In the operating areas and isolation wards of Charité Hospital in Berlin, Germany, an invisible “air gate” precisely defines the boundaries of life with Pascal values. This is one of the largest hospitals in Europe, where the positive pressure system in the operating rooms delivers more than 2 cubic meters of HEPA -filtered air per minute, while the adjacent negative pressure isolation wards for infectious diseases ensure that not a single potentially contaminated airflow leaks out. Behind this system lies the core physical law of modern biosafety.
What is Positive and Negative Pressure Control?
Positive and negative pressure control is the core principle of clean space design. Essentially, it utilizes minute pressure differences to achieve directional control of airflow.
In a typical design, the operating room maintains positive pressure (+15 Pascals), the adjacent anesthesia preparation room maintains a lower positive pressure (+10 Pascals), and the soiled waste disposal room maintains negative pressure (-5 Pascals). This pressure gradient ensures that air flows only from areas of high cleanliness to areas of low cleanliness, effectively preventing the backflow of pollutants.
These pressure values may seem small—15 Pascals is equivalent to only 0.15% of atmospheres or the pressure of a flat A4 sheet of paper—but in aerodynamics, this tiny difference is enough to overcome the resistance of gaps in doors and windows, achieving reliable control over airflow direction.
HEPA/ULPA: The “Purification Core” of Positive and Negative Pressure Systems
Whether it’s a positive or negative pressure system, its effectiveness relies heavily on high-efficiency air filtration technology. HEPA filters (High-Efficiency Particulate Air Filters) are the core component of these systems, with international standards requiring a minimum filtration efficiency of 99.97% for 0.3-micron particles.
In medical cleanrooms and high-level laboratories, more stringent medical-grade HEPA (≥99.99% efficiency for 0.3-micron particles) or ULPA (Ultra-High Efficiency Particulate Air Filters, ≥99.999% efficiency for 0.12-micron particles) filters are often used.
These filters work not only through physical interception, but also through a triple mechanism of inertial impaction, interception effect, and Brownian diffusion. Interestingly, due to Brownian diffusion, HEPA filters are even more efficient at filtering particles smaller than 0.3 micrometers, such as the novel coronavirus (approximately 0.1 micrometers), reaching over 99.99%.
Positive Pressure Spaces and Applications
Positive pressure spaces continuously supply filtered clean air, creating a slightly higher internal pressure than the surrounding area. This ensures that airflow is directed outwards, preventing external contaminants from entering.
In the medical field, positive pressure environments are commonly found in:
- Operating rooms (ISO 5-7, +15 to +25 Pascals)
- Burn wards (to prevent environmental microorganisms from infecting wounds)
- Intravenous preparation centers (to ensure sterile drug preparation)
- Neonatal intensive care units (to protect infants with immature immune systems)
In their design, positive pressure systems typically employ laminar flow technology, where air flows unidirectionally and parallel at a speed of 0.3-0.5 m/s, rapidly carrying away particles generated during procedures. Studies have shown that a good positive pressure laminar flow system can reduce surgical site infection rates by more than 50%.
Negative Pressure Spaces and Their Applications
Negative pressure spaces, conversely, use a greater exhaust volume than supply volume to create a slightly lower internal pressure than the surrounding areas. This ensures that airflow is directed inwards, effectively “locking” pollutants inside for safe discharge after high-efficiency filtration.
Negative pressure environments are a critical line of defense for biosafety and are commonly found in:
- Infectious disease isolation wards (e.g., tuberculosis, COVID-19 wards)
- P2/P3 level biosafety laboratories
- Pathology departments and anatomy labs
- Chemotherapy drug preparation rooms
The Coordination and Balance of Positive and Negative Pressure Systems
Modern medical and research buildings are often complex combinations of positive and negative pressure spaces. The key lies in the smooth transition between areas and the stability of the pressure differential.
A typical comprehensive hospital may include:
- 1. Core clean area (operating department, ICU): highest positive pressure;
- 2. General clean area (general wards, examination rooms): moderate positive pressure;
- 3. Potentially contaminated area (laboratory, radiology department): slight positive pressure or balanced;
- 4. Contaminated area (isolation wards, waste disposal): negative pressure;
From the Berlin medical center, in which Trenntech was involved, to the laboratory cluster at the University Hospital of Munich, the combination of positive and negative pressure systems with high-efficiency filtration technology has become the cornerstone of modern biosafety and infection control. This invisible technology not only safeguards every precise operation in the operating room, but also builds a defense against the spread of disease in isolation wards, and ensures the safety boundaries of high-risk research in high-level laboratories. Every directional flow of air, every precise interception of filter media, together constitutes the invisible yet robust wall between humans and the microbial world.