Last winter each morning I would wake up to find greyish goop in each of our dogs' eyes. Our elder dog Max had it the worst, and I had to clean out his eyes 4-5 times a day. I finally put two and two together and realized that their eye problems probably had to do with what was coming out of our air ducts when the heater was on. So last summer we had all the vents and air ducts cleaned out by a company I found on Angie's list that had high ratings. Finally last week we turned the heat back on when it hit the low 30s during the night and I got to test my theory. I am happy to report that the dogs have no more eye gunk problems! We've only lived in this house for a couple of years, but the previous owners did a lot of construction in order to get the house ready to sell. Most of what came out of our air ducts was construction material.