2010 marks the 40th celebration of Earth Day, a holiday that helped spark America's modern environmental movement when it was founded on April 22, 1970, by then-Sen. Gaylord Nelson of Wisconsin. In honor of this year's historic anniversary, MNN is taking a quick look back at the last four decades of planetary appreciation.
1970: Twenty million people celebrate the first Earth Day on April 22. A few months later, the U.S. Environmental Protection Agency opens its doors for the first time.
1976: The EPA starts phasing out PCBs, which can cause cancer and other health problems.
Russell McLendon is an associate editor at the Mother Nature Network.
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