Subject: Re: Is UV Ionizing Radiation?
From: rparson@spot.Colorado.EDU (Robert Parson)
Date: 14 Mar 1997 01:29:16 GMT
In article <01bc2e4c$d69ae980$139a6e82@kyle>,
Kyle Jones <firstname.lastname@example.org> wrote:
>Dennis Nelson <email@example.com> wrote in article
>> In various threads I have maintained that ultraviolet radiation is
>> ionizing radiation. Mr. Guthrie insists that it is not. Could someone
>> settle this dispute. If it is not, at what wavelength does ionizing
>> radiation begin? Thanks much for any help.
>Per the Health Physics and Radiological Health Handbook, ISBN
>"Ionizing Radiation - Any radiation capable of displacing electrons from
>atoms or molecules, thereby producing ions. Examples: alpha, beta, gamma,
>X-rays, neutrons and ultraviolet light..."
>Based on this definition, I'd have to say yes, UV can be ionizing
>radiation. However, the electromagnetic spectrum is plotted in the same
>reference showing a division between ionizing and non-ionizing radiation at
>a wavelength of about 3e-8 meters - right in the middle of the UVB range.
>Therefore, you're both right.
Hmm. The McGraw-Hill Encyclopedia's article explicitly defines UV as
_not_ ionizing radiation. Of course this all depends upon what
molecules you are trying to ionize.
In any event, I would argue that UV-B and UV-A, and probably most
of UV-C as well, should _not_ be called ionizing radiation because
the mechanisms by which they produce biological damage have nothing
to do with ionization. Instead they involve resonant absorption
by particular biological molecules and subsequent photochemistry.
For example, when DNA absorbs a UV-B or C photon, it forms an extra
chemical bond between two adjacent thymine or cytosine units on the
helix ("pyrimidine dimers"). This dimer screws up transcription:
when the DNA replicates a cytosine dimer matches with adenines instead
of guanines as it ought to. This writes a mutation into the new DNA.
The pyrimidine dimer itself can be repaired, by an enzyme called
photolyase, but the mutation that it produces is permanent.
If the mutation occurs within a gene that regulates cell division,
the cell becomes prone to malignancy. For example, squamous cell
carcinoma is caused by a UV-B induced mutation in the p53 gene.
There is a very readable article in the July 1996 _Scientific American_
about this. A more technical article is "Unraveling the molecular
pathway from sunlight to skin cancer", _Accounts of Chemical Research_,
_27_, 76, 1994.