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In June 2002, the U. S. Geological Survey published a report “Glacial Ice Cores Reveal A Record of Natural and Anthropogenic Mercury Deposition for the Last 270 Years” showing measured mercury concentrations found in ice cores in Wyoming from the period 1700 to 1998. Find this report here: http://toxics.usgs.gov/pubs/FS-051-02/
The figure that follows shows their data which is displayed as natural and human-caused (anthropogenic) mercury concentrations. These concentrations are given in nanograms (billionth) per liter. Assuming 1000 grams per liter, these concentrations are also given in parts per trillion.
Examining the graph shows natural sources of mercury are about 4 parts per trillion with add on concentrations due to volcanic eruptions–Mt. Tambora 1815, Krakatoa in 1883, and Mt. St, Helen’s in 1980.
Anthropogenic contributions show a rise from 6 parts per trillion in 1945 to a peak of 23 parts per trillion in 1985 and then a decline to 15 parts per trillion in 1998. The little increase shown on the graph from about 1992 to 1996 may be due to the eruption of Mt. Pinatubo in the Philippines in June 1991. This was a massive eruption that caused global cooling of about one-half degree for several years over the planet. The mercury emissions would have been vastly greater than Mt. St. Helen’s and it may be the anthropogenic contribution in 1998 may be 10 parts per trillion or less.
As the graph shows, anthropogenic mercury contributions have been falling since 1985 and may be following even more from 1998 to 2011. These reductions are due to less use of mercury in activities such as temperature measurements and thermostats, removal of mercury from effluents from power plants, more care in incinerating human remains(mercury in teeth), more care in disposal of fluorescent light bulbs, etc. Keep in mind these concentrations are a few parts per trillion; which is extremely small and essentially irrelevant.
More measurements should be made of current mercury concentrations before mandating laws to remove more mercury from coal-and oil-fired power plants with no measurable effect.
We see anthropogenic mercury concentrations are in a few parts per trillion. Let’s examine guidelines for mercury concentration permissible in the workplace and for children.
The Occupational Safety and Health Administration (OSHA) occupational exposure limit( 8 hr, 5-day week) is 100 micrograms per cubic meter. The National Institute for Occupational Safety and Health (NIOSH) recommended safety limit is 50 micrograms per cubic meter and the American Conference of Government and Industrial Hygienists (ACGIH) recommends 25 micrograms per cubic meter for the same conditions as OSHA.
Due to children having a risk of greater damage from exposure to mercury, the Agency for Toxic Substance and Disease Registry (ATSDR) recommends a maximum level of 0.2 micrograms per cubic meter for exposure of children on a continual basis.
To put these exposures on a basis for comparison to known concentrations, cubic meters of air must be converted to grams. A cubic meter of air has a mass of 1280 grams. Thus 1 micrograms per cubic meter is a concentration of 780 parts per trillion by mass.
The allowable exposures by the various agencies in parts per trillion are as follows:
OSHA 78,000 parts per trillion
NIOSH 39,000 parts per trillion
AGOG 19,000 parts per trillion
ATS DR 156 parts per trillion
These allowable exposure limits are way above the anthropogenic mercury concentrations shown by the Wyoming ice core data of 5 to 10 parts per trillion.
It is quite apparent the Environmental Protection Agencies requirement to reduce mercury emissions from coal- and oil-fired power plants through its Maximum Achievable Control Technology (MACT) rules will show little effect at a great cost to the utilities in plant modifications.