CO2 cleaning has found application in many industries and technical areas, including aerospace, automotive, electronics, medical, manufacturing, basic and applied research, and optics.[5][6] The different carbon dioxide cleaning methods can remove gross contamination, paint, overlayers, grease, fingerprints, particles down to nanometers in size, hydrocarbon and organic residues, and radioactive residues. Materials cleaned include metals, polymers, ceramics, and glasses.[4]: 270 The key limitation is that the contamination must be on the surface, not buried within the material. Porous materials are not good candidates for pellets or snow, but can be cleaned using liquid or supercritical CO2.