By using the recirculation function the chemical does not get diluted with more fresh air making it work better. I would at least pull out the cabin filter first and clean out any debris in there and bang out the filter if not replacing it. Just cover the passenger foot well with plastic sheeting before removing the filter to catch the mess. I have also sprayed the pollen filter with Dettol 3in1 spray as that leaves a pleasant smell in the filter.