What is PM2.5?

There are many pollutants present in the air. Particles that float in the air and resemble dust are known as Particulate Matter (PM). The size of these particulate matter varies. Particles with a diameter of ≤2.5 micrometers (μm) are called Fine Particulate Matter (PM2.5), measured in micrograms per cubic meter (μg/m³).
PM2.5 has a diameter less than 1/28th the thickness of a human hair. These extremely fine particles can penetrate the alveoli in the lungs and directly enter the bloodstream, circulating throughout the body. Therefore, the impact on human health and the ecosystem is significant and cannot be ignored.

Finer Particles Cause Greater Harm to the Human Body

Research conducted by the German National Environmental and Health Research Center has confirmed that smaller particulate matter in air pollution poses greater risks to human health. In PM2.5, a significant portion consists of particles with diameters below 1 micrometer, especially those ≤0.1 micrometers, which can cause harm to the human body.

These research findings provide a new direction, suggesting that we should focus more on even smaller particulate matter rather than just PM2.5.

12 Common Air Pollutants

1. Pollen: A common allergen that can cause nasal allergies and persistent runny nose. Severe exposure may also lead to redness and swelling in the eyes and other areas.
2. Hair: Not just human hair, but pet hair can also cause allergies, leading to sneezing and a runny nose.
3. Dust: Cleaning can easily stir up large amounts of dust. Additionally, cleaning air conditioner filters can also cause persistent sneezing.
4. Dust Mites: In Hong Kong’s humid environment, dust mites thrive in sofas, carpets, blankets, etc. Inhalation can trigger asthma.
5. Dust: Divided into airborne dust and settled dust, it is more common in newly decorated or under-construction houses and is a common allergen.
6. Toluene: Commonly found in paints, nail polish, and cosmetics. Special attention is needed in enclosed environments.
7. Formaldehyde: Newly decorated houses and newly purchased furniture may emit excessive formaldehyde, which is also a common carcinogen.
8. Viruses: Various viruses inevitably exist in the air. Using HEPA filters and similar can intercept some harmful particles.
9. Bacteria: Common indoor air pollutants, with bacterial concentrations in hospitals occasionally exceeding standards.
10. Mold: Humid environments easily breed mold, and spores dispersed in the air can extend its impact.
11. Odors: Cooking fumes and pet odors can be partially removed using activated carbon filters to improve air quality.
12. PM2.5: Fine particles that penetrate deep into the lungs, causing long-term effects. They are also the main target for removal by current air purifiers.

Why Use Air Filters?

Air Conditioning Systems Need Protection from Air Filters

Air conditioning maintenance personnel often encounter the following issues after prolonged use of the air conditioning system:

1. Fans, heat exchangers, dampers, ducts, and other components become severely dusty and emit unpleasant odors.
2. After several years of operation, airflow and heating/cooling capacity decrease, making it difficult to meet original design requirements.
3. Black stains appear around the supply vents of comfort air conditioning.
4. Indoor occupants complain of polluted air and physical discomfort.
5. High-efficiency air filters in clean room systems experience a rapid increase in resistance, sometimes requiring replacement within 1-2 years.
6. Currently used metal/nylon screens and synthetic non-woven fabrics in air conditioners do not show continuous resistance increase even without cleaning. The main cause of these problems is dust. The filtration capability of washable filters is limited. Besides shutting down the air conditioning system for cleaning or replacing expensive system components, a more effective measure is to install high-efficiency air filters.(There are very few companies in the country that offer air conditioning system cleaning services.) Investigations and studies conducted in developed countries have proven that glass fiber bag filters are the most economical and effective choice for users.

Various Cleanliness Levels of Clean Rooms Rely on High-Efficiency or Ultra-High-Efficiency Air Filters
The concentration of dust particles inside a clean room is thousands of times lower than outdoors (in terms of count alone). To achieve a certain level of cleanliness, high-efficiency air filters must be installed to filter the incoming air.

Selecting the Appropriate Filter

Determine the efficiency of each level of filter based on the usage requirements of the location.

Typically, the final stage filter determines the air cleanliness level. Initial stage filters protect the final high-efficiency filters, thereby extending the lifespan of the high-efficiency filters, reducing maintenance costs, and ensuring the normal operation of the air conditioning system.

Specific Process:

1. Determine the high-efficiency filter based on cleanliness requirements.
2. Choose to use high-efficiency filters in conjunction with medium-efficiency and initial-efficiency filters, typically called pre-filters (the first layer).
3. Select pre-filters by considering the usage environment, spare parts costs, operational energy consumption (currently, energy consumption is not a major concern for domestic users), maintenance, and supply factors.
4. Pay special attention to the following: Clean room end-stage high-efficiency air filters need to be protected by filters with an efficiency of no less than F8 (mainly for fresh air processing). Air conditioning systems also require protection from air filters.
5. Key points in selecting filters: High-efficiency filters must have reliable performance, initial and medium-efficiency filter specifications should be reasonable, and maintenance should be convenient.

