Category 4 Smoke requires which ventilation rate range and dispersion condition?

• A. Ventilation Rate 51,750–68,999; Daytime all hours and Nighttime burning with fair to excellent dispersion
• B. Ventilation Rate 34,500–51,749; 0900-1700 EST
• C. Ventilation Rate 0–17,249; No burning
• D. Ventilation Rate 17,250–34,499; 0900-1600 EST

Answer: (A)

Ventilation rate and dispersion conditions determine when a burn can produce smoke that meets a given category. A higher ventilation rate means the atmosphere mixes and dilutes smoke more effectively, reducing ground-level concentrations. Category 4 requires the strongest dispersion potential, so it uses the highest ventilation-rate range and allows the broadest burn windows under favorable dispersion.

That’s why the best match specifies a ventilation-rate range of 51,750–68,999 and allows daytime burning across all hours plus nighttime burning only when dispersion is fair to excellent. Under lower ventilation-rate ranges, dispersion is poorer, making Category 4 criteria not met, and the burn would be restricted or not permitted.