Sunscreens are substances applied to the skin that either absorb or reflect the sun's rays in order to prevent UVR from reaching the skin.
Very simply there are two kinds of sunscreens CHEMICAL or PHYSICAL.
The CHEMICAL screen contain absorbing chemicals that act as a filter,absorbing solar energy before it can penetrate the skin.
Chemical sunscreens are wavelength-selective, with a specific absorption spectrum. The UVA sunscreens absorb within a limited absorption spectrum, usually at shorter wavelengths of UVA (320 to 360nm). The UVB sunscreens, on the other hand, are effective in absorbing the entire UVB spectrum (290-320nm).
UVB sunscreens are more effective than UVA sunscreens for photoprotection but because the action spectrum of many photodermatosis is in the UVA spectrum, the therapeutic potential of these sunscreens is limited.
UVA penetrates more deeply into the dermis than UVB radiation and are also less erythemogenic than UVB. One can be exposed to excessive doses of UVA radiation without manifesting erythema. This can lead to oncogenic and immunogenic consequences.
UVA sunscreens UVA sunscreens include benzophenones, anthralinates and dibenzoyl-methanes. Both benzophenones and anthralinates are considered broad spectrum sunscreens as they absorb over 300 to 350nm, whereas butyl methoxy dibenzoylmethane (Parsol 1789) is predominantly an UVA absorber as it absorbs maximally at 340 to 350nm.
UVB sunscreens The oldest approved sunscreens for use is para-aminobenzoic acid (PABA). Contact allergy to PABA has been well documented over the years and the use of PABA as a chemical absorber has been replaced completely in recent times by other chemical absorbers such as cinnamates and salicylates or its esters (padimate A & 0). Many products now available commercially are PABA free. Cinnamates are widely used as a UVB sunscreen.
PHYSICAL screens are usually opaque and contain ingredients that do not absorb UVR, instead reflecting and scattering the rays away from the skin.
Physical blocks act as physical barriers that scatter radiation and light. When present in sufficient quantities, they will reflect all the ultraviolet, visible and infrared rays They are currently used with chemical absorbers to achieve high sun protection factors. Titanium dioxide and zinc oxide show strong absorption band in the UV spectrum and may also be considered chemical sunscreens.
The effectiveness of physical blocks depends on the size and the concentration of the particles. As a rule, physical blocks that are micronized and have a thick film are more effective in scattering uv radiation and visible light. Of the physical blocks available, titanium dioxide, iron oxides and zinc oxides have been used in a multitude of particle size and suspensions and are widely used in cosmetic formulations.
By Wayne Potter