Researchers from the University of Illinois, Chicago have discovered thousands of tons of the chemical polyhalogenated carbazoles (PHCZs) in the sediments of three of the Great Lakes.

Their findings, published in the journal Environmental Science and Technology, showed that approximately 3,000 tons of the dioxins-like chemical were lying in the sediments under Lake Michigan, Superior and Huron.

"Because the amount of PHCZs we found is so high, and because of their location in the lakes as well as in the sediment cores we took, we believe that most of the PHCZs in these lakes is the result of natural processes," said An Li, professor of environmental and occupational health sciences in the UIC School of Public Health and corresponding author of the study, in press release. "However, we see some PHCZs in sediment laid down in more recent years, which is very likely from man-made sources. These are considered chemicals of emerging concern and should be monitored carefully."

For the study, the researchers collected soil samples from the bottom of the lakes and core samples from beneath it. Using gas chromatography, in combination with mass spectrometry, the researchers tested the samples from the three Great Lakes for 26 different PHCZs.

The results of their tests estimate the total amount of PHCZs deposited in the Great Lakes to be 3,000 tons. The researchers classified the PHCZs by their location. PCHZs found in sediments in the deepest parts of the lake, away from the shore, were classified as Category 1. On the other hand, PHCZs found in the shallower parts of the lake were classified as Category 2.

Based on the location of the PHCZs, the researchers believe that Category 1 PHCZs were deposited before 1900, as a result of the normal breakdown of vegetation that settled in the deepest part of the lakes. Meanwhile, Category 2 PHCZs are believed to be deposited just recently and were most likely caused by human activities, such as manufacturing of organic semiconductors, dye and pharmaceuticals.

Being likened to the highly toxic substance dioxins, PHCZs are considered to be an emerging health threat. Previous studies showed that PHCZs could disrupt embryonic development of zebrafish. Additionally, PHCZs were also observed to exhibit dioxin-like activity in breast cancer cells.