Cleanroom pressure differential determines how air moves between a cleanroom and the spaces around it. Controlling that airflow keeps outside contaminants from entering or keeps contamination generated inside from escaping.
The right setup depends on what you’re protecting. Positive pressure safeguards the product or process while negative pressure contains airborne contaminants at the source.
Positive Pressure vs. Negative Pressure
| Factor | Positive Pressure | Negative Pressure |
| Pressure | Higher than the adjacent space | Lower than the adjacent space |
| Airflow direction | Outward | Inward |
| Main purpose | Prevent outside contaminants from entering | Prevent internal contaminants from escaping |
| Primary protection | Product or process | Personnel and surrounding environment |
| Common applications | Semiconductor, electronics, sterile manufacturing, non-hazardous pharmaceutical compounding | Hazardous processing, powder handling, containment laboratories |
| Air balance | Supply airflow is greater than return or exhaust airflow | Exhaust airflow is greater than supply airflow |
How Does Cleanroom Pressure Control Airflow?
Cleanroom pressure controls the direction air moves between connected spaces. Where there is an opening or leakage path, air moves from the higher-pressure area toward the lower-pressure area.
In a positive-pressure cleanroom, air moves outward. In a negative-pressure cleanroom, air moves inward. Pressure is always relative to another space, such as a corridor or airlock. For example, a reading of 10+ Pa only has meaning when the reference space is identified.
What Is a Negative-Pressure Cleanroom?
A negative-pressure cleanroom has lower air pressure than the surrounding space. Air therefore tends to enter the room through available openings instead of flowing outward. This arrangement is useful when a process can release powders, aerosols, or other airborne contaminants that should not spread beyond the controlled area.
Negative-pressure rooms are used in applications such as hazardous pharmaceutical processing, powder handling, certain biological laboratories, and other containment processes.
Negative pressure is also used in healthcare airborne infection isolation rooms. These rooms have their own ventilation requirements and should not be treated as identical to industrial cleanrooms.
Where hazardous materials are involved, negative room pressure may be only one part of the containment system. The design may also require dedicated exhaust, suitable filtration, airlocks, or controlled material transfer systems.
What Is a Positive-Pressure Cleanroom?
A positive-pressure cleanroom has higher air pressure than the space next to it. When a door gap or another opening is present, air moves out of the cleanroom rather than into it. Filtered air is supplied to the room. The HVAC system is balanced to maintain the required pressure difference.
Positive pressure is commonly used where outside particles could affect product quality. Examples include semiconductor manufacturing, electronics production, sterile pharmaceutical manufacturing, and medical device production.
For example, if a cleanroom is positive relative to a general corridor, air moves from the cleanroom toward the corridor. This reduces the chance of corridor particles entering the controlled space.
Positive pressure alone, however, does not determine the room’s ISO classification. Airborne particle concentration, filtration, airflow distribution, cleaning practices, personnel, and manufacturing processes also affect cleanliness.
Negative vs. Positive Pressure Key Difference
Positive and negative pressure cleanrooms differ in how air moves, what they protect, how their pressure relates to nearby spaces, and where they are used.
Airflow Direction
In a positive-pressure cleanroom, air flows from the cleanroom toward the adjacent space. In a negative-pressure cleanroom, air flows in the opposite direction, from the adjacent space into the cleanroom.
This direction controls where airborne contaminants are more likely to travel when air passes through door gaps or other openings.
What They Protect
Positive pressure is mainly used for product protection. It reduces the risk of particles and other contaminants from nearby spaces entering a sensitive production area.
Negative pressure is mainly used for containment. It reduces the risk of airborne materials produced inside the room spreading to workers or nearby areas.
Pressure Relationship
A positive-pressure cleanroom has higher pressure than its reference space, while a negative-pressure cleanroom has lower pressure.
The reference space may be a corridor, airlock, or another controlled room. This reference is important because a pressure reading only shows whether the cleanroom is positive or negative when compared with another space.
