Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

Table AX7-2.2.4. Summary Statistics of Ambient and Natural Levels of Lead
in Whole Organism and Liver Tissues
AX7-136
Tissue Pb (µg/g dry weight)
Whole Organism Liver
Statistic Natural Ambient Natural Ambient
% Censorship 50.54 39.16 89.16 71.20
N 93 332 83 559
Minimum 0.08 0.08 0.01 0.01
Maximum 22.60 22.60 3.37 12.69
Mean 0.95 1.03 0.28 0.36
Standard Deviation 2.53 1.74 0.54 0.96
Median 0.35 0.59 0.11 0.15
90th percentile 1.40 2.27 0.37 0.59
95th percentile 2.50 3.24 1.26 1.06
observed for each endpoint (Table AX7-2.2.5). For example, NCBP and NAWQA geometric
mean Pb concentrations were nearly identical (0.55 and 0.54 µg/g dw, respectively) and the
85th percentiles only differed by 0.5 µg Pb/g dw (NCBP, 1.10 and NAWQA, 1.60). The authors
acknowledge that a high degree of censorship is present in both of these datasets and no firm
conclusions can be drawn by comparing these means. The objective of this exercise was limited
to showing how the NAWQA data compare to other national datasets.
As was the case with surface water data, the high amount of non-detectable measurements
did not allow for a national assessment of spatial trends in Pb tissue concentrations. Instead,
areas with high Pb tissue concentrations were identified by classifying the data above and below
the 95th percentile. Similar to surface water and sediments, tissue concentrations were found to
be elevated in Washington, Idaho, Utah, Colorado, Arkansas, and Missouri; however, several of
the highest measured Pb concentrations were also found in study units in the southwestern and