Soot is the fine particulate residue combustion leaves behind — carbon plus whatever the fire burned, carried by smoke and deposited across surfaces far from the flames. It's why a small kitchen fire can produce cleaning work three rooms away: smoke travels, cools, and settles wherever air moved.
Not all soot behaves the same. Fast, hot fires tend to leave dry soot that vacuums and chem-sponges off relatively cleanly. Slow, smoldering fires — and burning plastics — leave wet, smeary soot that grinds in if wiped wrong. Protein fires (a pot of food left burning) leave a nearly invisible film with a persistent odor that ordinary cleaning misses entirely. Identifying the soot type before touching anything is the first professional step, because the wrong first wipe can set a stain permanently.
Soot is also corrosive. Combined with moisture, its acidic residues etch and discolor metals, stone, and finishes over days — one reason fire losses are stabilized quickly even when rebuild planning takes longer, and why crews pre-treat sensitive surfaces early.
Health-wise, soot is fine particulate matter and shouldn't be dry-swept or handled without protection. After any fire, keeping HVAC off until assessed prevents the system from redistributing residue through the house.