The sun: this friend, this enemy ...
The sun: benefits and dangers!
This motto was not chosen at random. Indeed, even if the sun is essential for life on Earth, it can be extremely dangerous for health (solar erythema, allergies, cancer…).
In order to benefit from the sun, it is therefore essential to know the risks associated with solar radiation depending on the exposure, but also the defense mechanisms of our body.
If the sun is essential to life on Earth, it is also a veritable thermo-nuclear power station which produces immense electromagnetic radiation. These radiations travel through space at a speed of 300,000 km / s (speed of light) and reach the earth at different wavelengths. We distinguish the different types of radiation by their wavelength: this is the solar spectrum.
Fortunately, most of the radiation is absorbed as it passes through the atmosphere. Only reach us:
Infrared (IR) : from 800 to 1400 nm, invisible, they heat the solid or gaseous matter they encounter.
Visible light : this radiation between 400 and 800 nm allows us to distinguish shapes and colors.
Ultraviolet A and B (UV) : wavelengths between 280 nm and 400 nm, they cause damage to cells without causing a feeling of overheating, which makes them dangerous.
There are different kinds of ultraviolet that can be distinguished according to their wavelengths.
Fortunately, UVCs, the most energetic, are mostly absorbed when they enter the atmosphere.
The most aggressive UV rays that reach us are UVB rays. They are the ones that cause solar erythema (sunburn).
UVA rays, although less energetic, produce skin changes more deeply than UVB rays. They are thus responsible for solar elastosis and actinic aging.
Prolonged or repeated exposure (cumulative) to both UVA and UVB rays can cause skin cancer.
The solar radiation to which we are exposed is actually the result of
Direct radiation which is all the richer in UVB as its path is short. The intensity and quality of this radiation are therefore a function of:
- the position of the sun (which varies with the hour, the season,
- altitude: the amount of UVB increasing by 4% in steps of 300
The radiation diffused by the sky which depends on the nature of the clouds
The radiation reflected by the ground (85% of the radiation arriving on
snow is reflected, against 17% on sand and 3% on grass).
We understand why the use of a parasol on the beach does not constitute
effective protection (reflection of sand). The possibility of a
erythema without direct sun exposure should therefore not be forgotten.