Answer to Question #2111 Submitted to "Ask the Experts"

Q

On a Web site, I've found frequency specs claiming that UV light is also ionizing. Is this correct and does that mean that it can penetrate deep into a human body if overexposed (say on a tanning bed)?

A

Recall that ultraviolet (UV) radiation is defined as that portion of the electromagnetic spectrum between x rays and visible light, i.e., between 40 and 400 nm (30-3 eV). The UV spectrum is divided into Vacuum UV (40-190 nm), Far UV (190-220 nm), UVC (220-290 nm), UVB (290-320), and UVA (320-400 nm). Visible light is in the nonionizing portion of the electromagnetic spectrum; x rays are in the ionizing portion. UV is in between.

The term ionizing radiation refers to radiation with sufficient energy to produce ionizations in a medium. The energy threshold is not precisely defined as it depends on the composition and phase of the medium. One energy threshold that is sometimes quoted is 10 electron volts (eV), which would mean that UV radiation with wavelengths shorter than 124 nanometers (nm) (mid-Vacuum UV) is ionizing radiation. Another characterization would be to say that the higher-energy (shorter-wavelength) portion of the UV spectrum can be considered ionizing radiation and the lower-energy (longer-wavelength) portion, nonionizing. Intermediate UV energies may be considered ionizing or nonionizing depending on the specifics of the materials and endpoints being measured.

UV radiation does not penetrate deeply in the body. Nor for that matter do low-energy x rays, which are also ionizing radiations. The ability to penetrate deeply into the body does not begin until the energy of the x rays is much higher, on order 50,000 eV and above.

Gary Zeman, ScD, CHP
Lawrence Berkeley National Laboratory