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In the hallowed halls of modern medicine, clean operating rooms are the last physical barrier protecting patients' lives. While we marvel at the advanced surgical techniques, we often overlook the air purification system that works tirelessly day and night overhead. This system is like the "respiratory system" of the operating room, and its core component---the air filter---is the "invisible defense" against bacteria and dust.

Primary filter: The vanguard in the battle.

The primary filter (pre-filter) is the first line of defense in an air purification system, typically installed at the fresh air inlet or mixing section of an air handling unit (AHU). Its role is like that of a strong and capable "vanguard," responsible for intercepting those visible "large" enemies in the air.

These filters are primarily made of non-woven fabric (synthetic fiber), metal mesh (stainless steel mesh), or nylon mesh, and have a relatively coarse structure. Their main task is to capture particles larger than 5 micrometers in diameter, such as airborne hair, lint, pollen, and large dust particles. Without the effective interception of the primary filter, these impurities will quickly clog subsequent, more sophisticated filtration devices. Therefore, primary filters require the most frequent maintenance, typically needing to be replaced every 1 to 2 months, or cleaned promptly when dust accumulation is severe to ensure sufficient fresh airflow.

Medium-efficiency filter: the "backbone" bridging the gap between upstream and downstream applications.

Although the air that has passed through the primary filter removes large particles, it still contains many fine dust particles and microorganisms. This is where the medium-efficiency filter (medium filter) comes in. Located after the primary filter and before the high-efficiency filter, it plays a crucial role in bridging the gap between the two.

Medium-efficiency filters typically employ a bag-type structure (pocket filter), filled internally with glass fiber or synthetic fiber, with a denser fiber arrangement. They effectively capture particles with diameters between 1 and 5 micrometers, such as fine dust, smoke, and some bacterial carriers. As the "backbone" of the system, medium-efficiency filters not only further purify the air, but more importantly, they protect the expensive high-efficiency filters at the end, preventing premature clogging. Generally, medium-efficiency filters should be replaced every 3 to 6 months, making them a crucial element in maintaining stable system operation.

High-efficiency air filters (HEPA) filters: the ultimate gatekeeper for victory.

If the first two stages of filtration are the foundation, then the high-efficiency air filters (HEPA ) is the "ultimate arbiter" of air quality in a clean operating room. It is usually installed at the air supply terminal in the ceiling of the operating room (ceiling HEPA) and is the last barrier before the air enters the surgical area.

HEPA filters are made of countless extremely fine glass fibers folded together, forming intricate nanoscale channels. They boast a filtration efficiency of over 99.97% for particles with a diameter of 0.3 micrometers or larger (including the vast majority of bacteria, viruses, and dust). This barrier ensures that the air delivered to the operating table is nearly sterile, significantly reducing the risk of postoperative infection. HEPA filters have a long lifespan, typically lasting more than 3 years, but require regular resistance testing (pressure drop test). Once the resistance exceeds 160% of the initial resistance, the filter must be replaced immediately.

Sub-HEPA filters: "Elite guardians" for specific scenarios.

In areas where cleanliness requirements are slightly lower than Class 100 or Class 1000 operating rooms, or as a pre-filter for HEPA systems, Sub-HEPA filters (Sub high efficiency air filter) play a unique role. Their filtration efficiency falls between medium and high efficiency, primarily targeting particles larger than 0.5 micrometers in diameter, with filtration efficiencies ranging from 95% to 99.9%.

Sub-HEPA filters are compact in structure and have moderate resistance, and are commonly used in Class III and IV clean operating rooms or clean auxiliary rooms. Like an "elite guardian," while not as stringent as HEPA filters, they are sufficient to meet the aseptic requirements of general surgeries. They are also often used as pre-filters in HEPA systems to further extend the lifespan of the final filter.

From the coarse primary filter to the precise HEPA filter, these four stages of filters work in tandem to construct a comprehensive air purification network. Though hidden in ceilings and machine rooms, working silently, they are an indispensable cornerstone of the modern medical safety system. Regular maintenance and scientific management of these "invisible defenses" safeguard the life and health of every patient.