Ventilation filters as sources of air pollution – Processes occurring on surfaces of used filters

Ozone concentrations were monitored upstream and downstream of used filter samples following 24hours of ventilation with ozone- filtered air. The ozone concentration in the air upstream of the filters was maintained at ~75 ppb while the concentration downstream of the filters was initially between ~ 35 and 55 ppb and slowly increased over time.

Within an hour the ozone concentration downstream of the filter had increased to ~ 70 ppb. In contrast, no change in ozone removal efficiency over time was observed for a new filter. The used filter samples were then placed for 24 hours in ambient air (ozone concentration <5 ppb); ambient air at an elevated temperature (100°C); or nitrogen.

When subsequently placed in the air stream containing 75 ppb of ozone partial regeneration of the filter’s ozone removal capabilities was observed. Regeneration was greatest for the sample that had sat at an elevated temperature. The process or processes responsible for regeneration remain to be determined