4.3.1.3.1 Gypsum board having a maximum flame spread index of 25 without evidence of
continued progressive combustion and a maximum smoke developed index of 50 when tested
in accordance with ASTM E 84, Standard Test Method for Surface Burning Characteristics of
Building Materials, or ANSI/UL 723, Standard Test Method for Surface Burning
Characteristics of Building Materials, shall be permitted to be used for negative pressure
exhaust and return ducts where the temperature of the conveyed air does not exceed 52°C
(125°F) in normal service