Change the filter then.
As mentioned before (a lot of times), over here, my experience is that you'd need to change it 6 months approx. Smell gets worse and worse. For 2013, 2014 and now 2015. 2013 and this year 2015 particularly bad. Culprit could be the ug/m3 load.
But some are not bothered by that. Hence 12 months. Push to 18 months also can.
I bought some second hand air purifier before. Eg LG LA-Q379SB. 1 year plus. I see the filter, pretty black...bad shape. Powered it on, WTF.....smell is crazy. Hence they wanna change to a new unit, there is no replacement filter available for the LG.
For me, I just customise Taobao HEPA for it.