What is ultraviolet light?

Ultraviolet (UV) radiation is a form of energy emitted by both natural sources – primarily the sun – and artificial ones. From a scientific perspective, it is a form of non-ionising radiation located on the electromagnetic spectrum between X-rays and visible light. One of its main characteristics is its wavelength, which ranges from approximately 100 nm to 400 nm.

One of the most frequently asked questions is whether ultraviolet light can be seen. The short answer is no: the human eye is incapable of perceiving these wavelengths, making it an invisible form of radiation to us. Nor can it be physically felt upon direct contact.

UVA, UVB, and UVC: types of ultraviolet radiation

To understand what ultraviolet light is used for and what its risks are, it is essential to classify it into three main groups based on its wavelength:

• UVA rays (315–400 nm): these are long-wave rays. They account for approximately 95% of the UV radiation that reaches the Earth's surface. They can penetrate deeply into the skin, reaching the dermis (the middle layer), and remain constant throughout the year.
• UVB rays (280–315 nm): these have a medium wavelength and higher energy than UVA rays. Unlike UVA, these rays primarily affect the epidermis, or outer layer of the skin.
• UVC rays (100–280 nm): these are short-wave rays and the most energetic in the UV spectrum. They are extremely dangerous to living organisms, but fortunately, the Earth's atmosphere filters them out completely.

Ultraviolet light and the sun: which rays reach the Earth?

The sun is the most powerful natural source of ultraviolet radiation in our environment. However, not all of the solar spectrum reaches us. The atmosphere acts as an essential protective shield, where ozone, water vapour, oxygen, and carbon dioxide absorb a large part of the harmful radiation.

Specifically, UVC rays do not reach the Earth naturally because they are entirely absorbed by the ozone layer.

As for UVB rays, the atmosphere absorbs approximately 90%, letting through only a small fraction, which is responsible for tanning and sunburn.

Conversely, UVA rays are barely affected by the atmosphere and reach us in massive quantities.

Environmental factors can alter the amount of radiation we receive:

• Altitude: the higher the altitude, the less atmosphere there is to absorb rays.
• Latitude: rays are stronger near the equator.
• Surface reflection: surfaces like snow can reflect up to 80% of radiation.

Black light: what it is and why it makes some materials glow

People often wonder whether black light is the same as ultraviolet light. In reality, a black light is a specific type of lamp that primarily emits long-wave UVA rays.

It is also technically known as a Wood's lamp, and its operation relies on a special dark glass that filters out most visible light, allowing only ultraviolet radiation and a small fraction of violet light to pass through.

The fluorescence effect occurs when UVA radiation excites the electrons of certain materials (phosphors), which, upon returning to their original state, emit energy in the form of visible light.

Everyday uses of ultraviolet light

The applications of ultraviolet light have been integrated into our daily lives in ways that sometimes go unnoticed:

• Counterfeit banknote detection: many banknotes incorporate fluorescent marks invisible to the human eye that are only revealed under a black light lamp.
• Forensic investigation: at crime scenes, it is used to locate biological fluids or fingerprints that are not visible under natural light.
• Art and entertainment: it is very common in nightclubs, museums, and shows to create striking visual effects by highlighting neon colours or white materials.
• Home hygiene: although their use is more limited, there are household UV torches that help detect stains and dirt.
• Artificial tanning: sunbeds use lamps that emit both UVA and UVB rays to simulate the effect of the sun, although health authorities warn that this significantly increases the risk of skin cancer.

Uses of ultraviolet light in medicine and health

Phototherapy is the medical application of UV radiation. It must always be carried out by healthcare professionals, as strict dosage control is required to ensure the benefits outweigh the risks.

• Vitamin D production: moderate exposure to UVB rays is essential for the skin to synthesise vitamin D3. This vitamin helps absorb calcium, making it essential for strengthening the skeletal and muscular systems.
• Treatment of skin conditions: UVA and UVB rays are used to treat conditions such as psoriasis, eczema, vitiligo, and certain types of skin tumours.
• Jaundice in newborns: UVA radiation has a therapeutic function in breaking down bilirubin in babies who have difficulty eliminating it on their own.
• Rickets: it has also been used historically to treat rickets due to its connection with vitamin D.

Ultraviolet light for disinfection: when it works and when it doesn't

Another application of ultraviolet light is disinfection. This relies primarily on the properties of UVC rays, which possess extremely high energy capable of destroying the genetic material of viruses and bacteria.

This method is highly effective for eliminating microorganisms that are resistant to traditional chemical disinfectants such as chlorine.

However, its effectiveness depends on direct exposure; that is, if the microorganism is in a shadowed area or under a layer of dirt, the light will not be able to reach it. Furthermore, it must be borne in mind that UVC rays can be very harmful to us if the skin or eyes are directly exposed. For this reason, they are not usually recommended for domestic use.

Ultraviolet light in water, industry, and research

In the industrial and research sectors, UV radiation is an indispensable technological tool:

• Water treatment: ultraviolet light is used in water treatment plants to safely sterilise drinking water without adding extra chemicals.
• Food industry: it is used in food irradiation to extend shelf life and eliminate pathogens.
• Climate control systems: some industrial facilities incorporate UV lamps to keep ductwork free from mould and bacteria.