A Comprehensive Analysis of Air Purification Systems in Clean Operating Rooms | Three-Stage Filtration + Airflow Organization + Precise Temperature Control (Including an Interpretation of the GB 50333 Standard)

If we compare the clean operating room department to a precision machine that safeguards human life, then the air purification and air conditioning system is its tireless heart.

The core of a clean operating suite is “controllability,” and this “control” relies almost entirely on this air purification system—invisible and intangible, yet working ceaselessly. Unlike surgical lights or ceiling-mounted towers, it is not immediately visible, yet it determines the air quality of every breath taken inside the operating room.

If surgical equipment is the weapon used to defeat disease, then the air purification system is the armor that protects both doctors and patients.

I. Standards First: The Regulatory Foundation Established by GB 50333 and WS/T 368

Before delving into technical details, it is essential to understand the regulatory framework that underpins the construction of the entire clean operating room department.

GB 50333, “Technical Standards for the Construction of Clean Operating Departments in Hospitals,” serves as the fundamental guideline for the construction of clean operating departments. On July 17, 2026, the Ministry of Housing and Urban-Rural Development issued Announcement No. 96 of 2026, approving the partially revised provisions of the national standard “Technical Specifications for the Construction of Clean Operating Departments in Hospitals” (GB 50333-2013), effective November 1, 2026. Concurrently, the title of the standard was changed to “Technical Standards for the Construction of Clean Operating Departments in Hospitals.” This revision was led by the China Academy of Building Research and jointly developed with authoritative institutions such as Tongji University, Peking Union Medical College Hospital, and the General Hospital of the Chinese People’s Liberation Army, as well as dozens of leading enterprises in the industry. It covers all disciplines, including architectural layout, cleanroom air conditioning, medical gases, electrical and fire protection systems, and construction acceptance. The revision integrates four major industry development needs: the widespread adoption of hybrid operating rooms, the prevention and control of respiratory infectious diseases, digital operating rooms, and energy conservation in line with the “Dual Carbon” goals.

Meanwhile, WS/T 368-2025 *Standard for Hospital Air Purification Management* was issued on July 30, 2025, and will officially take effect on February 1, 2026, replacing the original WS/T 368-2012 version. This standard specifies general requirements, management requirements, air purification methods for different departments, and monitoring of air purification effectiveness in hospitals. Also released at the same time was WS/T 855-2025, “Standard for Hospital Infection Control in Surgical Departments (Rooms).”

Clean operating rooms are classified into four grades—Grade I, Grade II, Grade III, and Grade IV—based on air cleanliness. Grade I clean operating rooms are suitable for surgeries with extremely high sterility requirements, such as joint replacement, organ transplantation, and cardiac surgery; Grade II is suitable for orthopedics, plastic surgery, and thoracic surgery; Grade III is suitable for general surgery, obstetrics and gynecology, and urology; and Grade IV is primarily intended for infectious and heavily contaminated surgeries. There are clear differences among the various levels of operating rooms in terms of cleanliness standards, scope of application, airflow patterns, air changes per hour, pressure differential control, and system configuration.

Weyuan Medical strictly adheres to the aforementioned national standards to ensure that every purification system—from design and selection to installation—meets regulatory requirements, providing clients with proven clean operating room solutions.

II. The System’s Three-Layer Filtration: Building Three Lines of Defense
Why is the air in an operating room so clean? The secret lies in a three-tiered filtration system that works like a sieve, progressively blocking dust and bacteria at each layer.

First Tier: Fresh Air Intake Filter—Guarding the Entry Point

The system draws in fresh air from outside, which is key to ensuring an adequate supply of oxygen indoors. However, outdoor air contains large amounts of dust and bacteria. The first filter is installed at the fresh air intake. Its job is to perform coarse filtration—acting like a large net to trap catkins, flying insects, and most large particles—serving as the system’s “gatekeeper.”