Principles of Air Filtration

1. Electrostatic EffectsDue to certain reasons, fibers and particles may carry charges, creating electrostatic effects. Electrostatic filters can significantly improve filtration performance. The reason is that electrostatic forces alter the particles’ trajectories, causing them to collide with obstacles more frequently and adhere more firmly to the medium. Materials that can hold an electrostatic charge for a long time are also known as “electret” materials. After charging, the filter material’s resistance remains unchanged while its filtration performance is significantly enhanced. Electrostatic effects do not play a decisive role in filtration but serve as an auxiliary function.
2. Chemical FiltrationChemical filters primarily involve the selective adsorption of harmful gas molecules. Activated carbon materials contain numerous microscopic pores, offering a large adsorption surface area. In grain-sized activated carbon, the internal surface area of the micropores can reach several square meters. When free molecules come into contact with activated carbon, they condense into liquid within the micropores due to capillary action and adhere to the material, sometimes chemically reacting to form solid substances or harmless gases, a process known as chemisorption. Physical adsorption, where no obvious chemical reaction occurs, can be reversed by heating or steam to release the adsorbed gases, allowing the activated carbon to be regenerated. However, the adsorption capacity of activated carbon diminishes over time, and the filter must be replaced once it reaches a certain level of saturation.

Recommendations for Selecting Filters

1. Choose pre-filters with sufficiently high efficiency.
2. Select filters with large filtration areas.
3. High-efficiency air filters must be tested individually.
4. For the same airflow, choose filters with lower initial resistance.
5. Use standard-sized filters for versatility.

Filter Parameters

Face Velocity and Filtration Velocity

Face velocity and filtration velocity reflect the filter’s ability to pass airflow.

• Face Velocity: Refers to the speed of airflow passing through the filter’s cross-section, typically measured in m/s. It is calculated as V = (Q/F) × 3600.
• Filtration Velocity: Refers to the speed of airflow passing through the filter material’s surface area, typically measured in L/cm²·min or cm/s. Filtration velocity indicates the filter material’s passage capacity and filtration performance. Lower filtration velocities generally result in higher efficiency but increase the filter material’s resistance.

Filtration Efficiency

The “filtration efficiency” of an air filter is the ratio of the amount of dust captured by the filter to the dust amount in the incoming air:

Filtration Efficiency = (Dust Captured by Filter) / (Dust in Upstream Air) = 1 – (Dust in Downstream Air) / (Dust in Upstream Air)

Although the concept seems straightforward, its meaning and numerical value can vary significantly depending on the testing method.

Factors influencing filtration efficiency include the definition of “amount” of dust, which can vary widely (total weight, particle count, specific particle sizes, all dust particles, etc.). This leads to diverse efficiency values.

Different testing methods by various countries and manufacturers result in varying interpretations and expressions of filter efficiency. Without a standardized testing method, filtration efficiency is not meaningful.

Penetration Rate

Penetration rate indicates the extent to which dust particles can pass through the filter after filtration:

K = (1 – η) × 100%

Resistance

The resistance of a filter refers to the opposition it poses to airflow. It consists of two main components: resistance from the filter material and resistance from the filter structure.

Experience shows that, given the filter material’s performance, the filter’s resistance is greatly influenced by its structure, potentially causing a resistance change of about 50 Pa.

As filters accumulate dust, their resistance increases. When resistance reaches a specified value, the filter must be replaced.

• Initial Pressure Drop: Refers to the resistance of a new filter.
• Final Pressure Drop: Refers to the resistance value at which the filter is discarded.

During design, a representative resistance value is often needed to calculate the system’s design airflow, typically the average of the initial and final resistances.

Dust Holding Capacity

Simply put, it is the filter’s ability to hold dust. The dust holding capacity is directly related to the filter’s lifespan.

It is usually defined as the dust mass on the filter when the final resistance reaches twice the initial resistance or when efficiency drops below 85% of the initial efficiency.

If the filter operates at airflow rates exceeding the rated capacity, resistance increases more rapidly with dust accumulation.

During the dust holding process, filters with low efficiency tend to show an initial increase in efficiency followed by a decrease. Low-efficiency filters accumulate more dust, have sparse filter materials, and dust particles can more easily penetrate the filter material or shed, causing secondary pollution. In contrast, high-efficiency filters generally maintain or increase efficiency as dust accumulates.

• For an airflow of 1000 m³/h:
  • Folded non-woven fabric filters have a dust holding capacity of around 100g.
  • Glass fiber filters hold 250-300g.
  • High-efficiency filters hold approximately 400-500g.

The dust holding capacity varies with filter size even within the same

Indoor Air Quality Standards for Various Indoor Air Pollutants

How Often Should Filters Be Replaced?

• Pre-Filters: Every 3 months
• Medium-Efficiency Filters: Every 6 months
• High-Efficiency Filters: Every 12 months

The replacement period may be shortened based on the pollution level of the usage area and operating conditions. To maintain the high-quality performance of the device, regular replacement is essential.

Do Air Filters Have an Expiration Date?

Store unused, brand-new air filters in sealed bags, keeping them in a dry, cool, and moisture-free place. They generally do not have strict expiration dates. However, it is conservatively recommended to use them within 1-2 years.