At extremely low frequencies (1 to 3000 cps), relatively slight amounts of ionization can produce notable effects on electromagnetic phenomena in the terrestrial system. Representative atmospheric conditions are examined to find minimum conditions that can produce such strong ELF effects at geophysically quiet periods. On the basis of analyses of ion production parameters, it is found that even for quiet nights, when only cosmic radiation is important for producing ionization in the mesosphere, large refractivities can be produced in this portion of the atmosphere below the ionosphere. For even a minimum electron detachment condition the refractive index of the mesosphere has the large magnitude of 4 at an altitude 75 km. For a more inclusive ion production condition with a maximum detachment factor that collates with VLF observations, strong refractivity effects occur at quite low altitudes: 35 km at 1 cps, 50 km at 10 cps, 55 km at 100 cps, and 65 km at 1000 cps. At very low frequencies (10 kc/s and above) effective altitudes for the nighttime lower ionosphere take on values normally assumed. During daytime and active solar periods it is indicated that strong refractivity effects will occur at altitudes below 30 km.