When it comes to boating, one person's pleasure is another's pollution.
That's becoming evident as the battle against smog moves closer to where people live and play. The fight began two decades ago with rules to control smokestacks, cars and trucks.
Now the U.S. Environmental Protection Agency is beginning to investigate a category of gasoline-powered polluters that includes one dear to the hearts of many Maryland boaters -- the outboard engine.
Boat engines are "much dirtier" than the engines of cars and trucks because boat engines are not equipped with catalytic converters that can filter pollutants out of their exhaust.
By the EPA's reckoning, the average 50-horsepower outboard produces as much pollution in one hour as a car driven 800 miles.
Even so, motor vehicles have been fingered as the main source of smog because far more people own cars and trucks than own boats and drive them with great frequency. Some 1.1 million motor vehicles are registered in Maryland, compared with 176,439 boats with motors.
Baltimore and its suburbs have the sixth worst smog level of any urban area in the country, the EPA says. Ozone, smog's chief gaseous ingredient, can cause shortness of breath, wheezing and chest pain, especially if inhaled while exercising outdoors. It is formed when hydrocarbons and nitrogen oxides in the air combine under a hot sun.
When Congress passed the Clean Air Act in 1990, it ordered the EPA to examine previously overlooked sources of smog and the potential for controlling them.
Motorboats are just one of the more than 80 different types of gas-powered engines, equipment, and off-road vehicles that are under scrutiny. "We're looking at how we can reduce emissions wherever we can," said Gay MacGregor, an EPA official in Ann Arbor, Mich., overseeing research into "mobile" pollution sources.
The EPA recently announced a plan with the electric utility industry to collect 1,000 used gas-powered lawn mowers from Baltimore and other cities so that their emissions can be studied closely. Rules are still a long way off, EPA officials say.
But when mowers are regulated, motorboats may not be far behind because as a group they are the next biggest source of pollution.
Maryland officials say they also are looking at the pollution caused by boat engines. But the state can do little on its own to limit emissions by marine motors. Besides, they say that cars and trucks are the chief culprits in causing smog.
"Cars [in Maryland] go 104 million miles every day," said Robert Perciasepe, the state's environment secretary. "These powerboats hang around all day. They don't go anywhere."
But the EPA estimates that with the Chesapeake Bay for a back yard,pleasure boaters in the Baltimore area spew out 16 tons of smog-forming hydrocarbons a day in their exhaust.
That is more than twice what state officials say comes from the the General Motors Corp. van plant on Broening Highway, the city's biggest industrial source of hydrocarbons.
In general, boaters like Joseph Bouchat, a Glen Burnie mechanic, are surprised to hear their motors are producing that much pollution.
"I only go out to relax," said Mr. Bouchat, saying he takes his runabout on the Patapsco River a couple of times a month to crab, fish and enjoy the water.
He has a 45-horsepower Chrysler outboard.
And, he said, he's not about to give up his relaxation:
"I just like going out on the water too much," he explained one day last week after a quick cruise from Middle Branch Park in South Baltimore to Key Bridge.
Commercial marine vessels in the area put another four tons of hydrocarbons a day into the air, the EPA estimates. But those boats, which use a different type of engine, also emit 16 tons daily of nitrogen oxides, the other major ingredient in ozone.
Most outboard engines are built to pack a lot of power for their weight, but the two-stroke combustion cycle they use lets up to one-third of the hydrocarbons in the fuel escape unburned, says John German, an EPA official.
Larger boats have inboard motors, which release fewer hydrocarbons because they are similar to four-stroke auto engines and burn gas more efficiently. But they produce as much nitrogen oxides as cars would if they did not have catalytic converters.
Besides helping to form ozone, nitrogen oxides also contribute to the pollution of the Chesapeake Bay when they fall out of the sky.
The EPA estimates that up to 30 percent of the nitrogen over-enriching the bay comes from "atmospheric deposition," or acid rain and nitrogen-laden dust particles.
Despite evidence that boats are significant polluters, the EPA's Ms. MacGregor said, "We've had a hard time, actually, getting environmentalists interested in this, and the states."