Just off the runway, beyond the scurrying passengers, lies a section of the Baltimore-Washington International Airport that pumps more than $500 million a year into the economy of Maryland and its neighboring states.

It is a vast space: eight sprawling buildings through which more than 420 million pounds of mail and freight moved last year, an essential commercial link between the mid-Atlantic economy and the rest of the world.

That capacity is about to double. Spurred by the rapid growth of cargo shipments during the 1990s, airport officials plan to add up to 358,000 square feet of space to existing cargo facilities over the next 18 months.

But the expansion coincides with an 18 percent drop in shipments in and out of BWI airport over the past half-year. Airport officials attribute the decline to a slowing of the nation's economy, the weakness in Asian markets following the Pacific region's widespread economic crisis, and a cutback in computer shipments as firms focus instead on fixing year 2000 software problems.

"What happens in one place around the globe eventually has effects here," said Frank Chambers, president of Aviation Facilities Co. in McLean, the parent company of the firm that will build and operate the new cargo area. Asian importers "just weren't able to buy the American goods like before and we're just seeing the effects now," he said.

Imports have fallen for the same reasons, he said. "A lot of companies over there folded so there's just not as much coming in."

The Midfield Cargo Complex, as the new cargo area will be called, is expected to generate about $1 million a year under a long-term contract with Aviation Facilities' AFCO Cargo BWI II LLC. AFCO also will buy and operate three of the existing cargo buildings, which all are leased, for $11.5 million. The funds from the sale will be used to reimburse some of the Maryland Aviation Administration's infrastructure costs for the complex.

Chambers said the state will lease the site to AFCO, which will rent to the warehouse tenants.

"If the economy falters it will be us that takes the hit," Chambers said. "But that's the nature of it. We have every confidence that BWI will be a growing cargo facility and we're excited about this."

Millions of tons of cargo travel through the area's three airports -- BWI, Reagan National and Dulles International -- creating thousands of jobs directly, and many more indirectly at businesses that depend on the airports, said Spencer Dickerson, executive vice-president of the American Association of Airport Executives.

And those who benefit most are the passengers and shippers who use the airports. Under Federal Aviation Administration rules, fees collected by the airports must be used to improve and maintain airport facilities.

"So it's not like we're making any money off of it," said William Castleberry, marketing and development director at BWI.

At BWI, cargo shipments produced $574 million in revenue last year for airlines and transportation companies that serve the airport. Cargo service accounts for almost 600 of the 10,140 airport jobs.

At Dulles and National, cargo revenue totaled a combined $12.4 billion last year, creating more than 2,200 jobs at the airports and at related businesses nearby. Last year Dulles began work on another cargo building that will add more than 43,000 square feet of cargo space.

In contrast with the steady growth in past years, 1999 isn't exactly a "banner year" across the country, said Steve Alterman, president of the Cargo Airline Association.

The new challenges include adapting to the growth of Internet commerce, he said. "Cargo airlines will be the ones to benefit as such start-ups struggle to ship their items in an efficient and timely matter."

BWI, Dulles and National form an airport triangle that produces intra-competition while combining to strengthen the region's economy.

Although Dulles is the fastest-growing of the three, analysts have not seen evidence that its success is hurting BWI on the cargo front.

"It's not as though we really consider BWI competition and it's not like they really consider us competition," said Richard Norris, air cargo development manager for Dulles and National airports.

Castleberry disagreed. "Dulles is our competition, but whatever business they do also helps our state, and that's what we want. If we can't do it, then someone else in the area can."

Larry Rosenstrauch, director of the Loudoun County Economic Development Department, said that the three airports together make the region more competitive with rival cities around the world.

"This region is a point of entry to the global economy," he said. "And because of that, we have to market ourselves to the world about what we have here."

Doug Gelfand, general manager of Emery Worldwide, a division of the $2.2 billion CNF Transportation, a global transportation company, says the strategic placement of BWI, Dulles and National airports spells better business opportunities for all three.

"We're able to get our freight out and put on individual routes around the area," he said. "This affects everyone, being able to ship things in a moment's notice."