Gas and Oil Home Heating Furnaces - Cause of dirty furnace filters

Simply put, my white replaceble air filters are turning black within seven days of changing them. I have a 2 story home with two furnaces, one in the attic and one under the house (we have no basement). This is my first full winter in this twelve year old house, and I began checking the filters more frequently after they choked off the furnace earlier in the heating season. I noticed they were black after checking them monthly, but now I find they are getting this way after only one week. We have gas logs, but have not used them recently. I'm hoping there is an obvious answer to this question.

leaks in the return duct, possibly pulling in dirt from unconditioned space. i have seen these filters turn dark quickly, but the capacity for holding more dust/dirt is still high.

Most likely your married or have kids.... This is soot...This happens quite frequently when they burn candles in the house..I saw a web site on this, I think by HB Smith boilers,...Think of the bright side of it....your not breathing the stuff. If I can find the website I'll post it. It turned the filter black in a matter of hours,...