It's interesting that the small compact fluorescents (cfl) and the newer
4-foot tubes (T8) both contain about the same amount of mercury (about 4
mg). Initially one would think the cfl could have less mercury, but I
think discharge in the cfl tube has to be much more intense and requires
a higher mercury concentration.
It is interesting that the mercury content goes down as the lamp ages
because it combines with the glass, phosphors, and electrodes. They
used to put excess mercury into lamps so mercury depletion in the tube
would not be the cause of failure. Now they have reduced the mercury so
low that mercury loss can be the cause of failure. The idea is to make
mercury loss become an important mode of failure at about the same time
as other parts would fail. I believe the tube could still work with a
bit less mercury than currently used, but that would be self-defeating
if the mercury depleted quickly enough that people had to replace the
lamps more often. Although research continues, some believe mercury is
currently about as low as we can get in terms of light output and lamp
life.
How bad is 4 mg mercury? The EPA limit for mercury in drinking water is
2 ppb. If the mercury in one cfl would be distributed in one liter of
water, that would be 4 mg/kg = 4ppm. That means one cfl could
contaminate 2000 liters of water to the EPA limit.
The question is, if mercury containing lamps are landfilled, how much of
the mercury will get into the ground water. Some say that in a good
landfill, none should get into the groundwater. Or alternatively, if
cfl are recycled to try to recover the mercury, what percentage is
recovered versus how much becomes fugitive emissions to air (then water)
or to water directly. Considering the energy and cost to recover just 4
mg from a lamp, is it best to try to recycle it, or is it best to send
it to a good landfill?
It's also interesting to note that if you break a cfl or T8 in your
house and an appreciable amount of the 4 mg end up in cracks in your
floor, the mercury sniffers used to look for mercury contamination in
ambient air will easily detect it, and you will exceed ambient
air-quality standards in the vicinity of the spill. The zeal with which
some mercury-phobic people pursue this would require that you spend
hundreds of dollars to decontaminate your house. Those at the opposite
end are shocked at how bent-out-of-shape some people get over ppb of
mercury. Reality is somewhere between the two, and also depends on how
air-tight you home is.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu