Dufour v. BP Expl & Prod

Case: 23-60361 Document: 78-1 Page: 1 Date Filed: 10/27/2026 United States Court of Appeals for the Fifth Circuit United States Court of Appeals ____________ Fifth Circuit FILED No. 23-60361 October 27, 2026 ____________ Thomas B. Plunkett, III Clerk Richard Allen DuFour, Jr., Plaintiff—Appellant, versus BP Exploration & Production, Incorporated; BP America Production Company, Defendants—Appellees. ______________________________ Appeal from the United States District Court for the Southern District of Mississippi USDC No. 1:19-CV-591 ______________________________ Before Richman, Oldham, and Ramirez, Circuit Judges. Priscilla Richman, Circuit Judge: Richard DuFour, Jr., sued BP Exploration & Production, Inc. and BP America Production Company (collectively, BP) for injuries allegedly caused by DuFour’s exposure to toxic substances during cleanup efforts following the 2010 Deepwater Horizon oil spill. The district court excluded DuFour’s causation experts and granted summary judgment in favor of BP. We affirm. Case: 23-60361 Document: 78-1 Page: 2 Date Filed: 10/27/2026 No. 23-60361 I As recounted in many decisions of this court, what has become known as the BP Oil Spill occurred as a result of a blowout of a well drilled by the Deepwater Horizon oil rig. The well released enormous amounts of crude oil into the Gulf of Mexico. DuFour was employed by BP Enterprises as a cleanup worker for almost three years. He worked in Pascagoula, Mississippi, performing shoreline cleanup and boom decontamination. He was exposed to crude oil and chemical dispersants through inhalation, as well as airborne and direct contact. DuFour alleges that the substances to which he was exposed were toxic and caused chronic worsening of his asthma. DuFour filed suit in the Eastern District of Louisiana pursuant to the Back-End Litigation Option (BELO) of the Medical Settlement Agreement (MSA) in the Deepwater Horizon class-action litigation. The parties filed a joint stipulation asserting that the appropriate venue was the Southern District of Mississippi, and the court transferred the case. Under the MSA, a BELO plaintiff must prove, among other things, that his injuries were “legally caused by” exposure to substances during response efforts. DuFour proffered two experts—Dr. Michael Freeman and Dr. Shawn French—to prove causation. BP filed motions to exclude their testimony and opinions and moved for summary judgment. DuFour requested an evidentiary hearing to present live testimony to prove the admissibility of his experts’ testimony. The district court denied the request for an evidentiary hearing, granted the motions to exclude Dr. Freeman and Dr. French, and granted BP’s motion for summary judgment. DuFour timely appealed. 2 Case: 23-60361 Document: 78-1 Page: 3 Date Filed: 10/27/2026 No. 23-60361 II An initial question is what law should be applied in resolving DuFour’s claims. The settlement agreement provides that “[n]otwithstanding the law applicable to the underlying claims, which the PARTIES dispute, this MEDICAL SETTLEMENT AGREEMENT and the RELEASE hereunder shall be interpreted in accordance with General Maritime Law.” DuFour’s exposure occurred in Pascagoula, Mississippi. However, neither party has argued that Mississippi law should apply. To the contrary, DuFour maintains, and BP agrees, that maritime law governs this case. DuFour asserts in his briefing in our court, as he did in the district court, that there is diversity jurisdiction and that “this Court has jurisdiction under 28 U.S.C. § 1331 based on Federal Question for the federal judiciary to hear all cases of admiralty and maritime jurisdiction.” His briefing in our court, as he did in the district court, additionally asserts “this Court has jurisdiction under 28 U.S.C. § 1333 and 46 U.S.C. § 30101 which extends the admiralty and maritime jurisdiction of the United States to ‘cases of injury or damage, to person or property, caused by a vessel on navigable waters, even though the injury or damage is done or consummated on land.’” In the district court, he asserted that “[t]his matter is governed by maritime law. As a result, under maritime law, if Mr. Dufour proves that this