The widespread presence of toxic substances found in the Great Lakes in the
1960s and 1970s was primarily the result of the increased commercial production
and widespread use of organic chemicals and metals that started during
World War II and accelerated afterward. Environmental records assembled
from radio-chemically dated sediment cores from Lake Ontario revealed the
presence of several organic chemicals starting as early as 1915, but sharply
increasing in the late 1940s and reaching peak levels in the early 1960s.21 The IJC
has verified that some 362 chemicals were present in the Great Lakes. One third
of these may have toxic effects.