For diseases that travel through the air, like COVID-19, special hospital rooms are essential for controlling pathogens and making sure they stay contained to one patient’s room, rather than infecting others nearby (as happened with COVID-19 on cruise ships). They’re called “airborne infection isolation rooms,” or negative pressure rooms. You’ve probably read about them in stories about hospitals treating acute cases. They create a crucial barrier between extremely infectious people and everyone else.
But how do they work exactly? And why aren’t they everywhere? To find out, we talked to Andrew Streifel, a hospital environment specialist who helped pioneer the design of these rooms in the 1980s, while trying to protect immunocompromised bone marrow transplant patients at the University of Minnesota Medical Center. He also contributed to the American Institute of Architects’ guidelines of hospital design published in 1996. Streifel has penned numerous papers, and visited roughly 400 hospitals around the globe, consulting on the design and implementation of isolation rooms over his more than 40-year career. Even though he’s retired, Streifel still consults, and his paper on converting stock hospital and hotel rooms into airborne infection isolation rooms is being referenced by healthcare providers around the world in response to the COVID-19 pandemic.
What are negative pressure rooms?
The goal of a negative pressure room is simple. If someone is exhaling a virus or other contagion into the air, “you create a vacuum—a rather low-[pressure] vacuum,” Streifel says. “You have to suck out more air than you are blowing in.”
That means an airborne infection isolation room is actually just one big vacuum. Generally speaking, it sucks in air through the gap under the door to the hallway. Yes, that means it’s sucking in air from a common space—meaning the air entering the room isn’t designed to be sterile. Then, an exhaust fan—often located in the bathroom—will eject the diseased air in the room to the outside.
These rooms have a few other features to maintain negative pressure. The windows can’t be opened. The doors close automatically. But most of all, these rooms need to spit out 30% to 40% more air than they are taking in at any moment. That’s what creates the pressure, or the one-way flow of air.
Wait, what happens to the virus when it’s outside?
It’s diluted by a whole lot of fresh air. And during the day, viruses are exposed to the UV light of the sun, which is generally regarded as a universal disinfectant—though its efficacy against COVID-19 is a question of some controversy that is still being studied.
That sounds simple!
Yes, the science behind negative pressure rooms is simple. But the execution of these rooms—both at the macro and micro scale—is trickier than it may seem.
Why is it so challenging to build negative pressure rooms?
In the 1980s, Streifel wanted to engineer a negative pressure room reliably. But he had a problem: He couldn’t track the near-invisible viruses floating in the air, so he couldn’t know how effective his designs were. That’s why he began using a smoke stick, which is a simple tool to track air flow and air leaks inside a building. (Smoke particles are larger than viruses, but smaller than bacteria.)
“What we started to do was learn to seal the room . . . it was my job to get on my hands and knees with finishing carpenters to find all the cracks,” says Streifel. Teaming up with the local carpenter’s union, he learned that builders already had some tools that he could use to construct these rooms. Fire walls, for instance, don’t just block fire but are designed to contain smoke. That meant isolation rooms were typically built with airtight, concrete walls rather than wood-framed drywall.
Also problematic were the wall outlets. As Streifel explains, the average critical care room has 10 to 12 outlets for electrical equipment. These needed to be sealed with caulk, as do the floorboards.
Of course, all the caulking had another problem at the floor level: water. Water is an omnipresent problem at many hospitals, due to leaks and frequent mopping. And when this water hit the caulk, it would quickly cause mold. So now, all caulking used is mold-resistant.
What about the ceiling?
The ceiling is trickier to handle, says Streifel, because your average drop ceiling isn’t going to be airtight, and the gaps above the drop ceiling can be shared between rooms, since they run all sorts of electrical and plumbing equipment.
Originally, Streifel says drop ceilings were sealed to create a negative pressure room. But maintaining them over time became difficult, because anytime someone needed to work on the plumbing or electrical, the ceiling had to be reopened then resealed.
Now, Streifel points to Lurie Children’s Hospital in Chicago, which he consulted on, as having the best modern approach: Each room is its own box (complete with a concrete ceiling and floors). This serves as an outer shell—a barrier between rooms. That way, you can still have a drop ceiling, but you can open and close it at will without breaking any seal.
This doesn’t sound that complicated. Can we convert more hospital rooms and hotels to negative pressure rooms?
Yes and no. As has been well-documented during the COVID-19 crisis, rooms for isolating airborne infectious disease are in short supply in hospitals. Only 2% to 4% of all hospital rooms in the U.S. are equipped for negative pressure, most probably because it’s an added expense. There is a need for more of these rooms, and hospitals have been improvising more of their own.
There’s just one problem. Negative pressure rooms, in practice, aren’t always maintaining proper negative pressure. In one government-funded study, Streifel analyzed hundreds of negative pressure rooms in Minnesota and found only a third met ventilation standards. The HVAC systems and room design simply didn’t mesh to create the proper one-way airflow to contain contagions.
One major issue with scaling negative pressure rooms is the source of the air. Yes, you can install a bunch of exhaust fans to spit out infectious air from individual rooms. That part is easy. The hard part is where all of your air comes from in the first place.
In temperate climates, it’s possible to suck in a lot of air into your building. But where it’s cold or hot, that air needs to be heated or cooled and humidified, and chances are, the building you’re in wasn’t designed for all of that extra capacity. And if you don’t have enough source air to feed into the vacuum, air will find another way in, through cracks in the walls or windows. You lose some control of air flow, and condensation can form on walls and windows, which will lead to mold.
Streifel has published best practices on how to convert a hospital or even a hotel room to be negative pressure. He suggests using tools like duct tape to seal rooms, and HEPA filters can help scrub the air when full exhaust isn’t possible. That means that these rooms will be pressurized but will ultimately recirculate some air. Since that air is filtered, though, it should be safer than if it isn’t filtered. “It’s not ideal,” says Streifel. “There are a number of things that aren’t perfect when it comes to this [improvisation]. But you have to do what you can.”