I live in northern California where the wildfires generated huge amounts of smoke this year. Subsequently, I moved one of our air400 units to our daughter's house to help protect the little lungs of our three grandchildren. They noticed a big difference, and I gave them the unit. This is my third unit, along with with multiples of the smallest unit (two of which I gave to our daughter so they could place them in the kids (6 and 2 yr old twins) rooms. It's pretty simple to me - ROI on these units is obvious. They are priced fairly, and they do the job. I also appreciate the great custome service that Bissell has provided me.