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.

Not anymore.

On Thursday, 40 years after that first Earth Day in 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.

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