Photoprotection Against Infrared Radiation
Finally, and as a separate subject, we will discuss the
question of protection against infrared (IR) radiation. IR
radiation (l760 nm-1 mm), which accounts for more than
half the solar energy that reaches the human skin, has
been divided into A, B, and C spectra. While IR-B and IR-C
radiation do not penetrate deeply into the skin, over 65%
of IR-A radiation reaches the dermis and 17% penetrates to
the subcutaneous tissue. Furthermore, it should be borne
in mind that one third of the radiation that reaches us from
the sun is in the IR-A spectrum.
Recent research has shown that exposure to IR radiation
or to heat induces angiogenesis and inflammatory cell
infiltration, disrupts the dermal extracellular matrix by
inducing matrix metalloproteinases, and alters dermal
structural proteins, thereby contributing to premature
aging of the skin.107 For this reason, it appears that
complete photoprotection should include protection against
IR radiation, and specifically against IR-A rays.
There are currently no specific physical or chemical
filters against IR radiation although some of the available
filters may provide such protection and this is currently
being investigated.
Antioxidants, especially those that target the
mitochondria, including epigallocatechin gallate and the
coenzyme Q derivative mitoquinone (MitoQ, Antipodean
Pharmaceuticals), may offer an approach to protection
against IR radiation. Topical application of these
antioxidants to human skin prior to exposure has been
shown to attenuate the detrimental effect on dermal gene
expression associated with IR radiation.107
In view of the above, some photoprotective products
have included protection against IR radiation according
to their labels. However, it is difficult to interpret such
assertions in the absence of any standardized method for
measuring the index of protection against these rays. Some
authors have proposed the minimal heating dose of the skin
as a measurement method,108 but this approach does not
appear very useful since IR-A, the most damaging portion of
IR radiation, unlike IR-B and IR-C, does not generate heat.