At concentrations of several ppm, and at room temperature, the half-life of ozone due to decomposition is of the order of 1 day. Only at temperatures above 500oC does the thermal decomposition reaction become important. Thus, in the very hot region adjacent to the welding arc this decomposition reaction can be very important, and the net quantity of ozone emitting from the arc can be greatly affected.
Decomposition can also be accelerated by contact with surfaces and catalysts.
Experiments have shown that the ozone concentration measured during the first minute of welding immediately reaches a peak, which decays after a few seconds, and then remains constant at a background level of between 40% and 60% of the peak value.