Filter Classes, Size and Grades

HEPA filters have found many important applications in the industrial, medical, pharmaceutical, and microelectronic sectors. These diverse applications have resulted in a number of industrial and governmental specifications. In general, these specifications can be grouped into five construction grades and three performance types that provide a range of materials, manufacturing techniques, performance characteristics, and costs for different applications and user preferences. A standard covering the grades and types of HEPA filters has been issued as IEST-RP-CC001.3 by the Institute of Environmental Sciences and Technology. This standard lists the following classifications.

Filter Construction Classification Grades

Grade 1 Fire-Resistant Filters. Filters of this grade must contain fire-resistant materials that may ignite when the filter is exposed to hot air or fire, but will not continue to burn once the ignition source is removed. The filter must exhibit a specified retention efficiency after exposure to no more than 700 50 degrees Fahrenheit (371 10 degrees Celsius). These filters comply with ASME AG-1, Section FC.2

Grade 2 Semicombustible Filters. This grade costs less, but provides a lower level of protection against elevated temperature than Grade 1. For this reason, the user should evaluate application of this filter grade with the individual fire propagation hazards in the area of use. This filter type will fail at temperatures much lower than Grade 1. These filters comply with UL 586.20

Grade 3 Combustible Filters. This grade covers filters required for certain service requirements that permit acceptance of the combustibility hazard. Grade 3 filters are readily combustible and are used only where high-value product recovery by incineration is desirable, disposal of volumes are critical, or exposure to chemical atmospheres might be incompatible with the use of a HEPA filter incorporating a medium of glass fibers. It should be noted that manufacture of a combustible HEPA filter medium formulated from asbestos and cellulose has been discontinued for more than a decade because of the hazards associated with its use and the resulting low demand. Specialty filter media for recovery of precious metals by incineration are still available. These filters comply with UL 900, Class 1.21