exposure caused a permanent exacerbation of his chronic condition, asthma, then Mr. Dufour would be entitled to [monetary compensation].” DuFour’s Second Amended Complaint alleges that “BP Defendants are at fault for the Oil Spill and Deepwater Horizon Incident.” It further alleges: As part of its response to the Deepwater Horizon Incident, BP Defendants directed the use of vessels to, inter alia, recover oil 3 Case: 23-60361 Document: 78-1 Page: 4 Date Filed: 10/27/2026 No. 23-60361 coming to the surface of the Gulf of Mexico; skim oil from the surface of the water; and conduct in-situ burning of oil that reached the surface of the water. BP Defendants also employed vessels to tow and deploy booms—floating barriers intended to contain, deflect, or hold back oil floating in the water’s surface. BP also employed an army of people to clean oil from the beaches and barrier islands. DuFour additionally alleged that “[a]s part of the Response Activities, BP Defendants purchased highly noxious chemical dispersants” that “were applied over large areas using various methods, including but not limited to . . . spraying from vessels, and subsea injection.” BP agrees that maritime law governs DuFour’s claims. Its Answer to the Second Amended Complaint said “BP Parties admit that Plaintiff’s claims sound in maritime law.” Its Answer also asserted that “[m]aritime law preempts all state law claims that are or may be asserted in this action.” DuFour has conceded on appeal that “[w]ithout an expert, DuFour could not prove the central issue in this litigation—the level, extent, and duration of his exposure to harmful substances during the relevant times of the Oil Spill.” He additionally states in his briefing in our court that he “was required to prove the following factors as to legal causation: (1) the scientific knowledge of the harmful level of exposure to a particular chemical, and (2) that he was exposed to such quantities.” In support, he cites an unpublished opinion from our court, Seaman v. Seacor Marine L.L.C., 1 that considered claims “under the Jones Act and general maritime law.” 2 _____________________ 1 326 F. App’x 721 (5th Cir. 2009). 2 Id. at 723. 4  Case: 23-60361 Document: 78-1 Page: 5 Date Filed: 10/27/2026 No. 23-60361 In Seaman, a captain of vessels alleged that he developed bladder cancer as a result of chemical exposures, 3 including exposure to diesel exhaust and Ferox for more than a decade. 4 In addressing causation, our court said, “[s]cientific knowledge of the harmful level of exposure to a chemical, plus knowledge that the plaintiff was exposed to such quantities, are minimal facts necessary to sustain the plaintiffs’ burden in a toxic tort case.” 5 We then said, “[a] plaintiff in such a case cannot expect lay fact- finders to understand medical causation; expert testimony is thus required to establish causation.” 6 For this latter proposition, we cited toxic tort cases, 7 but we also cited a Second Circuit decision 8 authored by then-JUDGE SOTOMAYOR, Wills v. Amerada Hess Corporation, 9 which adjudicated claims brought under the Jones Act and general maritime law. 10 In Wills, the widow of a seaman claimed that her husband’s cancer was caused, in part, by exposure to benzene and polycyclic aromatic hydrocarbons while working aboard Amerada Hess’s vessels. 11 The widow maintained that “by granting summary judgment in favor of defendants on the ground that she failed to proffer admissible evidence on the issue of causation,” the district court _____________________ 3 Id. 4 Id. at 725. 5 Id. at 723 (quoting Allen v. Pa. Eng’g Corp., 102 F.3d 194, 199 (5th Cir. 1996)); see also Allen, 102 F.3d at 195, 199 (affirming motions for judgment “as a matter of law” in favor of defendants sued by “a maintenance worker at Baton Rouge General Hospital” in a toxic tort case). 6 Seaman, 326 F. App’x at 723. 7 Id. at 723 n.6. 8 Id. 9 379 F.3d 32 (2d Cir. 2004). 10 Id. at 36. 11 Id. at 37. 5 Case: 23-60361 Document: 78-1 Page: 6 Date Filed: 10/27/2026 No. 23-60361 “undermined ʻthe right of the jury to pass upon the question of fault and causation,’ a right that in Jones Act cases ‘must be most liberally viewed.’” 