(CBS NEWS) NEW YORK -- Japan's tsunami last year sent an estimated five tons of debris into the Pacific Ocean. Experts say roughly a ton-and-a-half of debris is still afloat, heading toward Western U.S. shores. Some has already washed up in Alaska.
The earthquake-spurred tsunami resulted in an emergency at the Fukushima nuclear power plant, which sent untold amounts of radioactivity into the air.
But a leading scientist says the U.S. needn't be too concerned about radioactivity, as far as the floating debris is concerned.
Toxicity is the major worry, says M. Sanjayan, the lead scientist at the Nature Conservancy and now a CBS News science and environmental contributor.
He told "CBS This Morning" co-hosts Charlie Rose and Erica Hill that comparisons being made by some observers between the Exxon Valdez disaster and the approaching debris field are off-base.
"The people who are saying that," Sanjayan observed, "are really talking about the geographic extent of it and the tonnage of it. Of course, oil floats, so it's not that hard to imagine that this 1.5 million tons of debris is really heavier than (oil).
"So, is it a serious issue? It's a serious issue. But Exxon (Valdez) was concentrated in a small area, and so the impacts of Exxon in that small area were very much greater."
Asked if it's correct to say the danger comes from toxicity, not from radioactivity, Sanjayan responded, "That's right. The fishing boat that was just sunk - scientists got on that boat, they looked at radioactivity, and it was essentially background level. It was normal. So, I'm not particularly worried about radioactivity."
But toxicity - from gas, oil or chemicals and other elements washing ashore - is a real issue.
"Think about everything in your garage and imagine that dumping in the ocean," Sanjayan said. "Some of it is going to make it out here intact, so a barrel might contain something. If it's punctured, it would have been diluted by now. That's what I think people are worried about -- it showing up on a beach."
Sanjayan pointed out that, "It's great that people are concerned about this one particular issue, but, to put it in perspective, there's probably over 100 million tons of garbage, basically, floating in the Pacific Ocean alone. So, this represents one, one-and-a-half percent of what's out there."
The best start to cleaning it all up is to get Man to stop dumping in the ocean to begin with, Sanjayan stressed.