In 3 Nephi 10:12 it says that "it was the more righteous part of the people who were saved." According to Alvin Benson, in the more righteous cities, lethal concentrations [of volcanic gases] may not have been present a few feet above the ground allowing the more righteous to survive. [See the commentary on 3 Nephi 10:13] [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]