12 Similarly, the widow contended that, “in excluding her proposed expert testimony on the issue of causation,” the district court “failed to appreciate her reduced burden to prove causation.” 13 The Second Circuit rejected these arguments, holding “where an injury has multiple potential etiologies, expert testimony is necessary to establish causation, even in view of plaintiff’s reduced burden to prove causation,” pointing out that “this court has never held that a Jones Act plaintiff can survive summary judgment in a toxic tort case without admissible expert testimony on the issue of causation.” 14 In Gowdy, a published opinion involving claims under both the Jones Act and general maritime law, our court cited our unpublished decision in Seaman with approval for the proposition that “[i]n toxic tort cases, expert testimony is often required.” 15 In Gowdy, we drew a distinction between those cases in which “the causal link . . . can be understood by jurors based on everyday knowledge and experience” and those in which it cannot. 16 The causal link between chemical exposure and exacerbation of preexisting asthma is more akin to “developing cancer after years of toxic tort exposure” than to “breaking one’s leg after being hit by a car.”17 Our Gowdy decision also recognized that “in Wills v. Amerada Hess Corporation, the Second _____________________ 12 Id. at 45 (quoting Oxley v. City of New York, 923 F.2d 22, 25 (2d Cir. 1991)). 13 Id. 14 Id. at 46. 15 Gowdy v. Marine Spill Response Corp., 925 F.3d 200, 206 (5th Cir. 2019). 16 Id. at 208. 17 Id. 6  Case: 23-60361 Document: 78-1 Page: 7 Date Filed: 10/27/2026 No. 23-60361 Circuit explained that expert testimony was needed on the question of whether exposure to toxic chemicals caused squamous cell carcinoma because that causal link was ‘sufficiently beyond the knowledge of the lay juror.’” 18 Our decision in Seaman v. Seacor Marine L.L.C., 19 although unpublished, was well-reasoned and clearly in line with decisions in our circuit and in other circuits that have applied maritime law. We expressly agree with its holding. DuFour’s claims are among the class of non-readily observable injuries, or those with multiple possible etiologies, that traditionally require an expert to survive summary judgment on legal causation. We note that we have applied the toxic tort causation standard from Knight v. Kirby Inland Marine Inc. 20 in at least one published BELO decision, 21 as well as unpublished decisions. 22 This standard requires the _____________________ 18 Id. at 206 (quoting Wills v. Amerada Hess Corp., 379 F.3d 32, 46 (2d Cir. 2004) (SOTOMAYOR, J.)). 19 326 F. App’x 721 (5th Cir. 2009). 20 482 F.3d 347, 351 (5th Cir. 2007). 21 See, e.g., Williams v. BP Expl. & Prod., Inc., 143 F.4th 593, 598 (5th Cir. 2025) (“This circuit recently applied the toxic tort standard for causation to a case concerning an illness allegedly caused by the DWH oil spill,” requiring “ʻa plaintiff to show both general and specific causation in toxic tort cases.’”) (quoting Prest v. BP Expl. & Prod., Inc., No. 22- 30779, 2023 WL 6518116, at *2 (5th Cir. Oct. 5, 2023))). 22 See, e.g., Wunstell v. BP, P.L.C., No. 23-30859, 2024 WL 4100496, at *2 (5th Cir. Sept. 6, 2024) (“In toxic-exposure cases like this one, a plaintiff ‘must prove both general and specific causation.’”) (quoting Braggs v. BP Expl. & Prod., Inc., No. 23-30297, 2024 WL 863356, at *2 (5th Cir. Feb. 29, 2024))); id. (“The expert must show ‘that the chemical at issue is actually capable of harming individuals in the general population.’ To make this showing, the expert must ‘determine not only whether a chemical can cause certain health effects, but also at what level of exposure those health effects appear.’”(internal citations omitted) (quoting Braggs, 2024 WL 863356, at *2)); Barrington v. BP Expl. & Prod., Inc., No. 23-30343, 2024 WL 400191, at *1 (5th Cir. Feb. 2, 2024); Prest, 2023 WL 6518116 at 7  Case: 23-60361 Document: 78-1 Page: 8 Date Filed: 10/27/2026 No. 23-60361 district court to first “determine whether there is . . . admissible general- causation evidence,” then “determine whether there is admissible specific- causation evidence.” 