Municipal waste combustion ash can be classified as hazardous under RCRA if it exhibits characteristics such as toxicity from heavy-metal leaching. Bottom ash and fly ash from the same facility may be regulated differently. Importers must assess the specific ash type and its test results under EPA's TCLP (Toxicity Characteristic Leaching Procedure) before import, and may need to comply with consent-based import procedures for hazardous waste.

Seaweed ash, historically called kelp, is the calcined residue of burned marine algae and was historically a source of iodine and potash. It is classified here as a residual ash product rather than as a chemical or fertilizer. In modern trade it is a minor commodity, but its explicit mention in the heading text means any such material must be classified here rather than in headings covering seaweed in its natural or processed form.