The Steiner Tunnel Test – also known as ASTM E84, tests the flame spread rating of Interior Finishes and Building Materials, specifically wall and ceiling finishes. Don't confuse this with ASTM E648 Critical Radiant Heat Flux test for Interior Floor Finishes.
Building materials are rated from A-C according to how fast a flame spreads, with A being the most fire-resistant. Building codes use these ratings to specify the rating for materials used in an area by occupancy type.
Class A (1) flame spread index 0-25, smoke development index 0–450
Class B (2) flame spread index 26-75, smoke development index 0–450
Class C (3) flame spread index 76-200, smoke development index 0–450
The smoke development index determines how much visibility is maintained when a material is on fire and creating smoke, which affects an occupant's ability to see exit signs while evacuating a building. Any finish with an SDI over 450 would create too much smoke and, therefore, would typically not be allowed by the codes.