Applications
Positive pressure is commonly used in semiconductor, electronics, sterile pharmaceutical, and medical-device manufacturing, where contamination can affect product quality.
Negative pressure is used in hazardous pharmaceutical processing, powder handling, and certain biological containment areas, where airborne material must be kept from spreading beyond the room.
Can Negative and Positive Pressure Be Used in the Same Facility?
Yes. A facility does not always have to choose one pressure type for every room. A production area, for example, may use positive pressure to protect a sensitive product, while a separate room uses negative pressure to contain hazardous material.
Connected rooms can also have different pressure levels. This creates a pressure cascade. For product protection, a cascade may run from higher to lower pressure:
Critical cleanroom → support room → airlock room → corridor
For containment, the hazardous area can be kept at the lowest pressure so air from nearby spaces moves towards it. This approach allows one facility to manage both product protection and containment where the process requires both.
Do Negative and Positive Pressure Affect ISO Classification?
Pressure types do not determine a cleanroom’s ISO classification. ISO 14644-1 classifies cleanrooms according to airborne particle concentration. Positive and negative pressure describe the pressure relationship between connected spaces.
A negative-pressure cleanroom can therefore meet a specified ISO class. Likewise, positive and negative pressure alone does not make a room ISO classified. This means the required ISO cleanliness class and pressure type must be considered separately when designing the cleanroom.
What Pressure Differential Is Used for Positive and Negative Cleanrooms?
The required pressure differential depends on the cleanroom application, adjacent space, contamination risk, and applicable standard. For sterile medicinal-product manufacturing, EU GMP Annex 1 gives 10 Pa as a minimum guidance value between adjacent rooms of different grades. Other applications may require different values based on their contamination-control or containment needs.
In a positive-pressure cleanroom, the differential must maintain higher pressure than the specified adjacent space. In a negative-pressure cleanroom, it must maintain lower pressure. The final setpoint should support the required airflow direction during normal operation.
Which One Do You Need?
Choose the pressure type based on where the contamination comes from and what needs protection. Use positive pressure when outside particles or contaminants could affect the product or process. Use negative pressure when the process generates airborne contaminants that must be contained within the room.
When a facility needs both product protection and containment, separate positive and negative pressure zones can be used for different areas.
Wrapping Up
Positive and negative cleanrooms manage contamination in different ways, so the right setup starts with the specific risk in your process. The decision should account for what needs protection, the surrounding spaces, and the pressure relationship required to control air movement.
In some facilities, one pressure type is enough. Others need both positive and negative zones to manage product protection and containment within the same cleanroom system.
At Sunnyda Clean Room, we consider these requirements together with your cleanroom layout, filtration, and HVAC design to develop a pressure strategy that fits your application.
Frequently Asked Questions
Can a negative pressure cleanroom still have a high ISO classification?
Yes. Pressure direction and ISO classification are separate. ISO classification is based on airborne particle concentration, while negative pressure controls how air moves between the cleanroom and adjacent spaces.
What happens if a positive-pressure cleanroom loses pressure?
The intended outward airflow may weaken or stop. If the room falls below the pressure of an adjacent space, outside air and contaminants may enter through openings and leakage paths.
What happens if a negative-pressure cleanroom loses pressure?
Containment can be affected. If the required negative pressure is not maintained, airborne contaminants may move from the controlled room into surrounding spaces.
How is positive or negative pressure checked?
Differential pressure gauges, sensors, or transmitters measure the pressure difference between the cleanroom and a defined reference space, such as a corridor or airlock. Critical applications may also use continuous monitoring and alarms to detect pressure changes.
Can a facility use both positive and negative pressure cleanrooms?
Yes. Positive and negative pressure zones can be used within the same facility when different processes have different contamination risks. The pressure relationships between these areas must be planned so each zone maintains its intended function.
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