23 While the Knight standard derives from Texas law, 24 the Knight decision applied that standard to a Jones Act claim, 25 even though, as already discussed above, in Jones Act cases, there is a distinct body of law regarding the burden for establishing causation. 26 We further note that the Eleventh Circuit has “formally recognize[d] in the maritime context that non-readily observable injuries require medical expert testimony to prove causation” as a matter of general maritime law. 27 In that case, the Eleventh Circuit affirmed summary judgment in favor of the defendant because the plaintiff “failed to adduce sufficient medical evidence to satisfy proximate cause.” 28 The Eleventh Circuit discussed principles of toxic tort liability, including general and specific causation, and applied those _____________________ *2; Byrd v. BP Expl. & Prod., Inc., No. 22-30654, 2023 WL 4046280, at *2 (5th Cir. June 16, 2023); McGill v. BP Expl. & Prod., Inc., 830 F. App’x 430, 433-34 (5th Cir. 2020). 23 Knight, 482 F.3d at 351. 24 Id. (quoting Merrell Dow Pharms., Inc. v. Havner, 953 S.W.2d 706, 714 (Tex. 1997)); see also Ruffin v. BP Expl. & Prod., Inc., 176 F.4th 855, 860-61 & n.1 (5th Cir. 2026) (applying Knight while recognizing that “Knight’s general-causation standard is drawn from Texas law, which neither party suggests applies here,” but assuming arguendo that the BELO claim at issue arose under general maritime law, explaining that “we have still previously required general and specific causation for toxic-tort claims arising under general maritime law, and we see no reason to depart from that here.”). 25 Knight, 42 F.3d at 350. 26 See Brister v. A.W.I., Inc., 946 F.2d 350, 354 (5th Cir. 1991) ([T]he Jones Act contains a liberal causation requirement. If the defendant’s negligence played any part, however small, in producing the seaman’s injury, it results in liability.”). 27 Willis v. Royal Caribbean Cruises, Ltd., 77 F.4th 1332, 1338 (11th Cir. 2023). 28 Id. at 1333. 8  Case: 23-60361 Document: 78-1 Page: 9 Date Filed: 10/27/2026 No. 23-60361 principles to BELO claims in In re Deepwater Horizon BELO Cases. 29 As already discussed, our circuit has applied these same toxic tort principles in BELO cases. Moreover, the federal common law of admiralty is not immutable. As JUSTICE KAVANAUGH explained, writing for the Court in Great Lakes Insurance SE v. Raiders Retreat Realty Co., 30 “[u]nder the Constitution, federal courts possess authority to create and apply maritime law.” 31 The Great Lakes decision set forth how maritime law may be created and applied: “To maintain that uniform system, federal courts ‘make decisional law’ for maritime cases.” 32 “When a federal court decides a maritime case, it acts as a ‘federal common law court, much as state courts do in state common-law cases.’” 33 “Exercising that authority, federal courts follow previously ‘established’ maritime rules.” 34 “No bright line exists for determining when a federal maritime rule is ‘established,’ but a body of judicial decisions can suffice.” 35 “In the absence of an established rule, federal courts may create uniform maritime rules.” 36 We conclude that there is either (1) “a body of judicial decisions” that “suffice” to establish the maritime rule that causation principles that have _____________________ 29 119 F.4th 937, 940-43, 945-46 (11th Cir. 2024). 30 601 U.S. 65 (2024). 31 Id. at 69. 32 Id. (quoting Norfolk S. Ry. Co. v. Kirby, 543 U.S. 14, 23 (2008)). 33 Id. (quoting Air & Liquid Sys. Corp. v. DeVries, 586 U.S. 446, 452 (2019)). 34 Id. at 70 (quoting Wilburn Boat Co. v. Fireman’s Fund Ins. Co., 348 U.S. 310, 314 (1955)). 35 Id. (citing Bisso v. Inland Waterways Corp., 349 U.S. 85, 89-90 (1955)). 36 Id. (citing Norfolk S. Ry. Co., 543 U.S. at 23). 9 Case: 23-60361 Document: 78-1 Page: 10 Date Filed: 10/27/2026 No. 23-60361 been applied in certain types of non-maritime toxic tort cases for decades 37 apply under maritime law in cases like the present one, or (2) at least in this circuit, as well as the Second and Eleventh Circuits, federal courts have created that maritime rule. The district court did not err in applying widely accepted toxic tort causation principles as part of maritime law to the facts of this case. III As a preliminary matter, we also address how the toxic tort causation principles established in Daubert and subsequent Supreme Court decisions interact with the federal summary judgment rule embodied in Federal Rule of Civil Procedure 56 and the Supreme Court’s decision in Celotex Corp. v. Catrett. 