Second Line of Defense: Return Air and Pre-filters—Achieving Circulating Purification

To conserve energy, most of the air inside the operating room is recirculated. However, after circulating through the room, this air carries contaminants such as skin flakes from medical staff and particles generated by breathing. Medium-efficiency and high-medium-efficiency pre-filters installed within the HVAC unit are responsible for filtering out these internal contaminants. This step significantly reduces the burden on the terminal filters and is key to balancing cost and effectiveness. According to operational management standards, coarse filters are typically replaced every 1–2 months, while medium-efficiency filters are replaced every 3–6 months.

Stage 3: High-Efficiency Particulate Air (HEPA) Filter—Providing Ultimate Purification

This is the most critical stage. Before being delivered to the operating room, air that has passed through the first two filtration stages must pass through a high-efficiency or ultra-high-efficiency particulate air (HEPA) filter installed near the air supply outlet. Medical-grade HEPA filters have a filtration efficiency of at least 99.97% for particles as small as 0.3 μm (H13 grade), while H14-grade filters achieve an efficiency of 99.995% or higher. According to GB 50333 requirements, the retention rate of operating room filters for bacterial aerosols must be ≥99.9%. Bacteria typically adhere to particles with a diameter of ≥1 μm; therefore, HEPA filters are sufficient to block the vast majority of bacteria. It is precisely this final line of defense that ensures the air delivered to the core area of the operating room is ultra-clean.

These three lines of defense work in concert to form a dynamic, continuous air self-purification cycle.

III. The Magic of Airflow: How to Achieve Precise Coverage of the Operating Table?

Clean air alone is not enough; it must also be directed to where it is needed. This involves airflow organization, with the air distribution system at its core.

Centralized Air Distribution (Ceiling Diffusers)

This is standard equipment in high-level operating rooms. A massive air supply grille—with a minimum area of 2.4 m × 2.6 m for Class I operating rooms—is installed directly above the operating table, acting like an “air umbrella” that covers the entire surgical area. The airflow moves downward in a uniform, low-speed, piston-like motion, creating a main airflow zone of the highest cleanliness within the surgical area, which pushes contaminants emitted by the surgeon and patient toward the periphery and carries them away.

Class I clean operating rooms use a vertical unidirectional airflow pattern, with the average air velocity in the work area maintained between 0.20 and 0.25 m/s. The air change rate requirements for operating rooms of different classes are as follows:

Air Distribution Ceiling with Leak-Barrier Layer

This is a more advanced technology. It incorporates a leak-barrier layer—a low-resistance sub-HEPA filter material with a pressure drop of 40–80 Pa and filtration efficiency within the sub-HEPA range—beneath the air supply surface. This design breaks away from the conventional practice of placing high-efficiency filters at the end of the air supply duct, thereby reducing building floor-to-ceiling height and facilitating filter installation and maintenance. Even if minute leaks occur in the HEPA filters on the ceiling, the leak-containment layer ensures that unfiltered air does not leak downward, greatly enhancing the system’s safety redundancy.

Simply put: Good airflow design involves establishing a clean air protection zone in critical areas.

IV. Precise Environmental Control: The Art of Balancing Temperature, Humidity, and Pressure

A top-tier surgical environment controls not only cleanliness but also an invisible triangle: temperature, humidity, and pressure.

1. Temperature: Why Should the Supply Air Be Cooler Than Room Temperature?

Regulations require that the supply air temperature be lower than the room temperature. Why? Because cold air is denser and naturally sinks. This ensures that the clean airflow delivered from the ceiling is steadily directed toward the operating table, forming an effective protective layer. If warm air were supplied, it would rise, preventing the clean airflow from reaching the operating table and significantly reducing the purification effectiveness.

According to GB 50333, the temperature in a clean operating room should be maintained between 21 and 25°C, with a relative humidity of 30% to 60%. This temperature and humidity range applies to Class I through Class III clean operating rooms.

