Forest fires emit carbon monoxide (CO) that can be transported into the atmosphere far from the sources and reacts to produce ozone (O₃) that affects climate, ecosystems and health. O₃ is also produced in the stratosphere and can be transported downwards. Using a global numerical model, we found that forest fires can affect CO and O₃ even in the South Pacific, the most pristine region of the global ocean, but transport from the stratosphere is a more important O₃ source than fires in the region.