Air ion counters are hand-held meters that measure ion density-the number of ions per cubic centimeter (ion /cc) of air. It measures the number of positive and negative ions separately (+ and - ions are usually present together). The instrument is a true ion densitometer, based on the Gerdien condenser design, which contains a fan to move air through the meter at a calibrated rate. Standard models can measure up to 2 million ions /cc (for comparison, outdoor air usually has 100 to 1000 ions /cc, + and -). The 2 million range is sufficient for almost all situations, except when measured very close to ionizers or strong radioactive sources.