Smoke is an aerosol, and the size and colour of its particles depend on which combustion regime produced it. Smouldering and early pyrolysis release condensed tars and unburned droplets that are relatively large, roughly \( 0.3 \) to several micrometres, and pale grey or white; they scatter light strongly. Vigorous flaming produces soot formed in the flame itself, much smaller at roughly \( 0.02 \) to \( 0.2 \) micrometres, and black; these particles scatter visible light poorly but are numerous and readily attach to charged ions. This physical split is the reason two smoke detectors sitting side by side can disagree, since a scattering-based sensor responds best to the large pale particles and an ionisation-based sensor to the small ones, and it is also why steam, dust and cooking aerosols, which are large and pale, produce so many nuisance alarms. The confusion it resolves is treating smoke as a single substance with one detectable signature. After this Concept you can predict what kind of smoke a fire in a given regime will produce and what makes it easy or hard to see.
Smoke detection relies on two distinct sensing mechanisms, each tuned to a different particle-size regime produced by different combustion modes: ionization sensing, which detects disturbances to an …