Protecting Building Occupants from Exposure to Biological Threats

Minimum Efficiency Reporting Value (MERV)

MERV is a measure used to describe the efficiency with which particulate filters remove particles of a specified size from an air stream.[54] The higher the MERV designation, the better the efficiency of removal, particularly for smaller particles (Table 1). MERV levels 1 through 16 are determined using the ANSI/ASHRAE Standard 52.2-2007 test method by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE).[54] Standard 52.2-2007 does not address high efficiency particulate air (HEPA) filters or ultra low penetration air (ULPA) filters (MERV 17 – 20). HEPA/ULPA filters are assigned MERVs based on their performance in accordance with standards published by the Institute of Environmental Sciences and Technology (IEST).[51]

Table 1: MERV Parameters
	Composite Average Particle Size Removal Efficiency (%) in Size Range (μm) - ANSI/ASHRAE Standard 52.2-2007			Contaminants Typically Controlled	Typical Applications
MERV	0.3–1.0	1.0–3.0	3.0–10.0	Contaminants Typically Controlled	Typical Applications
1	—	—	<20	Particles >10.0 μm: pollens, dust mites, textile/carpet fibers	Minimum filtration; residential buildings
2	—	—	<20
3	—	—	<20
4	—	—	<20
5	—	—	20–35	Particles 3 - 10.0 μm: mold, spores, cement dust	Most commercial and better residential buildings
6*	—	—	35–50
7	—	—	50–70
8	—	—	>70
9	—	<50	>85	Particles 1.0 - 3.0 μm: Legionella, lead dust, coal dust, auto emissions	Superior residential and better commercial buildings
10	—	50–65	>85
11	—	65–80	>85
12	—	>80	>90
13	<75	>90	>90	Particles 0.3 - 1.0 μm: all bacteria, most tobacco smoke, droplet nuclei, most smoke	Hospital inpatient and general surgery; superior commercial buildings
14	75–85	>90	>90
15	85–95	>90	>90
16	>95	>95	>95
IEST Standards
17	>99.97 on 0.30 μm particles, IEST Type A			Particles <0.3 μm (viruses, radon progeny, carbon dust)	Cleanrooms and pharmaceutical manufacturing
18	>99.99 on 0.30 μm particles, IEST Type C
19	>99.999 on 0.30 μm particles, IEST Type D
20	>99.9999 on 0.10–0.20 μm particles, IEST Type F

* Minimum requirement under ASHRAE standard 62.1 (Ventilation for Acceptable Indoor Air Quality). Source: Adapted with permission from ANSI/ASHRAE Standard 52.2-2007: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. Atlanta, GA: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. 2007 © American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., www.ashrae.org

For more information see:

Guidance for Filtration and Air-Cleaning Systems to Protect Building Environments from Airborne Chemical, Biological, or Radiological Attacks (April 2003)
Risk Management Guidance for Health, Safety and Environmental Security under Extraordinary Incidents. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (January 2003)
Kowalski, W, Bahnfleth, W. Airborne-microbe filtration in indoor environments. HPAC Engineering 74(1); 2002: 57-69.

*Note: The information that appears on the pages collectively known as "Protecting Building Occupants" was up-to-date and accurate when published in 2008; the materials have not been updated since original publication. Please be sure to check current resources for the most up-to-date information on this topic.