Air filtration : classes and efficiency of filters (MERV, ISO)

Depending on their design, air filters (bag filters or cartridges) have different degrees of efficiency towards dust particles. This page aims at detailing the different norms for filter efficiency (MERV, ISO) and to help the reader chose the right filter specification depending on the application intended.

1. Dust filtration efficiency norms and charts

What are the MERV ratings for filters ?

1.1 General filters (filtration rated until 1 microns)

There are different norms in US, Europe, and international (ISO), it is thus sometimes difficult for companies and people to find their way in those different standards, read suppliers datasheets and properly specify their needs. The norms most often referred to are ISO16890 (recent international norm that is trying to unify how filter efficiency is defined), EN 779 in Europe and ASHRAE MERV in US. The different norms and those correspondences are given on the charts below :

ISO 16890

ISO Coarse

ISO Coarse

ISO Coarse

ISO Coarse

ISO ePM10

ISO ePM2.5

ISO ePM10

ISO ePM1

ISO ePM2.5

ISO ePM10

ISO ePM1

ISO ePM2.5

ISO ePM10

ISO ePM1

ISO ePM2.5

ISO ePM10

<50%

<50%

>50%

>50%

>50%

>50%

>60%

>50%

>65%

>80%

>70%

>80%

>90%

>80%

>90%

>95%

EN779 2012

G1

G2

G3

G4

M5

M6

F7

F8

F9

ASHRAE MERV

1

2-3-4

5-6

7-8

9-10

11-12

13

14

15

Table 1 : air filtration norms on dust - correspondence ISO, EN, MERV

The table is based on the new ISO norm which is actually defining, for each filter, an efficiency on particles of 1 micron, 2.5 microns and 10 microns. To be noted however that those filters are also often able to filter smaller particles to a certain extent. The table below gives an idea (it's not a norm here) on the filtration that can be achieved for those different filters. The actual efficiency will depend on the filter used and should be checked with the supplier.

ISO 16890	ISO Coarse	ISO Coarse	ISO Coarse	ISO Coarse	ISO ePM10	ISO ePM2.5	ISO ePM10	ISO ePM1	ISO ePM2.5	ISO ePM10	ISO ePM1	ISO ePM2.5	ISO ePM10	ISO ePM1	ISO ePM2.5	ISO ePM10
	<50%	<50%	>50%	>50%	>50%	>50%	>60%	>50%	>65%	>80%	>70%	>80%	>90%	>80%	>90%	>95%
EN779 2012	G1	G2	G3	G4	M5	M6		F7			F8			F9
AHSRAE MERV	1	2-3-4	5-6	7-8	9-10	11-12		13			14			15
Typical initial efficiency (% of particles captured)
0.1 μm					0-10	5-15		25-35			35-45			45-60
0.3 μm				0-5	5-15	10-25		45-60			65-75			75-85
0.5 μm			0-5	5-15	15-30	20-40		60-75			80-90			90-95
1 μm		0-5	5-15	15-35	30-50	50-65		85-95			95-98			>99
3 μm	0-5	5-15	15-35	30-55	70-90	85-95		>98			>99			>99
5 μm	5-15	15-35	35-70	60-90	90-99	95-99		>99			>99			>99
10 μm	40-50	50-70	70-85	85-98	>98	>99		>99			>99			>99

Chart 2 : comparison of filter classes, air filters efficiency on dust

1.2 High efficiency filters (EPA, HEPA, ULPA)

For specific applications requiring to catch the finer dusts, other kind of filters are required. Those filters are quite specific, for example in pharma, or to create atmospheres without particles in clean room, and may not be needed in most of the powder handling industries.

The norms that apply to this kind of filters are EN 1822 in Europe and ASHRAE MERV in US.

EN1822	E10	E11	E12	H13	H14	U15	U16	U17
AHSRAE MERV	16	17	18	19	20
Initial efficiency (% of particles captured)
0.3 micron	>85	>95	>99.5	>99.99	>99.999	>99.9999	>99.99999	>99.999999

Chart 3 : MERV and EN1822 equivalence and efficiency

2. How to chose the right dust filter efficiency ?

In the context of powder handling processes, the choice of a filter must be determined depending on the requirements of the application, and on the particle size distribution of the dust to treat.

Filter	Application	Kind of foreign bodies / dust captured
G1 G2	Insect screens	Insects Hairs Sands Spores, pollens Dust mites Textile fibers
G3 G4	1st filters for AHU, pre-filters for F class filters
M5	Air filter for industrial areas (warehouses, garages...)	Spores, pollens Bacteria and germs on host particles
M5 M6 F7	Main filters for AHU Pre-filters for E class filters
F7 F8 F9	Main filters for AHU in sensitive environments (hospitals...) Pre-filters for H class filters	Fumes, smoke
E10 E11 E12	Final filters high purity rooms (lab, pharma factories...)	Germs, bacteria, virus, smoke
E11	Final filters for clean room
E12 H13	Final filters for clean room	Fumes Radioactive particles
H14 U15 U16	Final filters for super clean room	Aerosols

Table 4 : How to chose a filter efficiency

For example, in air handling units it is usual to have 1st a G4 filter (ISO coarse) to catch all the big foreign bodies that may be carried by air, and then a F7 filter that will carry the actual fine filtration to ensure a clean air. The G4 filter is then used to protect the F7 filter and prolong its life span.

When handling a powder that must not escape to the environment, like a pharmaceuticals or some probiotics in food industries, high efficiency filters (typically H13) may be used.

Having the particle size distribution of the dust to filter also helps to avoid overspecifying a filter. For instance, if the PSD shows that there is no particles below 10 microns in the powder processed, a F7 filter may be sufficient to vent the process, not requiring a HEPA filter.