Stack Effect in wintertime encourages infiltration of outdoor air on lower levels and loss of warm air, due to high pressure, on the top levels of buildings
December is here. For those of us in the Midwest, that means it’s cold outside. Like really cold. It means that our windshields needed defrosting this morning, that snow lurks in the forecast, and that we’ve turned our heaters on high.
It also means that our healthcare clients should be adjusting their Infection Prevention measures.
Why? Because when the seasons change, our indoor environments change too. And this is never more true than in the wintertime.
I’ll explain, but first let’s review a few basic Infection Prevention facts:
- Bacteria, viruses and contaminated dust are everywhere.
- We can protect patients by eliminating (or at least controlling) these contaminants.
- The mode of transport for these contaminants is air flow.
- The success of Infection Prevention measures depends on whether or not we can control air flow.
So the problem we face is this: winter weather affects our ability to control indoor air flow due to a phenomenon called “the stack effect” that gains traction in the winter.
The Problem: Stack Effect
What is Stack Effect?
Warm air is ‘lighter’ than cold air so it naturally rises. Cold air naturally sinks. In buildings, where this air is trapped, you end up with a pressurization effect. Warm air gathers at the top of the building, creating relative high or “positive” pressure that pushes the air out of the building.
When warm air rises to the top, we’re left with relative low or negative pressure near the bottom of the building. The lower levels then act like a vacuum that sucks air inside through lower-level doors, windows and walls to balance the negative pressure.
This chain of events is known as the stack effect.
The Chimney Effect
If you’re lucky enough to have a working fireplace in your house, you’ve probably experienced the Chimney Effect, which works very similar to the Stack Effect. Warm air inside your house has higher pressure than the cold air outside. When you open the flu to start a fire, warm air rushes up and out the chimney to make up for the lack of positive pressure coming in from outside.
Why Is Stack Effect More Powerful in Wintertime?
The stack effect is compounded in the wintertime because there is a larger temperature difference than there is during the summer.
In summertime, the temperature difference between indoor temperatures ( 73°-79 °) and outside temperatures ( 85°- 95° Fahrenheit) might only be 20 degrees.
But in wintertime, the temperature difference is much greater. Indoor temperature remain between 68° and 75°, while outdoor temperatures can dip to more than 60 degrees below indoor temperatures.
This drastic temperature difference causes a much more extreme stack effect, with intense positive pressure near the top of buildings, and acute negative pressures near the bottom. Of course, cold air rushing in through ground-level doors and windows only multiplies the effect.
Pressurization Definitions
Building Pressurization: The air pressure inside a building in relation outdoor air pressure, which is determined by the balance of air entering in versus escaping the building.
Positive Pressurization: When there is higher air pressure indoors than outside, indoor air is pushed out of buildings through windows, doors and seams. Slightly positive pressurization is favorable for most facilities—especially hospitals—because it prevents unfiltered and unconditioned air from infiltrating the indoor air supply. The key to maintaining positive pressurization is to ensure you provide enough makeup air to your HVAC system.
Negative Pressurization: When indoor air pressure is lower than outside air pressure. Negative pressurization allows untreated outdoor air to infiltrate (or leak into) buildings. Infiltration is of particular concern for healthcare facilities, as patient health and comfort depends on maintaining control of air flow and contaminants.
How do we make this worse?
It’s not only natural phenomena increasing the stack effect. We also play a role.
First of all, we crank on the heat when it gets cold outside. Warm air that’s produced by our heaters rushes toward the top of building, adding to the positive (high) pressure on the upper levels. As this precious warm air travels up, the negative pressure on the lower levels is also compounded.
Many facilities have also been known to close outside air intakes in an effort to save energy. The rationale is that it takes less energy to treat air that’s already inside the building because it does not have to be warmed or filtered as much. But closing the air intakes actually compounds the problem: in the wintertime, air is rushing out the top levels much fast than it’s being re-introduced.
If new air cannot be introduced through air intakes, the stack effect is compounded. This is especially troublesome in healthcare facilities because air sucked in through the levels will likely be unfiltered and introduce new contaminants to the space.
Stack Effect & Revolving Doors
Did you know that revolving doors were created because of the stack effect in high-rise buildings? When we started building skyscrapers in the 1880’s, the stack effect was so strong that people could barely open the doors of these tall buildings. The lower levels had intense negative pressure compared to the outdoors, so outside air would try to rush in, making it difficult to push doors the other way. Revolving doors help minimize the amount of outdoor air that can rush in at one time.
Concerns: Stack Effect and Infection Control
We’ve established that winter weather can dramatically affect pressurization (stack effect) in buildings. So now the real question is: How does Stack Effect affect our approach to infection prevention in healthcare facilities?