2. Humidity: The Principle of Prioritizing Humidity Control

Humidity provides an ideal breeding ground for bacteria. Excessive humidity promotes bacterial growth, while insufficient humidity can cause static electricity, which may interfere with precision equipment and cause patients’ surgical incisions to dry out. Therefore, modern cleanroom systems follow the principle of prioritizing humidity control: first, they focus on thoroughly removing moisture from the fresh air, and then precisely adjust the temperature based on indoor conditions. This is akin to wringing out wet clothes before considering how to iron them—it ensures maximum efficiency and the most stable control.

Humidifiers in cleanroom air handling units should preferably use dry steam humidifiers; no water droplets should form during the humidification process, and the water quality must meet drinking water hygiene standards. During dehumidification, the system should discharge condensate within 3 minutes of operation.

3. Pressure: The Invisible Air Curtain 

This is a sophisticated design to prevent cross-contamination. By ensuring the supply airflow exceeds the return airflow, the air pressure in high-cleanliness rooms (such as operating rooms) is maintained at a level higher than that of adjacent areas (such as corridors). Clean zones should maintain positive pressure relative to the non-clean zones they are connected to; the minimum static pressure difference should be ≥5 Pa, and the maximum static pressure difference should be less than 20 Pa. The pressure difference should not cause whistling noises or interfere with door operation. Some bidding documents require a pressure differential of ≥8 Pa between operating rooms and clean corridors, and ≥12 Pa between operating rooms and non-clean areas.

This invisible air curtain ensures that when the door is open, clean air always flows outward, preventing contaminated air from entering from the outside.

V. System Maintenance: The “Lifeline” of Air Purification Systems

A good air purification system relies 30% on design and 70% on operation and maintenance. According to the requirements of WS/T 368-2025, “Standards for Hospital Air Purification Management,” air purification technology has specific management and maintenance requirements during operation.

Filter replacement intervals are central to maintenance:

· Pre-filters: Replace every 1–2 months
· Medium-efficiency filters: Replace every 3–6 months
· High-efficiency filters: Replace every 1–2 years (dynamically adjusted based on pressure drop and test results)

Differential pressure gauges should be installed upstream and downstream of each filter stage; measuring lines must be unobstructed and securely installed. Filters should be replaced promptly when the pressure drop reaches 1.5–2 times the initial value.

Regular testing is equally essential: Key parameters—including cleanliness level, settled bacteria, airborne bacteria, air velocity, pressure differential, and temperature and humidity—should be tested quarterly, with results recorded and traceable. The enclosure of the cleanroom air handling unit must be reliably sealed—when a static pressure of 1,000 Pa is maintained inside the unit, the air leakage rate of a system with a cleanliness level of ≥1,000 should not exceed 1%.

Weyuan Medical not only provides high-quality cleanroom system design and equipment integration but also offers customers full-lifecycle operation and maintenance support services to ensure the long-term, stable operation of the system.

VI. Weyuan’s Perspective: Systematic Thinking to Build a Reliable Life-Support System

As a professional service provider in the field of medical system integration, Weyuan Medical fully understands that a clean air conditioning system is far more than a simple assembly of equipment; it requires systematic design and integration capabilities.

Seamless Coordination with Operating Table and Light Towers

The overhead towers and surgical lights we provide feature streamlined designs that minimize obstruction of clean airflow and the generation of vortices. Our operating table layouts also take the positioning of return air vents into full consideration to ensure unimpeded airflow. All of this is done to ensure that the clean “blood” pumped by this “heart” flows smoothly throughout the entire system.

Guarantee of a Comprehensive Solution

Weyuan Medical collaborates with top-tier cleanroom engineering partners to ensure the entire system operates at peak performance—from design and equipment selection through construction. We are committed to using high-quality core components from leading domestic brands, which not only guarantee reliable performance and easy maintenance but also deliver long-term value to our customers through exceptional cost-effectiveness.

A good cleanroom system is one that is virtually imperceptible to both medical staff and patients, yet provides constant protection at all times.