Specific Extinction Spectra Of Sunscreen Filters
It is not uncommon to read that a specific filter is a blocker of a specific type of UV radiation. Indeed, most sources which provide information on cosmetics have a system of symbols which represents how well different filters block different types of radiation.
For example, on websites and databases which function to explain the function of cosmetic ingredients, one often sees systems like this:
I understand why filters are presented like this. It is good shorthand. I also take real umbrage with it. In that it is shorthand, it is reductive. The reality is about one trillion times more complex. My issues can be summarized:
It presents UV radiation as a “Yes” or “No”for each category of wavelength. If a filter is said to filter UVB, one would be led to incorrectly assume that the filter offers a consistent level of UV extinction through the 30 individual wavelength of UVB. This is literally never the case with sunscreen ingredients. UV is a spectrum, and each wavelength of UV light delivers a unique amount of energy. The molecular composition of different sunscreen filters cause each filter to respond to different wavelengths of radiation differently.
It provides no context for how efficient the UV filter is for each “category” of UV light. This is quite an issue. Some filters are much more absorptive than others. As in, to get any real protection from Meridamate, one needs to use a much higher quantity of it than one does Avobenzone.
The best way to understand the actual absorbing capability of a sunscreen filter is by looking at its specific extinction, which I’ve attempted to explain in the context of sunscreens here.
I will summarize briefly: specific extinction refers to the sunscreen ingredient’s absorbance in accordance with a fixed set of parameters: how much of each wavelength is made extinct through a 1cm long 1% concentration of the solubilized filter. Looking at specific extinction spectra provide a facile way to compare the effectiveness of isolated sunscreen filters. One can compare “apples to apples”: how well one filter did versus another, in accordance with the same set of parameters. Looking at a filter’s specific extinction allows one to determine which wavelengths it filters effectively, and the rate of that efficacy per filter.
It is worth stating that looking at the specific extinction of a filter will not give an absolute indication of how well the filter will perform in an actual sunscreen product. In real life, there are a myriad of other factors which will impact the protection of a sunscreen beyond the absorptive capability of the filters. Below is an incomplete list of things which can impact the actual absorbance of a sunscreen formula
- The vehicle cream
- Synergistic or photodegratory interactions between sunscreen filters
- The concentration at which the sunscreen filter is included in a product
- The filter’s photostability, and the rate of its potential degradation given the ambient UV in one’s environment
- Whether or not the filters are encapsulated, and if so, the encapsulation technology
- The concentration of emollients / antioxidants / other ingredients which impact the performance of the sunscreen filters
However, if one wants to look at the actual performance of a sunscreen formula – not a specific active ingredient – one must measure absorbance. Like specific extinction, Absorbance measures how much of the UV light failed to get from the start of a system to the end, but it allows for testing of sunscreen at the indicated dosage form, which Specific Extinction, obviously, does not.
That said, let’s look at the filters.
Broad Spectrum Filters
Solid UVB & UVA II Filters
Liquid UVB & UVA II Filters
Please be adivsed I have left off several filters which I have literally never seen in sunscreen products. They include Dioxybenzone, Cinoxate, PABA, Trolamine Salicylate, several more. These don't really play a role in sun protecting products as sold to consumers.
Note: I originally listed the regulatory allowance of each filter in this post. I have since done a better job of listing these regulations in this post.
Extinction Spectra from:
Osterwalder, U., & Herzog, B. (2009). Chemistry and Properties of Organic and Inorganic UV Filters. In Editor H. W. Lim & Z. D. Draelos (Eds.), Clinical Guide to Sunscreens and Photoprotection (pp. 33). New York, NY: Informa Healthcare USA, Inc