An environmental study says seaports are the largest urban polluters in the United States. An environmental group called the Natural Resources Defense Council rated 10 major U.S. ports, and calls many unsatisfactory.

The port of Houston gets a failing grade, say spokesmen for the group, which rated the ports by ranks from "A" to "F."

Savannah, Georgia, fares a little better, with a D+. The port of Oakland ranks the highest, with a B-.

The study looked at factors such as air and water quality, land use by the port, and the impact on neighboring communities.

The study says the ports of Los Angeles and Long Beach, which received a C- and a C respectively, emit as much diesel exhaust as 16,000 tractor-trailers that are running constantly.

Julie Masters, an attorney for the Natural Resources Defense Council, says much can be done to mitigate pollution. "Ports can install electric dockside power to run the ships while they are loading and unloading cargo, instead of idling massive diesel engines for hours at a time. They can use cleaner alternative fuels in their yard equipment. They can retrofit their older yard equipment with pollution control devices. There are a lot of things they can be doing," she said.

A successful citizen lawsuit against the Los Angeles port has prompted recent changes, and port officials in Los Angeles say they share the concerns of environmentalists.

"Diesel toxics are a major health concern, and we recognize that, and we're really working hard at it," said environmental scientist Ralph Appy, who oversees pollution controls for the city's port.

But he says the changes cannot be made overnight, and he disagrees with the finding that the port is not addressing the problem aggressively.

"We've removed toxic sediments from the harbor. We're getting major grant programs to look at storm water runoff into the harbor," he said. "We've sunk millions of dollars into clean air programs here at the port. Virtually all tug and workboats have been retrofitted here."

He adds that the port will soon open a new terminal that allows ships at dock to turn off their diesel engines and use onshore power.

Mr. Appy says Los Angeles officials have committed themselves to holding down emissions to their current levels as the port expands, while working on further reductions.

U.S. seaports are crucial to the economies of their respective regions. The joint ports of Los Angeles and Long Beach move more than 200 billion dollars in goods every year between the western United States and U.S. trading partners in Latin America and Asia.

This Los Angeles resident says that is no excuse for pollution. "Here we have this huge engine of the economy generating these billions of dollars for somebody, not us," he said.

Critics question say more needs to be done to curb emissions from seagoing tankers, port equipment, and the trucks and trains that carry the cargo inland.