic drug reactions, photosensitive dermatoses, photoaggravated dermatoses

  • Known sensitivity to any active sunscreen ingredient/ vehicle ingredient
  • Infants < 6 months of age
  • As the sole component of a sun protection program

  • Irritation
  • Contact urticaria (immune & non-immune)
  • Irritant contact dermatitis
  • Allergic contact dermatitis (ACD) (fragrances, preservatives, benzophenone 3)
  • Photosensitivity (photo-ACD)
  • Induce or exacerbate acne (vehicle)

Instructions to patients 

  • Wear protective clothing (tightly wove; greater protection), hat (covers face & neck) & sunglasses
  • Avoid sun 10 AM to 4 PM (strongest ray exposure)
  • May be needed year-round depending on latitude & climate (80% ultraviolet radiation reaches Earth on cloudy days)
  • Use a sunscreen with SPF 30 +, also broad spectrum
  • Apply 15 min prior to sun exposure to allow sufficient time for protection 
  • Amount (adult): 1-2 ounces to cover exposed skin
  • Reapply every 2 hours & after prolonged swimming or excessive sweating
  • Use a water-resistant sunscreen if swimming or excessively sweating
  • Not approved for infants < 6 months of age, & for older children, suggest using physical sunscreens only  

Sunscreen bans & coral reef

  • Hawaii enacted legislation that will ban the use of two major ingredients (Oxybenzone & octinoxate) used in the majority of sunscreens
  • Reason for ban → deleterious impact on coral reefs
  • New law goes into effect January 1st 2021 (1)