UVA, UVB and UVC explained

The sun's ultraviolet light comes in three bands, named by wavelength. The shorter the wavelength, the more energy it carries and the more of it the atmosphere absorbs before it reaches the ground.

• UVA: 315 to 400 nanometres
• UVB: 280 to 315 nanometres
• UVC: 100 to 280 nanometres

UVA

UVA (315 to 400 nanometres) is long-wave ultraviolet, sometimes called soft UV. It is the band closest to visible light, and blacklights give out UVA.

The ozone layer absorbs little of it, so UVA reaches the ground in the largest amounts: about 95 per cent of the ultraviolet reaching the ground is UVA.

It doesn't carry the energy to burn quickly, as UVB does, but it reaches deeper into the skin, where it can cause long-term damage and premature skin ageing. Small doses, day after day, switch on the enzyme that breaks down collagen and wear away the skin's elastic tissue.

UVB

UVB (280 to 315 nanometres) is medium-wave ultraviolet. The ozone layer absorbs most of it: Health Canada puts what reaches the ground at about 5 per cent. At high altitude near the equator, 10 to 20 per cent can get through.

What gets through is the part of sunlight that burns the outer layers of the skin. Getting sunburnt increases the risk of skin cancer.

UVB causes most sunburn, which is why the UV Index, the figure most weather forecasts show, is built mainly around it.

UVB is strongest in the middle of the day. UV levels are higher nearer the equator, and rise by about 10 per cent for every 1,000 metres you climb.

UVC

UVC (100 to 280 nanometres) is short-wave ultraviolet and carries the most energy.

Its upper range, 200 to 280 nanometres, is known as germicidal UV: lamps that give it out are used to disinfect, because it damages the DNA and RNA of viruses and bacteria so they can't reproduce.

The atmosphere absorbs all of it, so none reaches the ground.

What Solarpunk shows

Solarpunk forecasts the two that reach you, hour by hour for where you are: UVB as the UV Index, and UVA on its own scale. Our FAQ answers common questions about both.

Sources

• World Health Organization (2016). Radiation: Ultraviolet (UV) radiation. Questions and answers, 9 March 2016. who.int
• World Health Organization, World Meteorological Organization, United Nations Environment Programme, International Commission on Non-Ionizing Radiation Protection (2002). Global Solar UV Index: A Practical Guide. WHO/SDE/OEH/02.2. who.int
• Health Canada. Health effects of ultraviolet radiation. canada.ca
• US Food and Drug Administration (2025). Executive Summary, General Issues Panel Meeting on Germicidal Ultraviolet (GUV) Devices, 10 December 2025. fda.gov