Filters may be placed at the air intake, inspiratory limb, patient Wye, expiratory limb and/or exhaust port, however, placement at each of these sites does not provide equivalent function.  (Video of filter placement location)
Ideally a two filter setup should be used (Anesthesia Patient Safety Foundation):
Inspiratory/Patient filter: the first filter should be placed between the circuit Wye connector and the patient and achieves two things: 1) protects the ventilator and the room from exhaled gases from an infected patient, 2) protects a non-infected patient from a possibly contaminated ventilator. If using an active heat and humidification system, then this should be a bacterial viral filter (not an HME). If not using active heat and humidification, then this should be an HMEF or a bacterial viral filter in series with an HME (HME should be between endotracheal tube and B/V filter). In a dual-limb ICU ventilator, the inspiratory bacterial viral filter may be placed on the inspiratory limb at the takeoff from the ventilator.
Expiratory filter: A second bacterial viral filter is recommended on the exhalation limb before the exhalation valve, to protect the room environment and healthcare staff from stray particles (and to protect the device in a dual limb circuit setup).
Placement of filters between the circuit Wye and the patient’s endotracheal tube can add significant deadspace to the circuit, especially for pediatric patients.
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