LYNN – The scenario isn’t so far-fetched in the post-911 era when bombings and biological attacks routinely make world headlines.A potentially hazardous or lethal substance has contaminated hundreds of people living or working in Lynn, sending them scurrying toward Union Hospital.”People will not wait at the scene of the disaster. They’ll come here and our job is to control access to the hospital. We have to keep contaminated people outside. Otherwise, they’ll contaminate the emergency room and it will have to be closed down,” said James Weston, emergency preparedness coordinator for North Shore Medical Center. “To do that, we work with the Lynn Police Department and the hospital’s security personnel.”As Wednesday’s mock emergency unfolded, Lynn firefighters arrived at the hospital and quickly erected a long yellow tent fitted with nozzles capable of spraying a mist of water from nearby Fire Department pumper trucks.The tent, protective boots and suits and other decontamination equipment is stored in an on-site trailer, one of 60 statewide.”The concept is to keep the hospital from becoming contaminated. We know people don’t wait. They come straight to the hospital and get inside however they can – back door, front door, side door,” said Lynn District Fire Chief Joseph Zukas. “That’s why we have to bring the people through the tents before anything else.”Zukas said victims disrobe in the first tent, wash off in the second and redress in hospital patient attire or other loose-fitting garb in the third.”Ninety percent of the contaminants can be taken out just by having people take their clothes off,” he said. “The second tent uses a 3-percent baby shampoo solution so that the victims can scrub down. The nozzles spray them as they walk through. By the time they get to the third tent, they’re clean, dressed and ready to enter the hospital.”A system of table-height rollers inside the tents handles the non-ambulatory patients who most likely would arrive on backboards.Erecting the decontamination tents and ushering the victims through them takes 10 to 12 firefighters. Depending on the extent of the disaster, males and females would be separated during the process, the district chief said.The Fire Department has been practicing the decontamination drill every year for about eight years.”Next time we might make this a complete surprise instead of a planned drill,” said Zukas.He noted that Massachusetts is ahead of most states in planning for mass decontamination.A representative from the state Department of Public Health and members of the hospital’s maintenance and security staffs observed the drill.”About four years ago we thought we had the real thing,” said Zukas.He recalled how the Walmart in Danvers experienced a chemical leak and area emergency management agencies were put on alert after receiving reports of mass casualties.”The warning went out, but they didn’t have as many people down as they first thought.”