Ozone or O3 is an unstable molecule which is usually formed by energetic interactions with electricity or light. Ozone in the upper atmosphere, far away from people, forms a crucial shield against harmful UV radiation. Ozone at ground level is harmful to people even in low concentrations. Ozone can be produced by laser printers and copy machines, or from malfunctioning or poorly designed ionization air purifiers. Ozone has a pungent odor which in low concentrations is indistinguishable from disinfectants. Ozone is known to have effects on the immune system and brain function.

The EPA has established NAAQS maxim level 8-hour average of 70ppb. The CDC/NIOSH REL and PEL levels are the same value of 100ppb. Ambient outside Ozone levels are, depending on location, difficult to determine from published reports because Ozone is commonly included in a generalized Air Quality Index which includes other pollutants. Ambient levels can be quite high. For example, the EPA Air Quality index of “moderate” may include a level of Ozone from 55 to 70ppb.

The most accurate way of sensing Ozone currently available is an Electro-Chemical (EC) sensor. This type of sensor detects a change of resistance in a chemically active layer between two electrodes. The WellStat devices have the ability to compensate for changes in both offset and slope of the sensor response.