Polyurethane foam (commonly used in furniture) is placed over a hot
soldering iron. Such fires can smoulder for a long time, gradually
producing more and more smoke, before bursting into flames. If these
fires occur at night, you can inhale their toxic smoke while sleeping,
which makes waking – and escape – more difficult. Smoke inhalation is a
major cause of death in fatal household fires.
The smouldering and flaming fire scores are based on the level of smoke obscuration at the time the alarm activated. The heavier the smoke obscuration, the lower the score. If the obscuration was 50% (very thick smoke), the alarm scored zero. High scores in either of these tests indicate the alarm activated when the smoke was still very light; visibility would be very good in these cases and you'd be able to easily find your way out of the house, or even have time to fight the fire if it was safe to do so.