The ISS needs about 7 tons of fuel per year to hold orbit. I just tried to calculate the air density in the 400 km orbit with the scale height 8.5 km.
Air density there should be 1 / (e power (400/8.5) ) compared to the sea level. But this seems far too thin, because at this calculated air density there whould be to little air to cause 7 t fuel usage to hold orbit.
