China cleaned its air. It made the air deadlier and killed birds.

China rolled out a national air monitoring network. Local governments then cut one pollutant, but another one surged, increasing deaths and killing birds.

Governments often set targets for one specific thing, like reducing a single pollutant. This paper shows that approach can make other things worse, like increasing a different pollutant that kills more people and birds. It quantifies the hidden costs, making it harder to ignore the full impact of narrow policies.