riz has a point , but depending on your situation , a DIY filter is adequate . For example , my nearest neighbors are 1/4 mile away and my current filter exhausts into my house , not the outdoors . So for me I can get away with no filter if I so choose , but I keep using one for rare occasion of having company over .
For many people , it IS important to have zero smell because of proximity to others . As was said , if you DIY a filter , thouroughly test it .