We've had two: One in our old house, and a different one in our current house. One was the air cleaner with inch thick filter. I think the price of each filter for it was $45. You suppose to change the filter every year, but I changed it every 6mo. Some recommends getting a system that requires a thinner, cheaper filter and change it every month, but I was more comfortable with the thicker one, every 6mo. Our current one is also an air exchanger, and I like this one since I can set the intervals of air exchange according to the season, etc. We also change the filter every 6mo. We also have the humidifier for the whole house. I will not have the portable humidifier since I would be concerned about the mold growth.<br><br>
Beside all the equipments, my first recommendation is to get a lot of house plants. And open your windows everyday. I highly recommend reading the book "How to Grow Fresh Air." Especially, if you have the 'brown thumbs' like me.<br><br>
You can also improve your air quality by getting the synthetic materials out(PVC, VOC) and start using natural materials for cleaning your house.<br><br>
If you have an extra money, I really would take the carpet out, and change the drapes to cotton or silk curtains.