Performed furnace tuneup. 16x25x1 filter is very dirty and needs changed as soon as possible. Due to filtration issue, dust film is starting to accumulate on blower wheel. Buildup is in early stages and advanced cleaning not recommended at this time. Upgrading filtration to polarized media filter and adding an air scrubber will help combat this issue as well as changing filter every month. Monitored system with proper operation, passed carbon monoxide test at 0ppm.