Carbon / Charcoal

All these units have some blend of activated granulated carbon in them making them much more effective for absorption of odors, chemicals, VOC's, fumes, gases, etc. 

The more weight (pounds) of carbon in the filter, the longer it usually lasts before its saturated and in need of replacement. 

For serious chemical abatement or VOC concerns, you want a unit with a thick (inches) and heavy (pounds) carbon filter so it can handle your pollution concerns.