"Interviews with survivors and pathologic studies indicated that victims rapidly lost consciousness and that death was caused by CO2 asphyxiation.[21] At nonlethal levels, CO2 can produce sensory hallucinations, such that many people exposed to CO2 report the odor of sulfuric compounds when none are present.[21] Skin lesions found on survivors represent pressure sores, and in a few cases exposure to a heat source, but there is no evidence of chemical burns or of flash burns from exposure to hot gases."
-https://en.wikipedia.org/wiki/Lake_Nyos_disaster