UVA vs UVB vs UVC: Wavelength Guide for Each UV Band

Every physicist I know has had this conversation at a party: "UV light kills germs, right? So why doesn't the sun sterilize everything?" The answer is that "UV light" is not one thing. It spans wavelengths from 400nm down to 100nm, and the properties at each end of that range are as different as a flashlight and an X-ray machine. The three standard bands (UVA, UVB, UVC) behave so differently in terms of penetration, biological effects, and practical applications that treating them as interchangeable is like confusing AM radio with gamma rays.

The UV Spectrum: Where It Sits

Ultraviolet radiation occupies the electromagnetic spectrum between visible light (which ends at about 400nm on the violet side) and X-rays (which begin around 10nm). The International Commission on Illumination (CIE) divides the UV range into three bands based on biological effects and atmospheric transmission:

• UVA: 315-400nm (sometimes called "near UV" or "long-wave UV")
• UVB: 280-315nm (sometimes called "mid-wave UV")
• UVC: 200-280nm (sometimes called "short-wave UV" or "germicidal UV")

Below 200nm is the "vacuum UV" range, where air itself starts absorbing photons. That region exists but is not relevant to most terrestrial applications.

A useful way to think about it: as wavelength gets shorter, photon energy increases (E = hc/wavelength). UVA photons carry 3.1-3.9 eV. UVB carries 3.9-4.4 eV. UVC carries 4.4-6.2 eV. These energy differences drive the radical differences in how each band interacts with matter and biological tissue.

UVA (315-400nm): The Penetrator

UVA is the most abundant UV radiation at Earth's surface, accounting for roughly 95% of the UV energy that reaches ground level. The ozone layer is essentially transparent to UVA. It passes straight through.

Biological Effects

UVA penetrates deep into skin, reaching the dermis (the layer below the surface epidermis). This deep penetration is why UVA causes tanning: it reaches melanocytes, the pigment-producing cells located in the lower epidermis and upper dermis, and stimulates melanin production.

Chronic UVA exposure damages collagen and elastin fibers in the dermis, causing photoaging (wrinkles, loss of elasticity, leathery texture). The WHO classifies all UV radiation as a Group 1 carcinogen, and UVA contributes to skin cancer risk, though less directly than UVB. UVA's role in melanoma development has been increasingly recognized over the past two decades.

Acute UVA exposure at normal environmental levels does not cause sunburn. You can sit in UVA-only exposure for extended periods without erythema (reddening). This is why tanning beds, which emphasize UVA, were once marketed as "safe." They are not safe in terms of long-term cancer risk, but they do avoid the immediate sunburn response.

Applications

• UV curing: UVA-sensitive photoinitiators are used in industrial adhesive curing, dental composite hardening, nail gel curing, and 3D printing (SLA/DLP resin printers). The 365nm and 385nm wavelengths are the most common.
• Black lights: 365nm UVA causes fluorescence in many materials (certain dyes, minerals, body fluids, security features on currency). This makes UVA the basis for forensic inspection, counterfeit detection, and entertainment lighting.
• Insect attraction: Many flying insects are highly sensitive to near-UV. Bug zappers and insect traps use 365-395nm UVA sources.
• Phototherapy: UVA combined with psoralen (PUVA therapy) treats severe psoriasis and vitiligo. The psoralen drug sensitizes skin cells to UVA, enabling targeted therapeutic effects.
• Non-destructive testing: Fluorescent penetrant inspection in aerospace and manufacturing uses UVA to reveal surface cracks in metal parts.

LED Availability

UVA LEDs are the most mature and efficient of all UV LED types. Wall-plug efficiencies of 30-50% at 365nm are commercially available. This is because the AlGaN/GaN material system works well at these wavelengths, with manageable defect densities and good light extraction. UVA LEDs are cheap, efficient, and available from dozens of manufacturers.

UVB (280-315nm): The Sunburn Band

UVB is partially filtered by the ozone layer. About 5% of solar UV reaching the surface is UVB. That small percentage is responsible for most of the acute biological effects people associate with sun exposure.

Biological Effects

UVB is the primary cause of sunburn (erythema). It is absorbed strongly by the epidermis and causes direct DNA damage by forming pyrimidine dimers, covalent bonds between adjacent thymine bases in the DNA strand. The body's inflammatory response to this damage is what you experience as sunburn.

UVB is also the primary driver of skin cancer from sun exposure. The same DNA damage that causes sunburn can, if repair mechanisms fail, lead to mutations in tumor suppressor genes. Squamous cell carcinoma and basal cell carcinoma are strongly correlated with cumulative UVB exposure.

