WASHINGTON -- Pollution before the first Earth Day was not only visible, it was in your face: Cleveland's Cuyahoga River caught fire. An oil spill fouled 30 miles of Southern California beaches. And thick smog choked many cities' skies.
Forty years after that first Earth Day on April 22, 1970, smog levels nationwide have dropped by about a quarter, and lead levels in the air are down more than 90 percent. Formerly fetid lakes and burning rivers are now open to swimmers.
The challenges to the planet today are largely invisible -- and therefore tougher to tackle.
''To suggest that we've made progress is not to say the problem is over,'' said William Ruckelshaus, who in 1970 became the first head of the Environmental Protection Agency. ''What we've done is shift from the very visible kinds of issues to those that are a lot more subtle today.''
Issues such as climate change are less obvious to the naked eye. Since the first Earth Day, carbon dioxide levels in the air have increased by 19 percent, pushing the average annual world temperature up about 1 degree Fahrenheit, according to the National Oceanic and Atmospheric Administration.
''We've cleaned up what you can see and left everything else in limbo,'' said Kathleen Rogers, president of the Earth Day Network.
Improvements took shape in the form of the Clean Air Act, the Clean Water Act and changes in the way businesses treat the environment, said Denis Hayes. Those reforms, he added, grew out of the first Earth Day, an event Hayes helped coordinate.
''It is the most powerful, sweeping, societywide change America has had since the New Deal,'' Hayes said. ''The air is cleaner despite the fact that we have twice as many vehicles traveling twice as many miles.''
Nancy Sutley, head of the White House Council on Environmental Quality, said progress in the past 40 years is about more than just laws. It's also about innovation that made cleaner cars. And that innovation, Sutley said, ''is going to be the answer for tackling climate change.''
No place illustrates progress more than the Cuyahoga River.
Cleveland's main river used to periodically catch fire. On June 22, 1969, trash and an oil slick ignited. The river burned for half an hour, drawing national attention to water pollution nationwide.
People didn't swim in the river at the time, and anyone who fell in needed to be checked by a doctor.
''The river bubbled like a cauldron. There were all kinds of chemicals in there, and that was what was bubbling at the bottom,'' said Wayne Bratton, a boat captain then and now, and the first president of the Cleveland Harbor Conservation Committee.
On Tuesday, Bratton was aboard his boat, The Holiday. He looked over the starboard side at Collision Bend and described by telephone what he saw: ''I'm looking at a lot of gulls, there's a loon, a lot of black heron.''
People now fish in the river, which holds 60 species. There's a spiffy amphitheater on the river bank, which never would have been built when the water had a dreadful stench, Bratton said.
It's not just the Cuyahoga. In 1957, the Public Health Service declared the Potomac River unsafe for swimming. Now Rogers lets her children swim in it.
''I don't even wash them off any more,'' she said.