In a case where the plaintiffs’ own toxicologist testified that only those individuals with a unique susceptibility to the chemical in question would exhibit symptoms, which would manifest itself in less than 0.1 percent of the population, “determining whether any particular person was within this microcosm of the population would require an entirely individualized understanding of each person’s health, medical history, records, and other variables impacting exposure.”  Alexander v. Norfolk S. Corp., 2011-2793 (La. 3/9/12), 82 So.3d 1234, reh’g denied, 2011-2793 (La. 4/20/12), 85 So.3d 1277.