New 2016 — Public Proof About Zinc
Apart from simple allergy to a sunscreen chemical absorbed into the skin, concerns with chemical sunscreens mainly pertain to their potentially generating free radical damage in the skin when exposed to light.
Unlike physical sunscreens which reflect light, chemical sunscreens absorb light and convert it into heat or a safe form of light.
Failing this, the absorbed radiation is used in a chemical reaction within the skin resulting in generation of free radicals.
Studies have shown that in many instances sunscreens are formulated to avoid this occurrence — the radiation absorbed is re-emitted, however at a longer wavelength.
In any event, the net effect of chemical sunscreen on skin under normal exposure is overwhelmingly positive.
Rosacea symptoms lessen, sun damage and aging signs are partially prevented and reversed (under prolonged use) and skin cancer protected against.
The instability of chemical sunscreens under UV exposure has also been called into question as possibly harmful to skin.
Unlike physical sunscreens which, although prone to wearing off the surface of the skin, don't degrade in the presence of UV, chemical sunscreens are ultimately exhausted by exposure.
In the process, they have sometimes been shown to form by-products which may be harmful to skin.
Although it is true that certain individual sunscreen chemicals break down rapidly in UV, combinations of chemicals, particularly if combined with antioxidants and photostabilizers, are resistant to this phenomena.
Regulatory bodies such as the US FDA and Australian TGA regulate which combinations of sunscreens may be used together, and within which ranges of concentration, to prevent this problem.
In some instances, the precise combinations of sunscreens and other formulation technologies are patented.
Trademarked examples include Neutrogena's Helioplex and La Roche-Posay's Cell Ox-Shield, and the combination of Tinosorb with avobenzone.
For example, while avobenzone is the major chemical UVA chemical block, it is highly labile to degradation by light; however combined with Tinosorb, it becomes highly stable.
Combinations of sunscreen chemicals therefore protect both the skin and the formula from rapid breakdown.
Attend SPF 50+ is an excellent example, containing 8 sunscreen agents (I believe this is the maximum ever used).
Newer sunscreen chemicals, such as Mexoryl (Ecamsule), have been specifically designed to be highly photostable.
Skinceuticals Active UV Defense SPF 15 is an example of a sunscreen providing new-generation, highly photostable UVA filtering:
For these reasons, I do not believe that photostability of modern chemical sunscreens from reputable companies should be a concern for rosacea patients.
Irritation May Not Be Due to Sunscreen Agent
In case of skin irritation from a sunscreen, it is easy to jump to the conclusion that a certain sunscreen chemical or chemicals caused the skin's negative reaction.
In reality, likely alternate causes of irritation include any of the product's other ingredients, particularly the preservatives the product contains (for example parabens such as methylparaben and propylparaben and various alcohols) and underlying unhealthy skin (this is pertinent, irritation may be due to your skin's poor health). Skin in poor health may not be able to tolerate sunscreen until recovered.
Choosing a sunscreen free of preservatives, or one preserved by virtue its packaging or high antioxidant content, as is the case with certain specialty pharmacy and rosacea products (again I like the Attend SPF 50+ sunscreen for this reason), in preference to a mass-produced product with a longer shelf-life, typically resolves chemical sunscreen irritation in rosacea patients.
Sunscreen Chemical Encapsulation
Sunscreen chemicals need to be present in skin but not necessarily absorbed into the entire skin in order to be effective.
Just as manufacturers of modern titanium and zinc oxide products coat their microfine particles with forms of silicone to prevent their reaction with skin, several manufacturers of chemical sunscreens are encapsulating their sunscreening agents with inert materials such as silica.
As an added benefit, encapsulation and separation of certain sunscreen agents within a single formula has been shown to increase both the stability and protection of the sunscreen.
Author: Jeremy Cleckley