An ionizer for the ionization of oxygen during any type of oxygen therapy. The ionizer contains an ionizing unit inside a housing. The housing has a grounded contact safety screen mounted thereon by means of a conducting metal ring and a high-ohmic resistor. A glass flask is mounted or screwed onto the housing to serve as a connection between the face mask and the ionizer. The housing has a projecting socket enclosed in a conducting metal cylinder connected to the high-ohmic resistor. The ionizing unit includes ionizing needles located directly adjacent the contact safety screen. Sufficient ionization of oxygen occurs without the production of ozone.