The exception is a. Recirculating exhaust ventilation system.
Recirculating exhaust ventilation systems are not recommended for nitrous oxide scavenging. These systems filter and recirculate air back into the operatory, which would simply spread the nitrous oxide contamination rather than removing it. Effective scavenging systems must vent the nitrous oxide outside the building. The other options are all important components of a good scavenging system:
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