Negative pressure rooms and positive pressure rooms
Pressure controlled rooms, with positive or negative pressure, are used to contain the dispersion of contaminants and to protect certain areas of a building from the contaminant spreading. These rooms are mainly used in hospitals and chemical factories to isolate certain rooms which contain hazardous products and protect the rest of the building from potential contamination.
What is a negative pressure room
In a negative pressure room, the pressure is maintained lower than the rest of the building to force the external air to enter the room and prevent any air in the room from spreading in the building. The goal is to prevent any contaminated air from within the room to reach other parts of the building. The negative pressure forces the air to flow into the room even when the door is open and when people enter or leave the room. For this to happen, it is necessary to maintain the room at a lower pressure than the external environment at all times.
Negative pressure rooms are used in hospitals to prevent the spreading of airborne infections from areas where infected patients are. Many airborne pathogens (such as flu, measles, chickenpox and legionella) are spread by air and many patients require to be quarantined in “airborne isolation areas”, where the negative pressure helps reducing the risk of infecting other people via airborne transmission. Other applications of negative pressure room include Biosafety laboratories or rooms in the manufacturing process where pollutant are dispersed in the air and need to be contained in the same room.
How do negative pressure rooms work
The key to maintain the constant negative pressure in the room is to remove a bigger quantity of air than the one supplied. An active mechanical ventilation system is connected to multiple outlets in the room and it is activated by a pressure sensor which constantly monitors the average room pressure. The room is also sealed to avoid external air leaks and avoid additional unnecessary air removal. The location of the air outlets is also very important to drive the flow out of the room without contaminating the people in the room. Typically the air outlets are located on the roof of the room removing the air vertically above the patients.
The CFD simulation above, which has been done using SimWorks, shows the pressure levels in a typical hospital floor. Red areas show the over-pressure inside a positive pressure room, which is generally used as a waiting room, designed to protect the people inside from airborne transmission of diseases, while the area in blue is an operating theatre, which is a negative pressure room designed to confine any pathogen inside it. The dark cloud shows the spreading of the pathogen released by the patient which are extracted by an air ventilation outlet located inside the room.
In specific cases, such as biosafety levels rooms, the air is filtered before being released to the outdoor environment. When the exhausted air can contain nuclear isotopes, it must be checked for the presence of radioactivity and it is exhausted through tall exhaust ducts and released away from occupied spaces.
On top of maintaining a minimum pressure difference between the room and the surrounding areas the room requires a constant fresh air supply which has to be filtered using filters of specific spec. Regulations usually specify the minimum number of air exchanges per hour (ACH) required for a specific application, usually the number goes from 2 ACH in public corridors to 20 ACH in the operating room (see the regulations and guidance section).
In order to correctly dimension the ventilation system of such a room the best solution is to carry out a CFD (Computational Fluid Dynamics) simulation to calculate the actual mass flow rate required.
Negative pressure room simulation setup
As an example a full CFD simulation has been carried out highlighting the main flow paths in a pressure controlled environment. These simulations show the air flow quantity required inside each environment to achieve the desired effect. Most of the times the design requirements specify the amount of ACH (Air Changes per Hour) that is the amount of times the total volume of air inside the room needs to be replaced every hour. The starting point of the calculation is to multiply the room volume for the ACH value to get the mass flow rate required per unit time. This value determines the number and the size of the air outlets in the room, as generally design requirements specify a maximum air speed, and a CFD simulation with an initial layout of air inlets and outlets can be setup. Once the simulation is completed, the results can show the actual flow features within the room and, if required, the layout air inlets and outlets can to be adjusted to alter the air flow. Finally, another CFD simulation can be performed to verify the new configuration matches the requirements.
Also known as anterooms, those rooms are placed in front of the pressure controlled room and are used as a buffer to the remaining spaces. The presence of these rooms can be sometimes required to further reduce the risk of airborne disease spreading by creating a decontaminated environment in front of the negative pressure room. This requires that the doors of the decontamination room and the negative pressure room are not opened at the same time. In case of a pressure loss in the main negative pressure room, the decontamination room is also providing an additional layer of safety. Sometimes these rooms are used as changing rooms for the medical personnel before entering the controlled pressure room to assist patients.
Positive pressure rooms
In a very similar way, positive pressure rooms maintain a raised pressure with respect to the surrounding environment to protect an area from the ingress of external air. This is required whenever a portion of a building needs to be isolated. A typical example is an hospital waiting room that needs to be protected from external pathogens.
How positive pressure rooms work
In this case, a positive mass flow rate has to be maintained by introducing a greater quantity of air into the room with respect to that removed from the room. The room pressure level needs to be continuously monitored to avoid any unexpected loss of pressure.
The level of pressurisation depends on the specific regulation set applied. Typically the pressure difference is in the region of 5 or 6 Pascals (about 6e-5 atm). Positive pressure rooms are used for protecting an area and the people in it from contaminated flow coming from other areas of the building. It is very important to maintain high quality air in an high pressure room and there must not be any sources of contaminants within the room, as the air in the room can spread to other parts of the building.