Pressurization Effects on Contaminant Flow
Remember that bacteria, mold spores and dirt travel via air currents. If we can control air flow in health care facilities, we can control the flow of these contaminants.
Your facility’s current infection prevention measures probably include HEPA filtration, negative pressurization for certain cleanrooms, and positive pressurization for high-risk patient areas. All of these measures are put in place to help us manage air flow through the facility.
When Stack Effect enters the picture, we lose some control of the air flow in our facilities. Warm air is more forcefully drawn upwards, while unwanted outside air is sucked indoors near ground levels. This can lead to strong drafts on lower levels and the loss of temperature and contaminant control on upper levels.
Infection Prevention Requires Overall Positive Pressurization
All buildings want overall slightly positive pressure to keep unfiltered outdoor air from entering through doors, windows and seams. In healthcare facilities, overall positive pressure is also essential to:
- Preventing unfiltered air (and contaminants) from entering the facility.
- Allow us to maintain control of air flow inside the building.
The key word here is “OVERALL”. Stack effect leads to concentrated positive pressure at the top of a building, but it certainly does not lend itself to overall positive pressure due to the vacuum effect on lower levels. Instead, we need to focus on obtaining pressurization balance throughout the building.
How does Wintertime Construction Affect ICP?
Indoor air flow is more difficult to control in the wintertime due to more drastic pressurization. At any given time, you already have to consider increased pressure differences between different floors of your building, while also considering pressure needs for ICUs, ORs, and other ‘clean’ and high risk areas
Construction and renovation projects at your facility add another level of complexity to this balancing act. Your approach to balancing air flow will not only have to account for the upward movement of air due to stack effect, but also negative air needs for any renovation areas, and methods for controlling the flow of contaminants created during construction.
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Case Study: Effects of Negative Pressurization on Patient Safety
Stack Effect encourages infiltration of contaminated outdoor air and reduces our ability to control indoor air flow in our buildings.
Negative pressurization due to stack effect is a serious problem for healthcare facilities because it encourages infiltration of unfiltered air. But stack effect is not the only phenomena that can contribute to negative pressure. Construction can also cause negative pressure issues, either by amplifying the results of stack effect, or off-setting pressurization relative to surrounding areas.
At one hospital, these phenomena resulted in an aspergillosis outbreak among oncology patients being treated in a hospital adjacent to new construction.
The problem: Pressure relationships within the existing healthcare facility were correct. There was positive pressurization in high-risk areas/patient rooms, and lower pressure in hallways. However, the overall pressurization of the oncology unit was negative in relation to the building under construction.
The result: The relative negative pressurization of the oncology unit turned that area of the existing hospital into a vacuum that sucked unfiltered, dust-laden air into the unit. This lead to an aspergillosis outbreak among cancer patients in the unit.
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Stack Effect Solutions
Now that we know about stack effect, it’s time to find a solution. We know that overall slightly positive pressure is ideal. But we also need to balance out pressure from the top down. How do we do this?
1. Balance energy savings with positive pressure
The first step to combatting stack effect is to make your HVAC system is provided with adequate make up air. This means keeping air intakes open, so you can control the air brought into your building. The alternative is infiltration by cold, unfiltered air, which is just as costly to work against.
2. Consider facility design
The combined placement and design of stair wells, elevators, interior/exterior doors and windows in your facility will largely determine air flow. Consider how these features work for or against your goals.
3. Bring in an expert
Ask an indoor air quality or building science expert to survey your facility for proper pressurization, air flow, filtration and air quality. They can help you identify the source of facility issues impacting patient health, as well as other issues issues that maybe be flying under the radar. impacting patient health and safety. This may include pressurization studies to ensure proper air flow in your facility, and collection of particulate counts to ensure proper filtration of air in high-risk patient areas.
Additional Steps for Health Care Facilities
1. Adjust Infection Prevention Measures, including proper pressurization balance/relationships
Healthcare facilities will need to modify their infection prevention measures based on the information they gather about stack effect from facility studies, observations, etc. This includes adjusting pressurization relationships within the facility to that patient rooms and treatment areas fight against infiltration from all sources.
2. Minimize construction drawbacks
Ensure the proper barrier systems are installed and check that all construction areas have negative pressurization relative to the rest of the facility. You can also reduce contaminants levels introduced to your facility by:
- Minimizing construction traffic in/through your facility.
- Providing workers with restrooms, vending machines and other amenities in the construction area.
- Plan (and, if possible, provide) alternate construction entrances/exits.
- Closing all air intakes near construction zones.
