1991 Jeep Cherokee heater is blowing steam/smoke?

Can anyone give me an idea of why this is happening? When I turn my heat on it blows smoke/steam and fogs up the windows. It's been leaving a white residue. When the heat is off there is still that steam/smoke coming out of the vents. It does not matter if I have the defrost or the regular heat on and it smells. Any ideas of what this could be? Thank you!