mysterious fog on windshield that won't go away

There is this fog on my windshield that doesn't clear een when the defrosters are on. Visability becomes extremely poor, and I cannot see out the windshield. Is it a buildup of dirt or grime? What are some solutions to this problem?

Is your heater core leaking? I had 2 vehicles that experienced similar conditions and it was caused by a leaking heater core. It can be hard to clean off the windows. I ended up using some concoction I made up with vinegar to get it off and had to clean the windows about 4 to 5 times to finally remove all traces. There might be some newer products on the market now that can do a better job at cleaning the windows.