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RE: UV effects

  Roger Miller wrote in APD #987:

  > ... UV is divided more-or-less arbitrarily into long wave UVA and
  > short wave UVB.  If I remember right (not a good bet) UVA is about
  > 340 nm to 400 nm and UVB is about 280 nm to 340 nm.

Some books, particularly older ones, do talk only of UV-A and UV-B.
However, the accepted subdivisions are: UV-A 400-315nm, UV-B 315-280nm,
UV-C 280-200 or 180nm, 'vacuum UV' below 180nm or so (air itself is
absorbing).


  > UVA is used to make dayglow posters light up, and not much else.
  > It has very little biological effectiveness.

UV-A has important biological functions. It's essential to some animals,
including humans, to make vitamin D, and lack of it causes deficiency
states (rickets, osteomalacia), unless prevented by dietary supplements.
It causes tanning of the skin and hence helps to protect from UV-B
damage. The activity spectrum for the chlorophyll/carotenoids/flavanoid
complex in plants has (one of its) peaks in the blue, extending into
the near UV-A, so it contributes to photosynthesis. In many plants,
blue light (again extending into the near UV-A) has other specific,
nonphotosynthetic functions, such as stimulating phototropism, and
controlling the ratio of staminate to pistillate flowers. Many insects
can see into the near UV, which is then used by them for navigation,
for recognising particular flowers (eg, bees), and for recognising
species-specific patterns (eg, butterflies which mimic toxic species in
visible light to fool vertebrate predators, but have different patterns
under UV-A to allow recognition of conspecifics). In vertebrates,
substances other than vitamin D precursors can undergo UV-A initiated
chemical reactions, producing chemicals which may be involved in
circadian rhythm entrainment. These are only some functions I've heard
about - there are bound to be more.


  > UVB is biologically effective - it kills things.

UV-B does have deleterious effects - for example, it is thought to be
the main cancer-inducing band in sunlight. However, it's largely UV-C
that "kills things", due to the region at around 260 nm where nucleic
acids have a strong absorption line which results in broken bonds in
the primary structure of DNA. Fortuitously, low pressure Hg discharges
can be made to put out most of their energy at the 253.7nm line, which
is close enough to the DNA absorption peak to be very damaging.
"Germicidal" lamps therefore use UV-C, not UV-B.

Ross Drewe