What pressure drop across an air-conditioning filter is indicative of a need to replace or clean the filter as appropriate?

Correct Answer: Option B — 0.5 inches of water column

A pressure drop of 0.5 inches of water column across an air filter is the industry-standard threshold indicating significant particulate accumulation. Exceeding this value restricts airflow, increases fan motor load, and reduces system efficiency, necessitating immediate cleaning or replacement to maintain proper air volume and prevent potential motor overheating.