On the beneficial side, UVB triggers vitamin D synthesis in the skin. Specifically, UVB photons between 290-315nm convert 7-dehydrocholesterol in the epidermis to previtamin D3, which then thermally isomerizes to vitamin D3. About 10-15 minutes of midday sun exposure on arms and face produces roughly 10,000-20,000 IU of vitamin D in light-skinned individuals, though this varies enormously with latitude, season, skin pigmentation, and age.

Applications

• Phototherapy for skin conditions: Narrowband UVB at 311nm (NB-UVB) is the standard treatment for psoriasis, eczema, and vitiligo. It is considered safer than PUVA therapy because it does not require a photosensitizing drug. Clinical efficacy rates exceed 70% for plaque psoriasis.
• Vitamin D supplementation research: Some researchers are investigating controlled UVB exposure devices as alternatives to oral vitamin D supplements, though this remains experimental.
• Reptile husbandry: Reptile lamps with UVB output are essential for captive lizards, turtles, and other species that require UVB for vitamin D synthesis and calcium metabolism.

LED Availability

UVB LEDs are commercially available but less efficient than UVA LEDs. Typical wall-plug efficiencies for 310nm LEDs range from 1-5%. The AlGaN material system requires higher aluminum content at these shorter wavelengths, which increases lattice mismatch, defect density, and light extraction challenges. Prices are higher and output power lower than UVA, but the technology is improving steadily. Several companies now offer UVB LED modules suitable for phototherapy applications.

UVC (200-280nm): The Germicidal Band

No natural UVC radiation reaches Earth's surface. The ozone layer and atmospheric oxygen absorb it completely. Every UVC photon you encounter is artificial. This complete absence from the natural environment is actually why UVC is so effective at killing microorganisms: they have never evolved defenses against it.

Biological Effects

UVC causes severe acute biological damage at very low doses. Photokeratitis (a painful eye condition similar to "welder's flash") can occur after just seconds of direct UVC exposure. Skin erythema from UVC develops faster and at lower doses than from UVB.

The mechanism is the same as UVB (DNA damage via pyrimidine dimer formation) but more efficient because UVC wavelengths align more closely with the peak absorption of DNA, which occurs near 260nm. At 254nm (the emission wavelength of standard mercury germicidal lamps), DNA absorbs UVC roughly 10 times more efficiently than it absorbs 300nm UVB.

This extreme DNA absorption is precisely what makes UVC germicidal. Bacteria and viruses exposed to adequate UVC doses suffer enough DNA/RNA damage that they cannot replicate. A dose of 3-10 mJ/cm2 at 254nm achieves 99-99.9% inactivation of most vegetative bacteria and enveloped viruses.

The Far-UVC Subset

Within UVC, the 200-230nm range (called "far-UVC") has attracted enormous research interest because of its unique safety profile. Far-UVC at 222nm cannot penetrate the dead cell layer of human skin or the tear layer of the eye, making it potentially safe for continuous use in occupied spaces. This sub-band represents one of the most active areas of photonics research and commercial development today.

Applications

• Water disinfection: Municipal water treatment, point-of-use purifiers, and industrial process water. UVC reactors are replacing chlorination in many applications because they leave no chemical residuals. The transition from mercury lamps to UV LEDs is accelerating in this space.
• Air disinfection: Upper-room UVGI fixtures, in-duct HVAC systems, and standalone air purifiers. Hospitals have used upper-room UVC since the 1930s for tuberculosis control.
• Surface disinfection: Conveyor-based UVC systems sterilize packaging, medical instruments, and food contact surfaces. Pulsed xenon UVC systems are used for terminal cleaning in hospital rooms.
• Semiconductor manufacturing: Deep UV light at 193nm (from ArF excimer lasers) is the workhorse of advanced photolithography.

LED Availability

UVC LEDs remain the most challenging UV LED type to manufacture. Wall-plug efficiencies at 265nm are typically 2-5%, though recent breakthroughs are pushing this higher. The deep aluminum content required in AlGaN for these wavelengths creates severe challenges: high defect densities, poor p-type doping, and difficult light extraction from the high-refractive-index semiconductor.

Despite these challenges, UVC LED performance is improving rapidly. Commercial 265nm LEDs with output power exceeding 100mW per chip are available from multiple suppliers. Prices have fallen roughly 50% over the past three years. The medical and disinfection markets are driving strong demand for continued improvement.

