Polychlorinated biphenyls (PCBs), industrial pollutants, are known to be carcinogens and a great danger to natural ecosystems. As a result of several studies, PCB production was banned in the United States in 1979 and by the Stockholm Convention on Persistent Organic Pollutants in 2001. One study, published in 1972 by R. W. Risebrough, is titled ''Effects of Environmental Pollutants Upon Animals Other Than Man'' (Proceedings of the 6th Berkeley Symposium on Mathematics and Statistics, VI, University of California Press, pp. 443-463). In that study, 60 Anacapa pelican eggs were collected and measured for their shell thickness, in millimeters (mm), and concentration of PCBs, in parts per million (ppm). Following is a relative-frequency histogram of the PCB concentration data.

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0.30- 0.25 D0.20 a 0.15 0.10 F 0.05 30 70 110 150 190 230 270 310 350 390 430 PCB concentration (ppm)