A woman aboard a Southwest Airlines flight late Thursday became sick and began vomiting, and by the time the plane landed at Baltimore-Washington International Airport, the initial reports were that she was unconscious, that numerous passengers were ill and that the cause could be some sort of hazardous material or biochemical contamination.

It was not, however, the airport confirmed yesterday.

The initial passenger had been ailing -- "from a stomach virus or something like that" -- before boarding Flight 1283 in Houston, airport spokeswoman Holly Ellison said. Her subsequent nausea as the plane headed east triggered similar queasiness and upset in three other passengers. That ultimately was the extent of the medical situation.

But "not knowing all the information when the pilot called in," Ellison said, airport officials activated their emergency response plan and had a hazardous materials crew meet the flight when it landed at BWI at 10:26 p.m. By 11 p.m., the crew had searched the plane, found nothing amiss and allowed all passengers to be on their way.

"We have to err on the side of caution," Ellison said.

The woman was treated at North Arundel Hospital and released, according to the airport. The three other passengers declined medical care.

No flight operations at BWI were affected, Ellison said.

The close, inescapable quarters of an airplane cabin at 30,000 feet can provoke problems that might never arise on the ground.

Earlier this month, emergency crews at the Bremen airport in Germany went on high alert when passengers on a flight from Amsterdam began complaining of nausea.

The culprit turned out to be one passenger's leaking bottle of nail polish. The smell, circulated through the plane's air conditioning system, proved powerful.

The Associated Press contributed to this report.