ultravioletB

Ultraviolet B (UVB) is a portion of the ultraviolet radiation that reaches Earth's surface, with wavelengths roughly 280 to 315 nanometers. It lies between UVA (315–400 nm) and UVC (100–280 nm) in the spectrum and is more energetic than UVA, though UVC is mostly filtered by the atmosphere. UVB accounts for a modest fraction of solar UV but is biologically significant.

In the atmosphere, ozone strongly absorbs UVB, but some reaches ground level. The amount of UVB arriving at the surface depends on solar zenith angle, latitude, season, altitude, and cloud cover, as well as ozone concentration and air pollution.

Biological effects: UVB can directly damage DNA by forming cyclobutane pyrimidine dimers, contributing to skin cancer and photoaging with repeated exposure. It also stimulates cutaneous synthesis of vitamin D3 from 7-dehydrocholesterol, a physiological benefit at moderate exposure. UVB exposure can cause sunburn and ocular irritation; the eye is particularly sensitive.

Sources and applications: The sun is the primary natural source of UVB. Artificial sources include UVB lamps used in medical phototherapy (narrowband UVB around 311–313 nm is common for psoriasis, eczema) and equipment for phototherapy and tanning. Tanning beds emit UV radiation, often with substantial UVB content, though modern devices vary.

Measurement and safety: UVB contributes to the UV index, a rating of daily potential for skin damage. Protection includes applying broad-spectrum sunscreen that blocks UVB, wearing protective clothing, and limiting exposure, particularly for individuals with fair skin or a history of skin cancer or photosensitivity.