Formaldehyde is a toxic air contaminant that poses a risk of cancer and other adverse health impacts. Studies have shown that some fiberglass filters used in commercial heating, ventilation and air conditioning (HVAC) systems can be sources of formaldehyde. This study assessed formaldehyde emissions from residential fiberglass filters relative to synthetic filters, and estimated their contributions to indoor concentrations in California homes. The results showed that fiberglass filters emitted more formaldehyde than synthetic filters. At high relative humidity (RH), fiberglass filters became strong sources of formaldehyde, with emission rates in the range 500-3500 µg h-1 m-2, but emissions from synthetic filters were only 100-120 µg h-1 m-2.