38 Provided that the case is in the correct procedural posture, courts should apply Daubert to disqualify experts at the summary judgment stage of a proceeding. Toxic tort cases in federal district courts (and state courts for that matter) typically have a scheduling order. A district court may set deadlines for the conclusion of discovery, and for filing expert reports, and may set a date by which any motions for summary judgment must be filed. When such deadlines are in place, the plaintiff knows that if he does not have expert testimony sufficient to raise a fact question on causation by the deadline for filing dispositive motions, summary judgment may be rendered against him. If, at the summary judgment stage, the district court concludes the plaintiff’s expert’s report or testimony is inadmissible under Federal Rule of Evidence 702, the plaintiff could conceivably ask that the deadlines be extended to _____________________ 37 See, e.g., Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579 (1993) and its progeny. 38 477 U.S. 317 (1986). 10 Case: 23-60361 Document: 78-1 Page: 11 Date Filed: 10/27/2026 No. 23-60361 permit the plaintiff to obtain additional or different expert evidence. But, at some point, a district court may establish a deadline by which sufficiently reliable and otherwise admissible evidence must be identified, or else summary judgment may be entered. In the present case, the district court entered a scheduling order setting a discovery-cutoff date, deadlines for expert designations, and a date by which dispositive motions were to be filed. The order was amended several times. To survive BP’s motion for summary judgment, DuFour was required to present reliable expert evidence that his asthma was aggravated by the chemicals he alleged he came into contact with during the Deepwater Horizon oil spill cleanup. Rule 56 requires that he do so by “citing to particular parts of materials in the record.” 39 The district court excluded DuFour’s causation experts. DuFour did not contend in the district court, and does not contend on appeal, that with more time, he could present additional or different expert evidence. So, his claim rises or falls on the record presently before us. The Supreme Court expressly recognized in Daubert that if a trial court concludes that the scientific evidence “presented supporting a position is insufficient to allow a reasonable juror to conclude that the position more likely than not is true,” the court is “free to direct a judgment, and likewise to grant summary judgment.” 40 The Supreme Court cited the Sixth Circuit’s decision in Turpin v. Merrell Dow Pharmaceuticals, Inc., 41 and the _____________________ 39 Fed. R. Civ. P. 56(c)(1)(A). 40 Daubert, 509 U.S. at 596 (internal citations omitted) (citing Fed. R. Civ. P. 50(a), 56). 41 959 F.2d 1349 (6th Cir. 1992). 11 Case: 23-60361 Document: 78-1 Page: 12 Date Filed: 10/27/2026 No. 23-60361 Fifth Circuit’s decision in Brock v. Merrell Dow Pharmaceuticals, Inc., 42 for these propositions. 43 In Turpin, the district court had granted summary judgment in favor of the defendant after concluding that the expert evidence was insufficient to raise a fact question as to causation, 44 and the circuit court affirmed. 45 In Brock, our court reversed a district court judgment that, after a jury trial, had awarded $550,000 in damages. 46 We rendered judgment in favor of the defendant. 47 In Kumho Tire Co., Ltd. v. Carmichael, 48 the Supreme Court upheld a summary judgment that the Eleventh Circuit had reversed. 49 The Supreme Court examined in detail the testimony and opinions proffered by an expert in tire failure analysis, concluding that the district court did not err in excluding the testimony of the witness at the summary judgment stage. 