1. Physical (Barrier) Sunscreens:

• Function

-Scatter or reflect UV radiation.
-Block UV, visible, and infrared spectra.

• Ingredients:

- Titanium dioxide (TiO2), magnesium oxide, iron oxide, zinc oxide (ZnO).

• Characteristics

- Rarely cause allergic reactions
- Recommended for intense sun exposure and for people with sensitive skin. - Older formulations required thick application, melted in the sun, stained clothing, and were cosmetically unacceptable due to opacity.
- Newer micronized formulations are translucent, cosmetically acceptable, remain on the skin’s surface, and are not systemically absorbed.
- No reports of contact allergy to ZnO or TiO2.


• Microfine ZnO

-Absorbs more UV light in the long-wave UVA spectrum (340-380 nm). - Not photolabile.


• Microfine TiO2

-Effective against UVB (290-320 nm) and UVA2 (320-340 nm).
-Less effective than ZnO in the UVA1 range (>340 nm).
- Has a higher refractive index, making it whiter and harder to incorporate into transparent products.


• Concerns

-Metal oxides may produce oxygen free radicals when irradiated. - TiO2 is more photoactive and can damage DNA in vitro.
- Microfine particles are too large to enter the skin, minimizing biological activity.
- Companies often coat metal oxides with dimethicone or silicone to reduce photoreactivity.


2. Chemical Sunscreens:

• Function

-Absorb UV radiation, dissipating it as heat, light, or via chemical reactions.

• Characteristics

-Often combined with physical sunscreens or other chemical sunscreens for high-SPF products.
-Broadly labeled as UVB- or UVA-absorbing substances.
-Colorless and often odorless.
-Can cause allergic or photoallergic reactions.
-Some are unstable under UV radiation (e.g., avobenzone degrades under UV exposure).
-Some are systemically absorbed and detected in urine.
-Should not be used in children younger than 2 years of age.

• Concerns

-May create ROS or photoproducts that can harm the skin or other chemicals in the formulation.
-Systemic absorption poses a risk, especially for young children.

• Recommendations

-Physical sunscreens with micronized ZnO or TiO2 are recommended for sensitive skin types due to their safety profile and lack of systemic absorption.
-Chemical sunscreens are suitable for high-SPF needs but may pose risks of allergic reactions and instability under UV exposure.
-Use the highest SPF sunscreen tolerable and apply sufficient amounts to ensure effective protection.
-Check labels for UVA and UVB coverage to ensure comprehensive sun protection.