Beginning today, two Princess Cruises ships will reduce their air-pollution emissions by about 30 percent by shutting down their diesel engines while docked in Seattle.

Instead of idling or using generator power, the Diamond Princess and Sapphire Princess will plug into the city's power grid and run off electricity.

The action comes after years of studies showing that diesel pollution is worse for people than once thought, and that while trucks and cars get cleaner, ships are increasingly among the largest sources of smog-forming nitrogen oxides and sulfur oxides, which cause acid rain.

Meanwhile, the cruise industry in Seattle has grown from a few annual sailings six years ago to 170 last summer.

Plugging in the ships will cut the equivalent of a year's worth of diesel exhaust from 1,100 cars, but the change didn't come cheap. It required a $1.8 million investment by Princess and a $50,000 grant from the U.S. Environmental Protection Agency.

Princess will pay the same rates for power as other commercial City Light customers. Each ship draws the same amount of electricity as a medium-sized hotel, but the two ships are in port only about 30 days a year combined, said Sharon Bennett, a City Light spokeswoman.

The other seven cruise ships operating out of Seattle don't have the equipment to connect to city power. They have agreed, however, to use cleaner fuel.

"It's not [just] a long extension cord — it's the same kind of setup you need to get electrical power for a skyscraper," said Mick Shultz, spokesman for the Port of Seattle. "City Light had to get power from the street out to where the ship ties up."

The port of Juneau, Alaska, which Princess also uses, is the only other port in the nation that offers such an option for cruise ships. The Port of Los Angeles-Long Beach in Southern California offers a similar type of power, but only at one container-vessel berth.