The replacement filter (4in by 14in metal can filled wih activated carbon) is only $38. The up-front cost $100 is the price of the filter, plus a cloth filter w

The replacement filter (4in by 14in metal can filled wih activated carbon) is only $38. The up-front cost $100 is the price of the filter, plus a cloth filter which goes around it, and the duct fan. From what I understand, activated carbon works really well for both particulate and VOCs but, unlike say HEPA filters (or non-HEPA cheap HVAC filters), it cleans the air after many cycles. So the amount of air passing through it, and how many times it passes through it, matters a lot. I think HVAC activated carbon filters are likely to let more particulate get to your HVAC unit, which is not good for it, and will not clean as much due to HVAC not moving as much air as these portable units. But I'm just an amateur airhead.