50 With this background in mind, we proceed to consider whether the district court erred in excluding DuFour’s expert witnesses. IV DuFour maintains that the district court erred by declining to hold an evidentiary hearing on the admissibility of his experts’ testimony (commonly _____________________ 42 874 F.2d 307 (5th Cir. 1989). 43 Daubert, 509 U.S. at 596. 44 959 F.2d at 1349-50. 45 Id. at 1361. 46 874 F.2d at 308. 47 Id. at 315. 48 526 U.S. 137 (1999). 49 Id. at 142, 158. 50 Id. at 153-58. 12 Case: 23-60361 Document: 78-1 Page: 13 Date Filed: 10/27/2026 No. 23-60361 called a Daubert hearing). We review the district court’s decision not to hold a Daubert hearing for abuse of discretion. 51 The district court explained its reasoning: “Given the thoroughness of the parties’ briefs and exhibits, as well as the Court’s familiarity with the issues raised, the Court finds that neither a hearing nor oral argument would be helpful or otherwise warranted.” DuFour objects that this is a scientifically complex case involving a new set of expert opinions relying on new data. He contends the district court abused its discretion by denying his experts the opportunity to explain any deficiencies the district court saw in their proffered testimony. “[T]he law grants a district court the same broad latitude when it decides how to determine reliability as it enjoys in respect to its ultimate reliability determination.” 52 DuFour cites no case holding that a district court erred because it denied a Daubert hearing over a party’s assertion that the case is complex and the proffered expert could explain their testimony. Indeed, we have observed that “a district court is not always required to hold a formal Daubert hearing”—a district court must “perform its gatekeeping function by performing some type of Daubert inquiry and by making findings about the witness’s qualifications to give expert testimony.” 53 The trial court has “considerable leeway” to “decide whether or when special briefing or other proceedings are needed.” 54 _____________________ 51 See, e.g., Carlson v. Bioremedi Therapeutic Sys., Inc., 822 F.3d 194, 201 (5th Cir. 2016) (reviewing a decision not to conduct a Daubert inquiry for abuse of discretion). 52 Kumho Tire, 526 U.S. at 142. 53 Carlson, 822 F.3d at 201. 54 Kumho Tire, 526 U.S. at 152. 13 Case: 23-60361 Document: 78-1 Page: 14 Date Filed: 10/27/2026 No. 23-60361 The district court did not abuse its discretion by declining to hold a Daubert hearing. The district court explained that it could resolve the Daubert motions on the parties’ thorough presentations and its familiarity with the Deepwater Horizon litigation. Because the record included depositions, declarations, and reports from the relevant experts, the district court had an adequate basis to determine the reliability of the experts’ opinions without a live hearing. We see nothing that indicates the court abused its “considerable leeway.” 55 V DuFour contends the district court erred in excluding Dr. Freeman’s and Dr. French’s expert testimony. “We review ‘a district court’s exclusion of expert testimony for abuse of discretion,’” 56 and an “appellate court will not reverse . . . unless the ruling is manifestly erroneous.” 57 Rule 702 of the Federal Rules of Evidence governs the admissibility of expert testimony. 58 The Supreme Court’s decision in Daubert “provides the analytical framework for determining whether expert testimony is admissible _____________________ 55 Id. 56 Smith v. Chrysler Grp., L.L.C., 909 F.3d 744, 748 (5th Cir. 2018) (quoting In re Complaint of C.F. Bean L.L.C., 841 F.3d 365, 369 (5th Cir. 2016)). 57 Gen. Elec. Co. v. Joiner, 522 U.S. 136, 142 (1997); see also Salem v. U.S. Lines Co., 370 U.S. 31, 35 (1962) (explaining that “the trial judge has broad discretion in the matter of the admission or exclusion of expert evidence, and his action is to be sustained unless manifestly erroneous”); Congress & Spring Co. v. Edgar, 99 U.S. 645, 658 (1878) (“Cases arise where it is very much a matter of discretion with the court whether to receive or exclude the evidence; but the appellate court will not reverse in such a case, unless the ruling is manifestly erroneous.”). 58 Fed. R. Evid. 702. 14 Case: 23-60361 Document: 78-1 Page: 15 Date Filed: 10/27/2026 No. 23-60361 under Rule 702 . . . .” 