What is ozone's half-life, and why does it turn back into oxygen?

Why it decays

Half-life is the time taken for half of a quantity to disappear; for ozone it is the time for half the gas present to revert to ordinary oxygen, and it shortens with heat, humidity and contact with surfaces. The third oxygen atom in O3 is loosely bound. It detaches, leaving stable O2 behind. We describe ozone as activated oxygen that reverts to oxygen in a short period, and the short half-life is the reason ozone can be used where residual chemicals are undesirable — it does its oxidising and then it is simply oxygen again.

The figure in a sauna

In the high heat and steam of a sauna we estimate ozone’s half-life at 10 to 20 minutes. The generator also stops producing new ozone five minutes before the session ends, so what is left in the sauna is already decaying by the time the session finishes. Between the two, ozone residue is limited.

What that means in practice

• Ozone cannot be stored or shipped. It has to be generated at the point of use, which is why an ozone sauna contains a generator rather than a cylinder of ozone.
• Heat accelerates the decay. Ozone starts to disintegrate at 40 °C and the process accelerates at higher temperatures. That is a real constraint on running a hotter cabinet.
• Moisture changes the chemistry. Moisture inside the sauna converts ozone to hydrogen peroxide (H2O2), so relatively little ozone off-gasses — escapes from the cabinet into the room — after a session.
• Ventilation is usually enough. The combination of decay, conversion, the five-minute cut-off and an open door and window is why we do not think an expensive ozone destructor is justified for a sauna installation.

The same property in industry

Ozone’s short life is precisely why it is used to disinfect drinking water and treat air: it acts, then disappears, rather than accumulating as a residue the way a chlorine-based treatment does.