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An air ionizer is a device which uses high voltage to ionize, or electrically charge, molecules of air. These machines can be designed either to generate specifically charged ions (all positive or all negative), or to create both polarities indiscriminately^{[citation needed]}. However, most commercial air purifiers are designed to generate negative ions.^{[citation needed]} Negative ions are air molecules that temporarily contain an extra electron, causing the entire molecule to possess a negative electrical charge. Similarly, a positive ion is deficient by one electron and has an overall positive charge. Unfortunately, the high electric fields used to create the air ions can also generate ozone (an energetic allotrope of oxygen), and NO_x. Either can be toxic in sufficient concentrations. Even in relatively low concentrations, they can irritate lung tissues, cause chest pain, coughing, throat irritation, and can worsen the conditions of persons suffering from asthma.