Freelance journalist Ashoka Mukpo, who contracted Ebola in Liberia, arrived at the University of Nebraska Medical Center today, becoming the second patient with the deadly disease to be treated there.

Why is he being sent to Nebraska instead of some other facility? Because the hospital is home to the largest of four high-level biocontainment patient care units in the U.S.

The Nebraska Medical Center says the unit was commissioned in 2005 as a joint project with Nebraska Health and Human Services and the University of Nebraska Medical Center.

"It was designed to provide the first line of treatment for people affected by bio terrorism or extremely infectious naturally occurring diseases," the center's website says.

"The Ebola virus is very difficult to contract," says Dr. Phil Smith, medical director of the unit, on its website. "The risk it would pose to people outside the unit would be zero, and this is something that can be very safely treated without infecting health care workers."

The three other high-level biocontainment facilities in the U.S. are at St. Patrick Hospital in Missoula, Mont., the in Maryland and Emory University Hospital in Atlanta, where two infected patients were treated this summer.

Dr. Rick Sacra, 51, was treated last month at Nebraska Medical Center. He has since recovered.

In an interview with NPR in August, Bruce Ribner, director of Emory's Serious Communicable Disease Unit, says caregivers use "personal protective equipment designed to prevent ... staff from coming into contact with blood, body fluids and large respiratory droplets."

Ribner said that the doors at the facility don't need to be sealed "because all airflow goes into the patient room since the rooms are under negative pressure."

Gizmodo writes:

"[The] isolation unit in Nebraska is isolated from the rest of the general hospital. It runs on its own air circulation system, and the air is passed through a high-efficiency particulate air (HEPA) filter before it is vented outside of the building. That's the same kind of precautions that you would see in a biosafety level 4 lab (the highest) that works with deadly or highly contagious diseases.

"In addition, the biocontainment unit has negative air pressure, which means that air pressure inside the isolation rooms is slightly lower than that outside. Essentially, air is gently sucked into the room, so particles from inside the room can't float out when you open a door. As another line of protection, ultraviolet lights zap any viruses or bacteria in the air or on surfaces."

Wired says: "[Hospital] staff volunteers at Nebraska Medical Center run twice yearly drills with decontamination at their hospital's 10-bed biocontainment unit. It's the country's largest, opened in 2005 with $1 million in federal and state funding. 'It's built like a concrete box,' says Angela Hewlett, the unit's associate medical director. 'We want to keep our germs inside.' But like Missoula, Nebraska hasn't seen a single infectious disease patient. Sometimes they use it as overflow for the emergency room."

Note: We previously identified the location of Montana's biocontainment facility as Rocky Mountain Laboratories (RML) in Hamilton. We've since updated this post.

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