Air ions, charged atoms or molecules naturally existing in minute concentrations in the air we breathe, which are studied at different atmospheric conditions in India. Elevated negative air ion levels are widely reported to have beneficial effects on humans including enhanced feeling of relaxation, and reduced tiredness, stress levels, irritability, depression, and tenseness. Depleted ion levels and enhanced positive ion levels are reported to have no effect, or deleterious effects. The degradation of air quality is a major environmental problem that affects many urban and industrial sites and the surrounding regions worldwide. Average positive to negative air ion ratio at rural, coastal and mountain site is below one which is healthy for human being. Average positive to negative ion ratio is above 1.5 at urban and poultry-farm site (6.3), supposed to be harmful to human health. It is observed that not only temperature but other meteorological parameters also affect on the concentration of air ions. Graduate students and senior undergraduate students in atmospheric science, physics, engineering or medical who intend to undertake researches in air ion should find this book useful.