was used to calculate half-lives,  and , of 20.8 and 91.5 minutes for the reduction of formaldehyde during the catalytic phase of the process. Note that the kinetics of chemical processes at interfaces have previously been shown to fit biexponential decay models. Figure : The concentration of formaldehyde in the simulated OR containing a continually functioning air recirculation device changes in response to controlled events. During the first phase the background formaldehyde levels do not change in the unoccupied OR. In phase 2, open containers of formalin are placed in the center of the OR and a rapid increase in formaldehyde concentration occurs. After removing the formalin, the formaldehyde levels gradually reduce (phase 3). The rate of formaldehyde reduction does not increase upon incorporating an activated carbon filter into the air recirculating device (phase 4). Upon inserting a catalyst that is capable of transforming formaldehyde to CO2 and H2O, the concentration of formaldehyde in the OR rapidly decreases (phase 5). The rapid reduction in phase 5 was modeled with a biexponential decay fitting.