Dual LC₅₀ values for red oak (8 and 30 g) were observed in the combustion toxicity testing using the University of Pittsburgh combustion toxicity (UPITT) apparatus. No report of any material displaying a dual LC₅₀ value had been made prior to now.

The National Forest Products Association became interested in knowing whether this was an anomaly of red oak or a property of other materials. A probative investigation was initiated to determine the existence and extensiveness of a dual LC₅₀ value phenomenon using the UPITT apparatus. Since red oak was the first reported to have a dual LC₅₀ value, other woods were selected for testing. White oak, southern pine, and Douglas fir were chosen.

Another objective of the investigation was to determine if there were analytical characteristics of thermal decomposition that would correlate with the LC₅₀ values. Attempts to correlate the maximum concentrations and concentration x time (Ct) products of carbon monoxide, the ratio of carbon dioxide to carbon monoxide, and maximum ∂ temperature area with the LC₅₀ values were made.

Three LC₅₀ values were found for red oak (8.48, 15.0, 29.2 g) and southern pine (7.34, 12.39, 20.62 g), while one LC₅₀ value was found for white oak (36.52 g) and Douglas fir (25.38 g). Maximum carbon monoxide concentrations reached a plateau and offered no correlation with the LC₅₀ values. The Ct products of carbon monoxide did fluctuate but did not clearly correlate with the LC₅₀ values. For red oak, ratios of carbon dioxide to carbon monoxide demonstrated a very good correlation with its LC₅₀ values, but this was not so for the other woods. The maximum ∂ temperature area indicated that there was a continuous change in burning characteristics as the specimen weight increased, but it did not correlate with the LC₅₀ values.

These data suggest that multiple LC₅₀ values cannot be strictly related to nonflaming and flaming modes or char formation. Therefore, until an explanation for the multiple LC₅₀ values is known, there remains the possibility that other products could also exhibit this phenomenon.