By David Liscio/The Daily Item

LYNN - An explosion rocks an MBTA bus en route from the Liberty Tree shopping mall in Danvers to Central Square in downtown Lynn. The driver radios the emergency from his location on Route 1 near the Lynnfield line, then heads for Union Hospital where the Fire Department begins to erect a mass decontamination unit.

Some of the 12 passengers are falling ill, or complaining of skin irritation and burning sensations. It could be from tear gas or a pepper-like substance.

Had passersby not seen signs posted near the hospital, alerting all that a training exercise was in process Friday, the scene might have caused nightmarish feelings. A dozen volunteer victims filed into a long yellow tent fitted with shower nozzles. Once behind the first curtain, their clothing was cut away by emergency medical technicians, mostly firefighters dressed in protective suits and breathing from air packs. The constant stream of soapy water cleansed the chemicals from their bodies. Before emerging from the tent, they donned hospital Johnnies, blankets, sheets, Tyvek suits or whatever else was available to ensure their privacy.

The goal is to protect the hospital in the event of a mass chemical incident.

Fire Capt. Joseph Zukas, the city’s emergency medical services director, said studies have shown that when these disasters occur, many victims fail to wait for emergency responders to decontaminate them at the scene. Instead, the victims individually search out emergency care and contaminate local hospital emergency rooms.

To keep this from happening, the state a few years ago deployed 76 mass decontamination units (MDU) departments, like the one stored at Union Hospital. Another 14 MDUs were spread across the state in various fire districts, with Danvers selected as the North Shore site. Should the need arise, these units can be trailered to the scene of the disaster or the hospital involved.

“Fortunately the unit has not had to be used for a real incident yet,” said Zukas, stopwatch in hand as he recalled a situation last year at the Danvers Wal-Mart when noxious fumes sickened several shoppers and employees. “The Lynn and Salem units were activated in case victims had to be decontaminated.”

Deputy Fire Chief James Carritte said Friday’s training differed from previous exercises because heated water was flowed into the elongated tent and volunteer victims were actually decontaminated rather than simply walking through the process. “We get a little better at it each time,” he said. “We’re doing a lot of training these days.”

Decontamination experts estimated that 80 percent of the contamination is contained in the victims’ clothing.

Lt. Thomas Bogart, the safety officer, made sure each victim was given a wrist identification band before entering the tent. He also kept watch on the entire scene, which was laden with fire hoses, stretchers, medical equipment and boxes of decontamination gear as firefighters and medics scurried about.

District Chief Jack Barry quietly observed, making adjustments to the process where necessary, although with the exception of a stubborn water heater, the exercise went smoothly.

Still soaked from the ordeal, Frances MacDonald of Saugus, an emergency medical technician (EMT) with Atlantic Ambulance, said volunteering as a victim gave her a better perspective. “It’s my first time walking through an MDU. Most of this is out of our realm of training, but it’s good to experience it from the patient’s point of view.”

Atlantic EMTs Kristin McGregor of Danvers and Amery Merritt of Peabody agreed that volunteering as victims was worthwhile.

North Shore Medical Center spokesman Kevin Ronningen said Union Hospital medical staff, security, and personnel from its environmental services and maintenance department have participated in the decontamination drills.

“Hopefully we’ll never need to do this for real, but if we do, at least we’re ready,” Bogart said.