The U.S. Bureau of Mines performed experiments to determine if the primary gas toxicities evolved during the early (approx. 300 Deg c) and later (> or = to 400 deg c) combustion stages of mine conveyor belts could be predicted by a smoke particle characteristic. The experiments were conducted in an approximately 20-l furnace at set furnace temperatures of 150 deg, 250 deg, and 1,000 deg c, with a furnace airflow of 10 l/min. The variables studied, as a function of time, included hydrogen chloride and carbon monoxide concentrations, the average smoke particle diameter (dg) and number concentration (no), and the product of the average smoke particle diameter and concentration (dgno). Other variables included the sample and furnace temperatures and the sample mass weight loss. Results show that the inverse of the smoke particle diameter- concentration product (l/dgno) correlates directly with the hydrogen chloride and carbon monoxide toxicities released in large quantities during the early and later stages of combustion.