2. How does light, infrared and UV radiation interact with skin and eyes?
The SCENIHR opinion states:
Interaction with skin and eyes depends on the wavelength of the radiation
3.3. Physical and biophysical background to light sensitivity
3.3.1. Physical background
The power (energy emitted per second) of a radiant source is expressed in watts (W), but light is expressed in lumens (lm) to account for the varying sensitivity of the eye to different wavelengths of light. The derived relevant units are the radiance (luminance) of a source in W/m2 (lm/m2) in a certain direction per steradian (unit of solid angle; all around is 4 π steradians), and the irradiance (illuminance) of a surface in W/m2 (lm/m2 or lux).
The human eye does not register the exact spectral composition of light, but perceives colour on the basis of three kinds of receptors with different spectral sensitivities. Due to the importance of the sun, as a broad spectrum light source, all technical sources can be characterised by their ‘Correlated Colour Temperature’ which corresponds to the surface temperature of ‘black body radiator’ (sun or star) which generates a similar colour sensation on the human observer. Typical incandescent lighting is 2700K which is yellowish-white. Halogen lighting is 3000K and daylight is around 5000K.
The Correlated Colour Temperature is an important characteristic for the impact of light on the human observer and on the way the human observer film or digital cameras captures images of objects and scenery. Obviously, through vision, this also affects the recognition and perception of external stimuli which leads to a wealth of effects in humans.
Electromagnetic radiation such as light can, through a number of processes, interact with matter where elastic processes (i.e. without loss of energy in movement) are of very limited effect on the atoms and molecules, whereas inelastic processes will transfer photon energy (“photon absorption”), which may excite electrons to higher energy levels in atoms and thus lead to secondary processes such as:
- Heat Formation ("dissipation")
- Fluorescence / Phosphorescence / Radical Formation / Light induced chemical reaction
- Ionisation (electron emission from an atom or molecule)
Absorption of electromagnetic radiation is typically related to warming of the tissue exposed which has mostly indirect consequences. However, radiation of shorter wavelengths, due to the higher characteristic photon energy, can excite electrons such that chemical processes are initiated which may have detrimental side effects. A well known mechanism is the detrimental effect of UV radiation on living cells.
Ionizing radiation consists of high-energy photons that can detach (ionize) at least one electron from an atom or molecule. Ionizing ability depends on the energy of individual photons, and not on their number. The ability of photons to ionize an atom or molecule varies across the electromagnetic spectrum. X-rays and gamma rays can ionize almost any molecule or atom; far ultraviolet light can ionize many atoms and molecules; near UV, visible light, IR, microwaves and radio waves are non-ionizing radiation.
Ionisation starts with wavelengths shorter than 200 nm and needs at least 6 eV, but more likely up to 33 eV (Hall and Giaccia 2006). An exception is the ionisation by (pulsed) lasers with high intensities (>1011 W/cm2; Robinson 1986). There are significant biological effects of ionisation where the most critical target is the DNA (strand breaks and chromosomal aberrations). Such DNA damage may lead to mutations and therefore cancer induction. Importantly however, ionisation is not generally produced by radiation in the visible/IR/UV range at wavelengths that are longer than 200 nm.
3.3.2. Light-tissue interactions
Like sunlight on water, UV, visible and IR radiation can be partially reflected from the outer surface of the skin and eyes, and as it penetrates the tissue it can be scattered in various directions (including backwards) from microscopic particles and structures such as fibers (e.g., present in the dermis of the skin). In the tissue, radiation may also be absorbed by various molecules. In comparison to UV and long-wavelength IR radiation, visible radiation is generally not strongly absorbed by the bulk tissue, but it is strongly absorbed by certain components like pigments and blood. The net result of backscattered and absorbed visible radiation determines skin color, the white of our eyes, and the multi-colored irises that we see (too little light re-emerges from the pupil, except on photographs taken with a strong flash light directed straight into the eyes). The long-wavelength IR radiation is not scattered but strongly absorbed by water – the main constituent of soft tissues – and this contributes to the heat sensation when the skin is exposed to sunlight. Ultraviolet radiation, especially with short wavelengths, is strongly absorbed by bulk tissue, i.e. by organic molecules like proteins, lipids and DNA. Most of the UV-B radiation is therefore absorbed in the outermost superficial layer (the epidermis of the skin). The absorbed energy from UV radiation is not only converted into ‘heat’ (i.e. thermal energy from increased movement of molecules), as is the case with IR radiation, but it can also drive photochemical reactions. In the eye, visible radiation is absorbed by special photo-pigments that trigger electrochemical stimuli to optical nerves, enabling us to see, but potentially also mediating adverse effects.
With a few exceptions (most notably the formation of pre-vitamin D3), most photochemical reactions caused by UV radiation in the skin and eyes are detrimental: proteins and DNA become damaged and dysfunctional, either by directly absorbing UV radiation or by being damaged through an intermediary step, such as reactive oxygen species generated from another UV-absorbing molecule. Hence, UV radiation can be considered harmful. Overly damaged cells will die and disassemble in a well-orchestrated manner (a process dubbed apoptosis). Large numbers of cells in apoptosis may cause notable defects that literally surface after a few days in a process we know as ‘peeling’. Fortunately, our skin is well adapted to UV-induced damage which also arises upon exposure to the sun. Cells react, alarm signals are produced (i.e. stress responses mediated through cascades of molecular reactions), and the damaged molecules and cells are repaired or replaced. The UV-induced damage and alarm signals can evoke an inflammatory reaction (attracting immune cells from the blood to the site of the toxic insult) as part of a normal sunburn reaction in the skin, or snow blindness (or welder’s flash) in the eyes (the redness is caused by widening of superficial blood vessels, and some swelling occurs because of a higher permeability of the vessel walls facilitating the trafficking of white blood cells). In some cases such sunburn reactions may already arise after extremely low UV exposures, revealing an enhanced UV toxicity. Alternatively abnormal allergy-like skin reactions may occur, indicating a pathologic immune response to UV exposures.
Source & ©:
3. Scientific Rationale, Section 3.3. Physical and biophysical background to light sensitivity,
Subsection 3.3.1. Physical background, p. 12 - 14