According to Alvin Benson, in 3 Nephi 10:13, inference can be drawn that some people died from suffocation from "the vapor of smoke and of darkness." Warren and Ferguson record that when the ash from a volcanic eruption "begins to fall back toward the earth, it is accompanied by many gases, including hydrochloric acid, hydrofluoric acid, carbonic acid, carbon dioxide, and ammonia. If the ash fall is heavy, people will naturally suffocate, not only from the ash content itself but from these gases, which are lethal in large quantities" (42). In several modern cases, volcanic gases have collected in low spots after an eruption, killing people, animals, and vegetation (Montgomery 105-106; Macdonald 251-52, 257) . . . Because most volcanic gases are heavier than air, they tend to hug the ground; hence, at ground level, concentrations could have been high enough to prevent ignition of the Nephites' dry tinder. However, in the more righteous cities, lethal concentrations may not have been present a few feet above the ground allowing the more righteous to survive. [Alvin K. Benson, "Geological Upheaval and Darkness in 3 Nephi 8-10," in The Book of Mormon: 3 Nephi 9-30, This Is My Gospel, p. 65] [See the commentary on 3 Nephi 8:21; 10:12]
Geographical Theory Map: 3 Nephi 9:1 -- 10:9 Many Cities Destroyed (34 A.S.)