Interaction with Common Materials

Each UV band interacts differently with everyday materials, which determines both hazard levels and engineering design requirements.

Glass: Standard soda-lime glass transmits UVA but blocks most UVB and essentially all UVC. This is why you do not get sunburned through a closed car window (though you can accumulate UVA damage on long drives). Quartz glass transmits all three bands down to about 180nm, which is why UV equipment uses quartz optics.

Plastics: Most plastics are damaged by UV exposure, with shorter wavelengths causing faster degradation. UVA causes slow yellowing and embrittlement over months to years. UVB accelerates this process. UVC can rapidly degrade polymer surfaces within hours of direct exposure. UV stabilizers added to outdoor plastics primarily target UVA and UVB absorption.

Water: Pure water is transparent to UVA and UVB but absorbs UVC weakly below 240nm. This absorption is why UVC water purification works: the UV-C penetrates the water column sufficiently (several centimeters at 254nm) to reach pathogens, while natural absorption limits the required reactor size.

Sunscreen: Sunscreens are rated by SPF (primarily UVB protection) and broad-spectrum claims (UVA protection). SPF 30 blocks about 97% of UVB. Broad-spectrum formulations add UVA-absorbing chemicals (avobenzone, zinc oxide, titanium dioxide). No commercial sunscreen addresses UVC because there is no natural UVC to block.

Safety Standards and Exposure Limits

The ACGIH (American Conference of Governmental Industrial Hygienists) publishes Threshold Limit Values (TLVs) for UV exposure that vary dramatically by wavelength:

• UVA (315-400nm): 10,000 mJ/cm2 per 8-hour day (relatively permissive)
• UVB (297nm, near peak erythemal): 3.7 mJ/cm2 per 8-hour day (very restrictive)
• UVC (254nm): 6.0 mJ/cm2 per 8-hour day
• Far-UVC (222nm): 161 mJ/cm2 per 8-hour day (comparatively permissive due to low penetration)

The thousand-fold difference between UVA and UVB limits reflects the dramatic difference in biological impact. A UVA source must be orders of magnitude more intense than a UVB source before it poses equivalent risk. This is important context when evaluating UV product safety: a "UV" device emitting at 365nm (UVA) is fundamentally less hazardous than one emitting at 280nm (UVB/UVC boundary) at the same power level.

The LED Revolution Across UV Bands

The shift from traditional UV sources (mercury lamps, deuterium lamps, excimer lasers) to LED-based sources is happening at different speeds across the three bands.

UVA LEDs are already dominant in many applications. UV curing has largely transitioned from mercury arc lamps to 365-395nm LED arrays. The advantages (instant on/off, no mercury, cooler operation, longer life) outweigh the remaining cost premium in most industrial settings.

UVB LEDs are reaching a tipping point for phototherapy. Clinical studies have validated 311nm LED devices for psoriasis treatment, and several medical-grade UVB LED panels are now commercially available. The LED advantage of precise wavelength control (narrowband output without filtering) is particularly valuable in phototherapy, where dosimetry accuracy directly affects treatment outcomes.

UVC LEDs are in active commercial deployment for water and air disinfection, but mercury germicidal lamps still dominate on cost-per-watt and total UV output. The crossover point, where UVC LEDs match mercury lamps on total cost of ownership, is approaching. Estimates range from 2026-2028 depending on the application and duty cycle. Mercury phase-out regulations (the Minamata Convention) are accelerating this transition by restricting new mercury lamp production.

Choosing the Right UV Band for Your Application

The decision tree is usually straightforward once you know what each band does:

• Need to cure adhesives, inks, or coatings? UVA (365-395nm). Match the photoinitiator's absorption peak.
• Need to detect fluorescence? UVA (365nm). Most fluorescent materials respond to this range.
• Need to treat psoriasis or vitiligo? Narrowband UVB (311nm). Established clinical protocol.
• Need to kill bacteria, viruses, or mold? UVC (254-265nm). Peak germicidal wavelength.
• Need germicidal action in occupied spaces? Far-UVC (222nm). Safe for human exposure within limits.
• Need to pattern semiconductor features? Deep UVC or EUV (193nm and below). Photolithography territory.

The wrong UV band for a given application is not just suboptimal: it usually does not work at all. UVA has negligible germicidal effect. UVB does not cure most photoinitiators. UVC does not penetrate deep enough into skin for phototherapy. Wavelength specificity is not a preference. It is a physical requirement.