Earth Day began in 1970, when 20 million people across the United States—that's one in ten—rallied for increased protection of the environment.
"It was really an eye-opening experience for me," Gina McCarthy, the U.S. Environmental Protection Agency administrator, who was a self-described self-centered teenager during the first Earth Day rallies, told National Geographic. (See pictures: "The First Earth Day—Bell-Bottoms and Gas Masks.")
"Not only were people trying to influence decisions on the Vietnam War," she recalled, "but they were beginning to really focus attention on issues like air pollution, the contamination they were seeing in the land, and the need for federal action."
At the time, she said, the environment was in visible ruins—factories legally spewed black clouds of pollutants into the air and dumped toxic waste into streams. (Learn more about air pollution.)
"I can remember the picture of the Cuyahoga River being on fire," she said, referring to the Ohio waterway choked with debris, oil, sludge, industrial wastes, and sewage that spectacularly erupted in flames on June 22, 1969, and caught the nation's attention.
Although members of the public were increasingly incensed at the lack of legal and regulatory mechanisms to thwart environmental pollution, green issues were absent from the U.S. political agenda.
First Earth Day "Took Off Like Gangbusters"
The environment's low profile frustrated U.S. Senator Gaylord Nelson of Wisconsin, whose campaigns to protect it during the 1960s had fallen flat.
In 1969 Nelson hit on the idea of an environmental protest modeled after anti-Vietnam War teach-ins.
"It took off like gangbusters. Telegrams, letters, and telephone inquiries poured in from all across the country," Nelson recounted in an essay shortly before he died in July 2005 at 89. "The American people finally had a forum to express its concern about what was happening to the land, rivers, lakes, and air—and they did so with spectacular exuberance." (Related: "Earth Day Pictures: 20 Stunning Shots of Earth From Space.")
Nelson recruited activist Denis Hayes to organize the April 22, 1970, teach-in, which today is sometimes credited with launching the modern environmental movement.
By the end of 1970, the U.S. Environmental Protection Agency had been established, and efforts to improve air and water quality were gaining political traction
"It was truly amazing what happened," Kathleen Rogers, president of the Washington, D.C.-based Earth Day Network, told National Geographic News in 2009. "Blocks just tumbled."
Since the first Earth Day, environmentalism has moved from a fringe issue to a mainstream concern, Amy Cassara told National Geographic News in 2010, when she was a senior associate at the World Resources Institute in Washington, D.C.
"As many as 80 percent of Americans describe themselves as environmentalists," Cassara said.
Environmental problems today, however, are less immediate than dirty air, toxic water, and a hole in the ozone layer, she said. For example, the effects of global climate change are largely abstract and difficult to explain "without coming off as a doomsday prognosticator." (See pictures of Earth Day stunts.)
"As we become more industrialized and our supply chains become less transparent," she added, "it can be more difficult to understand the environmental consequences of our actions."
McCarthy is in a new battle to protect Americans from modern environmental threats such as global climate change, which she called "one of the most significant, if not the most significant, public health issue of our time." (See a map of global warming effects.)
As EPA chief, she is charged with implementing large portions of President Barack Obama's controversial climate action plan, such as carbon pollution standards for new and existing power plants, which the agency says will help protect millions of Americans from the dangers of a warming planet.
People in the modern-day environmental movement, McCarthy noted, should remember the power of the grassroots activism that spurred the first Earth Day in 1970.
"It wasn't so much about demanding national action," she said. "It was about demanding that individuals get engaged, that would then push national action." (See your pictures of Earth.)
McCarthy is especially keen to hear more voices from minority and low-income communities, which are disproportionately affected by environmental hazards. "This is an issue where we need everybody to speak up," she said.
Though huge problems remain, McCarthy noted, the impact of that first Earth Day has been profound. Since then, the nation's air and water have become dramatically cleaner, and lead has disappeared from gasoline—while the economy has more than doubled in size.
"Really, it all began with Earth Day," she said, "and the ability to have a grassroots movement that demanded that we keep people safe while we continue to grow the economy."