Checks and guidance of positive and negative pressure rooms
The room sealing and the capacity of the ventilation system to maintain a constant pressure difference with respect to the other rooms has to be regularly checked to guarantee the safety of the system.
The Centers for Disease and Control and Prevention (CDC) issued in 1994 a recommendation to check Tuberculosis (TB) isolation room daily while being used for TB isolation, while the test can be done on a monthly bases if those rooms are not in use to control the spreading of the disease.
The smoke test uses a capsule of smoke placed near the bottom of the room door to inspect the direction of flow. In a negative pressure room, the smoke flows inside the room and does not escape out of it. On the contrary, in a positive pressure room the smoke should not enter the room. This test is inexpensive and can be easily carried out. On the other hand, it does not measure the actual pressure levels, meaning that the room pressure difference can be higher than required leading to a waste of energy. Moreover, this test can be repeated at regular intervals but it cannot detect if a problem in the ventilation arises in between two tests.
An automated and continuous monitoring of the pressure levels is required. A pressure sensor monitoring in the room can provide a continuous monitoring of the actual pressure conditions inside the room and promptly signal a possible loss in pressure. The sensor has to be separate from the main sensor driving the ventilation system to avoid dangers of false readings, which can related to an incorrect sensor calibration or sensor malfunctions. The sensor itself has to be regularly checked and calibrated.
Rooms network and overall flow requirements
Isolation (negative pressure) rooms and positive pressure rooms are part of a building layout and the choice of the location is crucial for maintaining the desired isolation effect. Moreover multiple controlled pressure rooms can be present at the same time on the same floor, in this case it is important to calculate in advance the main flow directions and the corresponding pressure differences between environments.
The location of positive and negative pressure rooms is critical to maintain the desired flow directions, especially when the doors are open. Areas that share similar pressure levels should be located next to each other to avoid local high pressure gradients, which could potentially trigger high velocity flows from one room to the other. Such conditions should be avoided in indoor ventilation systems to guarantee the air velocity stays within user comfort limits, which are generally 3 to 4 m/s. Flow velocity higher than those limits could also become dangerous in controlled environments like hospitals.
Regulations and guidances for pressure differences
The Center for Disease Control and Prevention recommends that the minimum pressure difference needed between the pressure controlled room and the rest of the environment should be equal or more than 2.5 Pa. Similarly the UK Department of Health recommends 5 Pa as a minimum for negative pressure isolation and allows a positive pressure in the outside corridor of 8 to 12 Pa (nominally 10 Pa).
The ASHRAE Applications Handbook 2011  defines the levels of pressure differences required for specific applications and values of Air Changes per Hour (ACH):
While the design requirement is to maintain a pressure which is a certain amount above or below the building average pressure of the building, the actual room pressure can change rapidly when a door is opened. The pressure sensor will emit a signal and activate the ventilation system to compensate for the sudden pressure drop or increase. While the ventilation system is reacting, the pressure difference value can temporarily fall below the required value. For this reason in the negative pressure room simulation we imposed a pressure difference of just 5 Pa instead of the design requirement of 10 Pa.
Those oscillations are perfectly normal and even if the instantaneous pressure difference becomes less than the design requirement, the prevalent flow is always in the same direction as intended. For example, the CFD simulation results of the room with open doors presented above show that the flow is leaving the positive pressure room and it is entering in the negative pressure room, this way any contaminant from within the room does not diffuse in the surrounding environments.
Positioning of inlets and outlets
In a CFD simulation, it is possible to predict the flow patterns within the room. For example, in the emergency department room analysed above, there is a plum of contaminated air moving upwards from the patient. This is caused by the breathing of the patient and the difference in density between the ambient air in the room, which is at a temperature of 24 degrees Celsius, and the air exhaled from the patient, which is at 37 degrees Celsius. In this case, the ideal positioning of the outlet is close to the ceiling on the wall next to the patient bed so that the prevalent air flow will go from the patient directly towards the air outlets minimising the contamination probability for other people present in the room.
If the contaminated flow is mostly colder than the surrounding air, it will naturally tend to move towards the floor of the room. In the case of a biosafety laboratory, where the contaminated samples are kept refrigerated, the ideal positioning of the outlets is at the floor level. This will prevent the flow to stagnate and potentially spread the contaminant within the room.
The positioning of the inlets has a smaller effect on the overall flow directions, in the case of the negative pressure room the inlet was placed next to the entrance. This way the colder flow delivered by those inlets was creating a clean air curtain which naturally tends to fall down due to the lower temperature with respect to the rest of the room:
Again only a CFD simulation can highlight any dangerous recirculating flow regions allowing the ventilation system designer to optimise the system to minimise risks.
The results and the description provided above where defined to describe physical behaviours under certain circumstances. They should not be considered a medical guidance and do not account for environmental variants such as humidity or wind.
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