Background: I have maintained a prediction calendar since 2010 and will be creating markets of the (least ridiculous) predictions in there. Most of these will not be my own predictions. I will not be betting in any of the markets I create.

For this prediction, the thesis is that air quality in the coming century will improve in such a way that people in 2123 will look back at smog as an oddity from the past.

Resolves YES if in 2123. smog is indeed a historical phenomenon, no longer an issue of that day. Resolves NO if smog is still an ongoing environmental and health problem, anywhere on the planet.

Possible operationalisation issues are about the definition of smog, which may change within the coming 100 years as a consequence of new chemicals polluting the air. Within the spirit of this prediction, such new chemicals would also qualify as smog: the YES prediction would mean that air is indeed clean.

Another issue concerns the scope of the problem and its solution. Specific towns may still struggle with smog, while it is no longer a concern of the world at large. Or perhaps certain developed regions got rid of smog, while others have fallen behind. For the purposes of this prediction, the problem counts as solved if it has been solved across the world.