Initial studies of oxidation processes on filter surfaces and their impact on perceived air quality

Used filters can be a strong sensory pollutant source. Oxidation processes, especially those initiated by ozone, may contribute to the pollutants emitted from such filters. In the present study, ozone was added to the airstream passing through used ventilation filters. Two. flow rates were examined. While the upstream ozone concentration was similar to 75 ppb, the concentrations downstream of the filter were initially 35-50% lower. However, within an hour downstream concentrations were only 5-10% lower than those upstream. These filter samples were then placed for 48 h in nitrogen, ambient air containing less than 5 ppb ozone, or ambient air at an elevated temperature. This resulted in partial regeneration of the ozone removal capability of the filter. In analogous experiments, lower ozone removal occurred when the filter samples were first ventilated for 24 h with ozone-free air before making the measurements. Samples from a new filter removed < 10% of the ozone in the airstream, and removal remained relatively constant over time. In companion studies, human subjects assessed the air passing through various used filter samples. In the initial evaluation each of the four filter samples, taken from the same filter and ventilated for 24 h, were assessed to be equivalent. The next evaluation was immediately after the samples had been kept for 24 h in either nitrogen, air, air at an elevated temperature or ozone. The nitrogen-treated filter was assessed to be best, while the ozone-treated filter was assessed to be the worst. The final evaluation occurred after ambient air had passed through the 'treated'. filters for 2 h. All such ventilated filters were assessed to be more acceptable than immediately after the 24-h treatments; the ozonized and air-treated filters were the most polluting of the four.