Carbon soot is a problem with diesel engines and in school buses. The particular concern is with the nanoparticle soot that is implicated in the rise of childhood asthma. The particles of concern are 2.5 nanometers or less in diameter and are difficult, if not impossible, to remove with mechanical filters and traps.

The best way to remove these particles is with hydrogen injected into the diesel engine to burn them into carbon dioxide gas.