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March 2016

MERV Ratings Lack Important Filter Efficiency Details

By: Robert Martin, CAFS, Associate Category Manager  |   03.23.16

When evaluating filtration efficiency, filter specifiers typically turn to the filter’s MERV (Minimum Efficiency Reporting Value) as determined by the ASHRAE 52.2 Test Standard. A MERV indicates the filter’s minimum fractional particle size efficiency. A MERV 1 is the least efficient, and MERV 16 is the most efficient. 

MERV provides a simple basis for choosing a filter, but it doesn’t always provide the most complete picture of a filter’s in-use performance. To get that complete picture, it’s necessary to look at the filter’s efficiency in capturing submicron particles. 

Why are submicron particles so important? Lung-damaging dust can be as small as 0.5 microns while some bacteria can be as small as 0.3 microns. Airborne particles under 2.5 microns in size are most likely to travel to the deepest part of the lungs, where they can cause a variety of respiratory health problems. 

E1 & E2 Particles

Information on a filter’s efficiency in removing E1 and E2 particles is available in the filter’s full ASHRAE 52.2 test report (specifically the Fractional Particle Size vs. Particle Diameter Curve), which provides the efficiency of the filter over three particle size ranges: E1 (very fine particles in the 0.3 to 1.0 micron range), E2 (fine particles in the 1.0 to 3.0 micron range), and E3 (coarse particles in the 3.0 to 10.0 micron range).

 

 

Standard 52.2 Minimum Efficiency Reporting Value

(MERV)

Composite Average Particle Size Efficiency, % in Size Range, µm

Average Arrestance, %

 

Range 1

0.30-1.0

Range 2

1.0-3.0

Range3

3.0-10.0

 

 

 

1

n/a

n/a

E3<20

Aavg<65

2

n/a

n/a

E3<20

65<Aavg<75

3

n/a

n/a

E3<20

70<Aavg<75

4

n/a

n/a

E3<20

75<Aavg

5

n/a

n/a

20<E3<35

n/a

6

n/a

n/a

35<E3<50

n/a

7

n/a

n/a

50<E3<50

n/a

8

n/a

20<E2

70<E3

n/a

9

n/a

35<E2

75<E3

n/a

10

n/a

50<E2<65

80<E3

n/a

11

20<E1

65<E2<80

85<E3

n/a

12

35<E1

80<E2

90<E3

n/a

13

50<E1

85<E2

90<E3

n/a

14

75<E1<85

90<E2

95<E3

n/a

15

85<E1<95

90<E2

95<E3

n/a

16

95<E1

95<E2

95<E3

n/a

Source: Minimum Efficiency Reporting Value (MERV) Parameters Chart, National Air Filtration Association, 2014.

 High E1 and E2 efficiencies are critical for providing good IAQ and helping building occupants avoid illness due to poor IAQ. However, many pleated filters today (especially at commonly used MERV 8) have very low E1 and E2 efficiencies. In fact, under the ASHRAE 52.2 Standard, there was no minimum requirement for E1 particulate capture below a MERV 14 and no minimum requirement for E2 particle capture below a MERV 9 until recently. 

Recent testing showed that MERV 8 filters using electret-treated filter media performed on average 20 percentage points higher in E1 and E2 efficiency than mechanical filters on the market today. 

 

Filter A

Filter B

Filter C

Media Type

Mechanical

Mechanical

Electret-Treated

Advertised MERV

8

8

8

Tested MERV

9

7

9

E1 Efficiency

8.5

8.3

28.4

E2 Efficiency

53.9

40.9

67.1

E3 Efficiency

78.9

66.5

77.2

Testing conducted in June 2014 by LMS Technologies, Inc. Data are an average of two initial 52.2 tests and one full 52.2 test per filter model.

The electrostatic effects created in an electret-charged synthetic filter media are particularly useful in increasing the capture efficiency for submicron particles. Mechanical-only media, on the other hand, depend solely on using the physical structure of the media to stop the flow of airborne particles, and this can create airflow resistance in the filter, which can lead to higher energy use/cost. 

Conclusion

Providing superior indoor air quality (IAQ) by removing submicron particles from the breathing air can improve health, work performance and school performance, as well as reduce health care costs and consequently be a source of substantial economic benefits. By looking beyond a filter’s MERV to its efficiency in removing E1 and E2 particles, specifiers will obtain the information they need to select the most efficient filter.

 

Comments


Previous Comments


A further suggestion is that attention be paid to documenting the amount of ventilation air actually delivered to the building occupants, as VAV boxes serving conference rooms are typically causing ventilation deficiencies.
By: David Bearg