Could titanium dioxide—a common ingredient in sunscreen—help buildings act as air purifiers? That’s the goal of Pureti, a Cincinnati-based manufacturer that offers a façade-cladding treatment in which titanium dioxide nanoparticles are sprayed onto the components of a building’s exterior. When ultraviolet rays hit the treated surface, they set off chemical processes that burn away grime and convert contaminants such as nitrogen dioxide into nontoxic minerals and water vapor. The result: a self-cleaning structure that also cleanses the air around it.

The...