My wife suffers from asthma and allergies and claimed our house was killing us. Over the last 20 years we had done every thing from replacing the ducts, cleaning the supply vent boxes, replacing most of the carpet with hardwoods and tile. We also replaced all 21 recessed lights(the one's with the 6 slits) with LED lights with no slits. We replaced the 8 wall returns with two large duct ceiling returns. All of it helped by did not cure the problem. Air & Heat Hospital to to the rescue and now we have the best air quality in the city. Happy Wife Happy Life! Thanks David!!