We actually have a whole house air exchange system with a Hepa filter, but my husband is always congested, so I thought I would try the Sans air purifiers to see if it helped his allergies. We bought a large one for the main level and a smaller one for our upstairs bedroom. When we first got them, they always showed levels of 002-004, so we were wondering if they either didn’t actually work, or if we just didn’t need them due to our air exchange system. However, within the last few weeks, we’ve had a lot of wildfire smoke in our area from fires in other parts of our state and we got a lot of smoke in our house, and of course, we had to turn off our air exchange system. We finally got to see how well the Sans purifiers work. The numbers were out of range for good air quality and they cleared the smoke out of our house quickly. We had actually considered returning them earlier, but never did it - now we’re so glad we kept them!