In a record from 12,370 carbon-14 years before the present to the present time, Ref. 59 finds that there was a decreasing lead-206/lead-207 ratio in deposited lead starting 3000 carbon-14 years ago. This is an indicator of lead mining
a. If the pre-anthropogenic deposition rate is 0.01 milligram per square meter per year, andthe highest rate found was 15.7 milligrams per square meter per year in 1979, determine therelative effect of humans in 1979
b. How much more lead is being used in 1979 relative to 100, if the deposition in the year
100 were 0.12 milligrams per square meter per year?
a. The size of the effect must be proportional to the exposure. The relative effect is
equal to the relative exposure: 15.7 mg/0.01 mg = 1570 . Presumably, we modern humans
have a poisoning effect from lead about 1600 times as great as the effect on our ancestors
(these appears to be no lower threshold to the damage done; any lead exposure that can be
measured apparently causes damage).
b. In the year 100, humans were using lead at a rate far above natural release—by a factor of
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