Four people became ill last night on an airliner headed for Baltimore-Washington International Airport, prompting a flurry of anxiety as initial reports suggested that the situation was considerably more severe.

Of the four who reported symptoms after the Southwest Airlines jetliner from Houston landed, one woman was taken to a hospital, where her condition was described as non-life-threatening. The other three declined hospitalization.

The cause of the illness was not clear, but officials said the woman appeared to have been ill when she boarded the plane.

An initial report from the jet while it was en route to Baltimore indicated that as many as 12 people onboard might have been suffering from nausea and vomiting, according to Richard L. Alcorta, Maryland's emergency medical services director.

It was reported in an early news account that 10 people on the plane had been overcome by fumes, with four of them in critical condition and six others in serious condition.

More than a half-dozen ambulances were sent to the airport, where the plane landed about 10:25 p.m., about 20 minutes ahead of schedule. Personnel in containment suits, designed to protect against hazardous environments, were the first to board the plane, authorities said.

Amid the flurry of activity sparked by the first reports from the plane, those who responded included a hazardous materials team, two paramedic units and a medical task force. In all, more than a half-dozen ambulances were dispatched.

The woman taken to North Arundel Hospital in Glen Burnie was described as a 48-year-old Texan.

Authorities said it appeared that she had been ill before boarding the flight at William P. Hobby International Airport and had thrown up while en route.

They also said it appeared that others around her who reported becoming lightheaded and nauseated may have experienced those symptoms as a response to the woman's illness.

"One started throwing up, and others sitting nearby" did so afterward in a "reactive response," said BWI spokeswoman Cheryl Stewart. An airline spokeswoman said the others may have suffered from "sympathy nausea."

Alcorta said the plane was checked for such matters as oxygen levels, presence of toxic gases or powder residues.

So far as could be learned early today, all the checks had proved negative.

However, in the first minutes of the incident, personnel at Anne Arundel Medical Center were placed on disaster alert.

They were told that a number of passengers were on their way there, according to spokeswoman Martha Harlan. But as it turned out, none arrived.

The Texas woman who was taken to North Arundel Hospital had apparently fainted on the plane. She was expected to be released today, according to hospital spokeswoman Kathy McCollum.

A spokeswoman for Southwest said it appeared that the woman had a preexisting illness that needed attention.

"I think it was more anxiety than anything," said Jim Brown, 48, of Austin, who was on the plane.