Efficiency: (1) The ability of a device to remove particulate or gaseous material from an airstream by measuring the concentration of the material upstream and downstream of the device. (2) In the ASHRAE 52.1 Standard test method, it is a measure of the ability of a filter to remove the staining portion of the atmospheric dust from the test air. This is officially termed Atmoshpheric Dust Spot Efficiency.

Electret: Filter media to which an electrostatic charge is applied during its formation.

Electronic Air Cleaners (Two Stage): Two-stage electrically-powered filters. In the first stage the particles are charged and in the second stage they are captured.

Electrostatic Precipitators: The type of air cleaners which gives particles of dust a charge by passing the dust-laden air through a strong (50-100 kV) electrostatic field. This causes particles to be attracted oppositely charged plates so they can be removed from an airstream. These devices are primarily used for stack gas cleaning.

Electrostatic Filters (Passive): A mechanical filter whose collection efficiency is augmented by the development of an electrostatic charge on the media by other than a continuous external power source. The electrotatic charge may be imposed at the time the media is manufactured (electret) or it may be generated by the flow of dry air through the media.

ETL: ETL Testing Laboratories. A testing laboratory for various types of equipment. ETL has facilities for testing air filters and room recirculators using different standard performance and safety test methods.

Evaporation: The process by which a liquid is changed to a gas. Heat is absorbed in the process.

Exfiltration: The controlled or uncontrolled leakage of air from a conditioned space.

Exaust air: Air is removed from a space and not reused therein.

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