HEPA Filter Size Chart

In scientific research, maintaining a controlled and sterile environment is vital to ensure accurate and reliable results. High efficiency particulate air (HEPA) filters are designed to efficiently remove airborne particles and contaminants, making them indispensable tools in laboratories and cleanroom facilities. Laminar flow hoods use HEPA filters to create the contaminant-free workspace essential for sensitive experiments, pharmaceutical production, and other critical scientific processes.

Some filters claim HEPA status simply because the filter has a most penetrating particle size of 0.3 µm. However, HEPA filters should be certified against a well-defined standard in order to gain this title. The HEPA filter efficiency chart outlines the HEPA classes and groupings in terms of actual filtration efficiency.

HEPA Filter Efficiency

---

The exact definition of HEPA filter efficiency can lead to some confusion in both ASME and ISO standards. A common misconception is that the efficiency rating applies to all particles of all sizes (e.g., the filter will remove 95% of all particles from the air). Alternatively, the efficiency is often misunderstood as describing the best possible efficiency that a filter can achieve.

HEPA filter efficiency is defined at a particular particle size, known as the most penetrating particle size (MPPS). The MPPS is the particle size which will most readily passes through the filter material. For EPA, HEPA, and ULPA filters, the MPPS is around 0.3 µm in size. Therefore, the stated efficiency is often higher for both smaller and larger particles. It is counterintuitive that below the stated particle size the efficiency should increase, but it does.

Below are graphs that show the typical filtration efficiency curve of a HEPA filter. The minimum filtration occurs at 0.3 µm. This curve is produced by overlapping filtration mechanisms. Some mechanisms work only on particles greater than the size of the gaps within the filter while others only work on smaller particles.

HEPA Filter Efficiency Chart

---

HEPA filters should adhere to specific well-defined standards such as:

• ISO 29463-1:2011 High-efficiency Filters and Filter Media for Removing Particles in Air
• ASME AG-1 Code on Nuclear Air and Gas Treatment

For each standard, filters much achieve a minimum level of particle filtration to achieve HEPA status. For the ISO standards, the minimum filtration that is counted as ‘high efficiency’ is 99.95%. This means that 99.95% of particles are filtered from the air. For the ASME standard, HEPA filters must remove at least 99.97% of particles, before they are classed as high efficiency.

The ISO standard provides a range of filter groups that include EPA, HEPA, and ULPA filters, and defines the minimum filtration efficiency for each group. The table summarizes the different groups along with the commonly used filter classification that each group falls under.

Filter Class	Filter Group	Efficiency	PPM Penetration
E11	ISO 15 E	≥95%	50,000
E11	ISO 20 E	≥99%	10,000
E12	ISO 25 E	≥99.5%	5,000
E12	ISO 30 E	≥99.9%	1,000
H13	ISO 35 H	≥99.95%	500
H13	ISO 40 H	≥99.99%	100
H14	ISO 45 H	≥99.995%	50
H14	ISO 50 U	≥99.999%	10
U15	ISO 55 U	≥99.9995%	5
U15	ISO 60 U	≥99.9999%	1
U16	ISO 65 U	≥99.99995%	0.5
U16	ISO 70 U	≥99.99999%	0.1
U17	ISO 75 U	≥99.999995%	0.05

The ‘PPM Penetration’ value is a percentage that represents the maximum amount of MPPS particles that can completely penetrate the HEPA filter. This value is shown in parts per million (ppm) and is directly related to the efficiency. The ‘Efficiency’ represents the percentage of MPPS particles that a filter will absorb or intercept.

Laminar Flow Hood

Browse now