Ultraviolet (UV) light is a form of electromagnetic radiation that occupies the spectrum just beyond visible light, lying between X-rays and visible light. The UV spectrum is defined by wavelengths between 100 and 400 nanometers (nm) and is categorized into three primary bands based on energy level and penetration ability. UVA (315–400 nm) is the least energetic, allowing it to penetrate deep into materials. UVB (280–315 nm) is more biologically active in superficial layers. UVC (100–280 nm) is the most energetic and damaging, though it is entirely filtered by Earth’s atmosphere, allowing UV light to be harnessed for various applications.
Ultraviolet Light in Sanitation and Purification
The energetic nature of UVC radiation makes it effective for Ultraviolet Germicidal Irradiation (UVGI), a process widely used in sanitation and purification. This technology primarily employs UVC light, often generated by low-pressure mercury-vapor lamps or modern UV-LEDs. The mechanism involves the absorption of this high-energy light by the genetic material of microorganisms (DNA and RNA). This absorption leads to a photochemical reaction that causes adjacent molecular bases, particularly pyrimidines, to bond together, forming structures called pyrimidine dimers.
The formation of these dimers physically distorts the structure of the DNA or RNA molecule. This damage prevents the organism from accurately replicating its genetic code and carrying out essential cellular functions. The UVC light renders the microbe—including bacteria, viruses, and fungi—non-infectious and unable to reproduce. This achieves inactivation without the use of chemical agents, which is valuable in sensitive environments where disinfectants might be undesirable.
A primary application of this germicidal technology is in water treatment and purification systems. UVC lamps are integrated into municipal, industrial, and domestic water supplies to disinfect flowing water before consumption. The light eliminates pathogens, providing a safe alternative to chlorination in some contexts. This is particularly useful in industries like food processing, where water quality is paramount for products like juices and beverages. UVC is also a common method for treating wastewater before water is returned to the environment.
UVC light is also employed for air quality control, often built directly into a building’s heating, ventilation, and air conditioning (HVAC) systems. As air circulates through the ducts, the UVC radiation neutralizes airborne pathogens, preventing their distribution throughout the enclosed space. This continuous air disinfection is utilized in hospitals, laboratories, and commercial buildings to enhance occupant health. Furthermore, UVC prevents microbial buildup on cooling coils and drain pans within HVAC units, which helps maintain system efficiency and reduce maintenance costs.
Surface disinfection represents another significant use, particularly in healthcare and controlled laboratory settings. Mobile or fixed UVC units are used to sterilize surfaces in operating rooms, isolation wards, and biosafety cabinets when the areas are unoccupied. Since UVC light is highly damaging to human skin and eyes, its application is strictly limited to enclosed systems or spaces where human exposure can be prevented. Research into far-UVC wavelengths (200–235 nm) has shown promise, as these shorter wavelengths penetrate human tissue less effectively while maintaining germicidal potency.
Diagnostic and Authentication Applications
The uses of ultraviolet light extend beyond sanitation into visualization and verification, relying primarily on fluorescence. When certain substances absorb UV energy, they immediately re-emit that energy as visible light, causing them to glow. This process, often induced by UVA wavelengths (commonly known as blacklight), allows examiners to visualize materials that are otherwise hidden under normal lighting conditions.
In forensic science, UVA analysis is a non-destructive method for detecting evidence at a crime scene. Biological fluids such as semen, saliva, and urine naturally contain compounds that fluoresce when exposed to UV light, making traces visible. Investigators also utilize fluorescent powders that adhere to latent fingerprints, allowing the prints to become visible and photographable under UV illumination. This technique increases the rate of evidence recovery compared to simple visual inspection.
Authentication and security represent a major field for UV application, especially concerning currency and official documents. Modern banknotes, passports, and identification cards are manufactured with embedded security features, such as fluorescent threads, fibers, or inks. When exposed to UVA light, these features glow in predetermined patterns or colors, instantly verifying the document’s authenticity and helping to detect forgeries. This method is standard practice used by financial institutions, border control agencies, and law enforcement.
UV light plays a role in the conservation and inspection of artwork and historical artifacts. Restorers use UV light to detect areas of repair or forgery on paintings, as newer varnishes, pigments, or glues often fluoresce differently than the original, aged materials. This contrast provides a map of the artwork’s history and condition, guiding conservation efforts.
In industrial settings, fluorescent dyes are introduced into closed-loop systems to quickly locate leaks. The escaping fluid fluoresces brightly under UV light, pinpointing the breach for repair.
Therapeutic and Controlled Biological Uses
The interaction of UV light with biological tissue is not limited to destruction; controlled exposure to specific wavelengths is intentionally used to elicit positive physiological responses in medicine. The most familiar natural biological effect involves UVB radiation (280 to 315 nm). When UVB light penetrates the epidermis, it initiates a photochemical reaction where a cholesterol precursor, 7-dehydrocholesterol, is converted into pre-vitamin D3. This compound then converts into Vitamin D3, which is essential for calcium regulation, bone health, and immune system function.
In a clinical environment, this principle is leveraged through phototherapy, where UV light is carefully administered to treat various dermatological conditions. Narrowband UVB (NBUVB) therapy, typically using a wavelength around 313 nm, is a well-established treatment for chronic skin diseases like psoriasis, eczema, and vitiligo. The targeted exposure helps manage these conditions by slowing the excessive growth of skin cells and inducing apoptosis in the overactive immune cells responsible for inflammation.
Another therapeutic approach combines UVA light with a photosensitizing drug called psoralen, known as PUVA therapy. Psoralen is administered orally or topically before the treatment, making the skin more sensitive to the UVA radiation (315–400 nm). This combination allows the longer-wavelength UVA to penetrate into the deeper layers of the skin where the disease process occurs, achieving a therapeutic effect. These medical applications utilize specific, measured doses and wavelengths under supervision to maximize benefit and mitigate the dangers associated with uncontrolled solar UV radiation.