Experts' Conclusions for Juries to Weigh, Not Courts — NM Supreme Court
The reliability of an expert's methodology is within the court's province, but not the conclusions drawn from it, the New Mexico Supreme Court has ruled, reversing the district and appellate courts' exclusion of a toxicologist's expert opinion.
In Acosta v. Shell W. Expl. & Prod., Inc, NMSC, 2016 Mar 3, more than 200 residents of the Westgate subdivision in Hobbs, New Mexico, alleged that toxic chemicals from an unlined storage pit used by Shell for its oil drilling operations caused their autoimmune disorders. Shell had put toxic hydrocarbons in the pit from the 1940s until the 1960s, when the pit was covered with fill dirt, unbeknownst to the developer who later built the subdivision. Massive hydrocarbon contamination of the soil, extending sixty-five feet below ground level and into the aquifer, was discovered in 1997, as well as a hard layer of hydrocarbon contaminants one to two feet below the ground surface of the subdivision.
The subdivision residents' toxicology expert witness, Dr. James Dahlgren, proffered expert testimony that the lupus and other autoimmune disorders the residents suffered from were caused or aggravated by long-term exposure to a mixture of benzene and other organic solvents, hydrocarbons such as pristane and phytane, and mercury, all of which are toxic chemicals found in crude oil.
To support his causation opinion, Dahlgren provided numerous animal and human studies linking the chemical agents to immune system disruption, autoimmune diseases, and lupus. He obtained medical records and questionnaire responses from all the residents, samples of blood from a subgroup of them, samples of house dust from some of their homes, and analysis results of the air monitored by Shell at multiple locations in the neighborhood. Dahlgren found elevated levels of pristane and phytane in the residents' blood samples and elevated levels of mercury, pristane and phytane, benzene, and other hydrocarbons in their homes. He also compared the occurrence of diseases in the residents with a nonexposed control community.
While Dahlgren acknowledged that he could not point definitively to any human cases of pristane-induced lupus, he referenced peer-reviewed studies demonstrating that mercury can cause immune system disruption in animals and humans and that benzene adversely impacts the immune system in humans, as well as studies establishing that pristane exposure in mice induces autoimmunity and lupus, and other research finding that pristane could be involved in cases of human lupus.
The district court excluded as unreliable Dahlgren's study of blood pristane data and his determination of the residents' cumulative dose from prolonged exposure. It concluded that Dahlgren's study had "limited value as a basis for a causal connection opinion" because it was a "hypothetical" study "admittedly done only for comparison purposes". The court also excluded the animal studies Dahlgren had referenced, reasoning that the studies failed to show general causation between the mixture of identified chemicals and lupus. Even though Dahlgren's expert opinion considered the dose-response relationship necessary for inferring causation in humans from toxicological results in animals, the district court found that he had not, ruling that the animal studies were "so dissimilar to the facts presented" that they were not relevant. Having excluded Dahlgren's causation opinion, the district court then granted Shell's motion for summary judgment.
In review, the state Supreme Court relied on the Bradford Hill guidelines to assess whether Dahlgren's epidemiological study could support an inference of causation. The Bradford Hill factors include:
- temporal relationship,
- strength of the association,
- dose-response relationship,
- replication of the findings,
- biological plausibility (coherence with existing knowledge),
- consideration of alternative explanations,
- cessation of exposure,
- specificity of the association, and
- consistency with other knowledge.
These factors are used to measure the ability of an epidemiological study to determine whether an association found by the study is sufficient or whether the association is merely spurious.
Reviewing Dahlgren's methodology at length, the state Supreme Court found that Dahlgren's epidemological study was consistent with the Bradford Hill guidelines. As well, and contrary to the assertions of the lower courts, the state Supreme Court found that Dahlgren had presented evidence that the results of the animal studies adjusted by extrapolation to the humans could provide a scientifically valid basis for his inference of causation in the residents of the Westgate subdivision. His methodology reflected that the harmful dose of pristane the mice received, adjusted for the weight of a human, was comparable to the residents' dose determined by their daily inhaled exposure, adjusted for the prolonged duration of that exposure. This result of Dahlgren's extrapolation from the animal study is evidence that supports an inference of causation, the state Supreme Court said.
As a key reason for excluding Dahlgren's general causation testimony, both the district court and the Court of Appeals cited General Electric Co. v. Joiner, in which the United States Supreme Court affirmed a district court's determination to exclude an expert's causation testimony on grounds of relevance, noting that "[a] court may conclude that there is simply too great an analytical gap between the data and the opinion proffered." 522 U.S. 136, 146 (1997).
In reversing the district and appellate courts, the state Supreme Court noted that New Mexico has never adopted the Joiner rule, and it refused to do so in this case. The lower courts improperly blurred the line between the court's province to evaluate the reliability of Dahlgren's methodology and the jury's province to weigh the strength of Dahlgren's conclusions. New Mexico law leaves credibility determinations and the weighing of the evidence to the jury.
The supreme court reversed the district court's exclusion of Dahlgren's expert causation testimony, his study, and the animal studies on which his study relied, and also reversed the summary judgment granted on the basis of those exclusions, remanding the case to the district court.