HEPA filters in Pharmaceutical Industry

High efficiency particulate air (HEPA) filters and ultra-low particulate air (ULPA) filters are air filters designed to trap a vast majority of very small particulate contaminants from an air stream.

HEPA Basics

Definition and Efficiency

Air filters must satisfy certain standards of efficiency — most commonly those developed by the US Department of Energy (DOE) — in order to qualify as a HEPA filter. The US standard (DOE-STD-3020-2005) requires that a HEPA filter be capable of removing 99.97% of contaminant particles 0.3 μm in diameter. Most standards also specify that HEPA filters must feature minimal pressure drop and maximum airflow when in operation.

 A filter's percent efficiency can be calculated using the simple equation below.

                                                                                                                                                                                                              where:

E = percent efficiency

D = downstream concentration (of contaminants)

U = upstream concentration (of contaminants)

Particle Size and Filtration Method

While the US HEPA standard usage of 0.3 micrometer particles to describe efficiency may seem arbitrary, particles of this size are actually the most difficult to filter, rendering them a kind of "worst-case scenario" reference particle. The reasons for this difficulty in filtration are described below.

HEPA filter media is made up of countless randomly-arranged fibers which together form a dense mat; when air flows through the filter, the media captures and contains contaminant particles throughout its depth.

Filter fibers trap contaminants using three primary methods:

 • Interception takes place when a contaminant particle passes within the distance equal to one particle's radius of a filter fiber, resulting in it touching the fiber and being removed from the airflow. Particles further than one particle radius from a fiber will not be trapped.

•Inertial impaction occurs when a large particle, unable to adjust to the change in air direction near a filter fiber, becomes trapped on the fiber. The particle's inertia ensures that it continues along its original path instead of circumventing the fiber, resulting in its capture.

•Diffusion relies on the Brownian motion of gas particles. Small particles (typically 0.1 μm or less) tend to travel on a streamline in an erratic fashion, making random motions as they interact with gas molecules. This erratic motion causes the contaminant particles to become stuck to filter fibers

Understanding these three methods makes it clear why particles around 0.3 micrometers are most difficult to filter. Particles less than 0.1 micrometers are easily trapped due to diffusion while particles larger than 0.4 micrometers are trapped by inertial impaction. Particles between 0.1 and 0.4 μm are therefore too large for effective diffusion and too small for inertial impaction and efficient interception, so that the filter's efficiency drops within this range. By specifying a HEPA filter's efficiency at 0.3 μm, standards bodies are really describing a variant of the filter's minimum efficiency.

ULPA Basics

 

Ultra-low particulate (or sometimes "penetration") air (ULPA) filters are closely related to HEPA filters but are even more efficient. ULPA filters are specified to remove 99.999% of contaminants 0.12 μm or larger in diameter. The chart below shows the overlap in the capabilities of ULPA and HEPA filters.

 European Standards for HEPA and ULPA

 

The European Union standard for both HEPA and ULPA filters — EN 1822 — classifies filters into different classes depending on their efficiency. All EN 1822 specifications are based on a filter's ability to trap and contain the most penetrating particle size (MPPS) particular to the filter. The MPPS is typically determined by a laser spectrometer or electrostatic classifier.

 

European filter classes and relevant specs are listed in the table below. Note the radical difference between the European and American definition of HEPA efficiency, particularly that the EU standard permits HEPA filters with efficiencies as low as 85%.

 

Classification	Filter type	Percentage efficiency at MPPS
E10	HEPA	≥ 85
E11	HEPA	≥ 95
E12	HEPA	≥ 99.5
H13	HEPA	≥ 99.95
H14	HEPA	≥ 99.995
U15	ULPA	≥ 99.9995
U16	ULPA	≥ 99.99995
U17	ULPA	≥ 99.999995

Construction

There are several types of HEPA and ULPA filters. As aforementioned they can be classified to a standard class based on efficiency, but they are also subdivided by the construction of the filters itself. Some of the most common construction types include the following:          

                      

Box / Rigid cell filter – These filters are composed of either a box or rigid cell that is filled with filter media.   

 

Cartridge filter – These filters have a cylindrical housings that contains a fabric or paper filter cartridge. The cartridge is typically replaceable.

Fan filter – Filters made from fiberglass or mesh-type materials and are designed to prevent the accumulation of lint and other large particles around the motor and fan.

Filter mat – Filters made of fibers that trap particles entrained in the flow of air or gas. Common configurations include bulk rolls, pads, and re-loadable frames.

Panel filter – Filters that are typically made of fiberglass or mesh-type materials. They are designed to prevent the accumulation of lint and other large particles in and around heating and ventilation systems.      

Pre-filter – Filters positioned before the main filter in a filtration system. These filters reduce particulate loading in the system and allow the filter to operate more efficiently due to lighter particulate loading. 

Test for HEPA filter

HEPA filters are installed in any area to purify the air to maintain the cleanroom classification, so HEPA filters should be routinely monitored for any type of leakage or filter damage. Because if any leakage is present in grass kit of HEPA filter or if HEPA filter is damaged then air quality will be poor and it will contaminate the cleanroom.

DOP Test (Dispersed oil particulate)

To check, the integrity of HEPA filter DOP test is used in which aerosol of poly alpha-olefin is used. In previous practices, DOP (Dioctyl phthalate) was used but it was found to be carcinogenic so its use is prohibited but the name DOP is still in use and it represents Dispersed oil particulate test. In DOP test aerosol of PAO is generated and it is scanned by prob of photometer first on upstream position and later on downstream position. The scanning is done by keeping a probe about 25mm away from the HEPA filter face. Scanning results should not exceed 0.01%. If the downstream position value is above 0.01% then filter needs inspection, means there may be any leakage in filter or gas kit so after rectification again scan with scanning prob and if the value is within range filter is pass and if again value exceeds, the filter should be replaced with a new filter. The leaked filter may be repaired but the repaired area should be only 0.5% of total filter surface.

According to PIC/S HEPA filter of class A and B should be tested for leakage after 6 months and class C and D after 1 year.

Cleaning of HEPA filter

It is a general question that can we wash the HEPA filter? The answer is that washing of HEPA filter is not recommended because washing with water damages the glass fibers arranged in HEPA filters.

Some people Clean the HEPA filter using Vacuum cleaner or compressed air but it is also not recommended because there are chances of damage to the glass fibers with vacuum cleaning or compressed air pressure. So the best practice is to replace the HEPA filter with a new filter.

When to Change the HEPA filter

No guidelines define the period after which HEPA should be replaced generally it is mentioned the word free from leakage. So HEPA life span depends upon its usage. Many HEPA filters can be functional for year's and some may be changed after months. Pressure drop is the main indication which gives an alert to replace the HEPA filter.