On April 28 a Metro press release announced the purchase of 175 buses that run on compressed natural gas (CNG). Jack Requa, director of Metro's bus division, said the buses would be "a big part" of his agency's "commitment to improve the region's air quality."

This purchase, along with a purchase of 75 more buses, would bring the number of Metro CNG buses in July to 414, roughly 30 percent of the system's fleet. Metro's plan also calls for the agency to buy 200 more CNG buses for fiscal 2006 to 2008 and to build a third natural gas fueling station in Montgomery County.

But on May 6 Metro's "commitment to improve the region's air quality" was nearly thrown out when Board Chairman Robert Smith proposed freezing the CNG bus program at 414 buses and canceling the new fueling station. Instead of acquiring more CNG buses, he favored buying 196 diesel buses and 50 diesel-electric hybrids. Fortunately, a tie on the Metro board tabled his proposal. On Thursday the board again deferred making a decision.

Smith claimed that new diesel buses and diesel hybrids would be just as clean as CNG buses. But no diesel engine manufacturer has yet built an engine that can meet the federal nitrogen oxide standard that will be phased in between 2007 and 2010. Two natural gas engine makers say that they can meet the standard. Further, hybrids are an experimental technology. Only 36 are in service in the United States, and most have been in limited use for just two or three years.

Metro's CNG buses emit 53 percent less nitrogen oxide -- a primary ingredient in smog -- than its cleanest diesel buses, according to the U.S. Department of Energy. They also emit 85 percent less particulate matter -- i.e., soot -- than Metro's cleanest diesel buses.

Our region has failed to meet federal health standards for ozone smog for many years; now it is failing to meet the federal standard for fine particulate matter. These pollutants cause asthma attacks, respiratory disease and even premature death. The percentage of District residents who suffer from asthma is twice the national average. Diesel exhaust also has been designated a carcinogen by the California Air Resources Board.

Natural gas has other advantages over diesel. It is a bridge technology to hydrogen, meaning that the CNG facilities Metro builds today will be relatively easy to convert to hydrogen when that technology becomes available. Natural gas is produced in North America, while a growing percentage of diesel fuel comes from unstable regions of the world.

At the May 6 meeting of the Metro board, Dennis Smith of the Energy Department said that Metro's facilities in Rockville and Landover are good candidates for housing natural gas fueling stations and CNG buses because they are much newer than the Bladensburg and Arlington sites, where Metro built its first two CNG fueling stations. That means building at Rockville and Landover should be less expensive than building at Bladensburg or Arlington. Metro's original CNG plan called for 200 natural gas buses and a fueling station in Rockville, and it would make sense to carry out that plan and broaden it to include the Landover facility too.

Expanding the CNG fleet to Rockville and Landover would allow Metro to wait and see if diesel engine manufacturers can meet the 2007 nitrogen oxide standard and if diesel-electric hybrids become more reliable. The Energy Department recommended that Metro go slow on hybrids, testing five to 30 buses during the next few years.

If the Metro board really is committed to improving the region's air quality, it will seriously consider the Energy Department's recommendations. It should continue to install retrofit technologies and exhaust-treatment devices on its older diesel buses while choosing the cleanest proven fuel technology available for new buses. For the near future, that means natural gas.

-- Elliott Negin

-- Mark Wenzler

are, respectively, Washington communications director of the Natural Resources

Defense Council and chairman of

the Washington chapter of the Sierra Club.

ENegin@nrdc.org; mwenzler@net.org