59 Under Daubert, the trial judge must “determine whether the expert testimony is both reliable and relevant.” 60 Even were substantive Mississippi law applicable rather than maritime law, the Mississippi Supreme Court has applied Daubert-based principles in toxic tort cases. 61 In a benzene case, that court upheld the exclusion of expert testimony or opinion when several of the studies relied upon did not provide a dose-response ratio. 62 The Mississippi court concluded that “a dose- response ratio is needed to indicate the level of exposure to benzene of the subjects of the study [and] is crucial under the case-study methodology to show specific causation so that [the plaintiff’s] level of exposure could be specifically compared to subjects with similar exposure.” 63 Dr. Freeman relied on three studies. The district court observed that different studies that examine the same exposure-disease relationship generally should yield similar results. The district court found it “problematic” that the three studies referenced in Dr. Freeman’s original report did not examine the same exposure-disease relationship. The district court concluded that the three studies upon which Dr. Freeman relied did not all address the same conditions, and none of them “appear[ed] to address the specific worsening of preexisting chronic asthma that is the central issue in DuFour’s case.” _____________________ 59 Burleson v. Tex. Dep’t of Crim. Just., 393 F.3d 577, 583 (5th Cir. 2004) (quoting Pipitone v. Biomatrix, Inc., 288 F.3d 239, 243 (5th Cir. 2002)). 60 Id. at 584. 61 See Watts v. Radiator Specialty Co., 990 So.2d 143, 147, 149-50 (Miss. 2008). 62 See id. at 146-48. 63 Id. at 147 n.9. 15 Case: 23-60361 Document: 78-1 Page: 16 Date Filed: 10/27/2026 No. 23-60361 Nor were the studies based upon the same exposures. “Dr. Freeman’s analysis focused on one study’s findings concerning inhalational exposure from oil spills,” “while the other two studies did not appear to be limited to inhalation exposure, nor did they differentiate between methods of exposure.” The district court concluded that “[t]he discrepancies in the underlying data used to support the exposure-disease relationship in the Original Report, along with Dr. Freeman’s ipse dixit statements” “indicate that Dr. Freeman did not reliably apply the relevant criteria.” The district court concluded, “There is simply too great of an analytical gap between the data and the conclusions in the Original Report that DuFour’s chronic asthma worsened due to exposure to chemicals associated with his oil spill cleanup efforts.” The district court considered Dr. Freeman’s reliance on the Lawrence Study. The court observed, “Dr. Freeman himself undermines confidence in the Lawrence Study by pointing out that ‘[o]ne limitation of the study is possible misclassification and over reporting of asthma due to the utilization of self-reported wheezing in the case definition.’” When self-reported wheezing was excluded from the definition of asthma, the Lawrence Study concluded, and Dr. Freeman admitted, that the results were “largely attenuated.” The district court also observed that the Lawrence Study included certain adults who participated in oil spill response and cleanup work, but it excluded “those who reported an asthma diagnosis prior to April 20, 2010, the onset date of the disaster.” The district court reasoned that, “[t]here is no dispute that DuFour has long suffered from asthma, including as a child” and “that he would therefore have been excluded from the Lawrence Study.” The district court also concluded, “The fact that the Lawrence Study would not have included DuFour as a participant because of his preexisting asthma demonstrates that he is not on common footing with 16 Case: 23-60361 Document: 78-1 Page: 17 Date Filed: 10/27/2026 No. 23-60361 any participant in that Study, and as such Dr. Freeman did not reliably apply the Lawrence Study to this case.” The district court additionally explained that “the dose-response discussed by Dr. Freeman in his Original Report concerned the reporting of symptoms, not the diagnosis or worsening of asthma or any other chronic conditions.” The district court pointed out that “Dr. Freeman acknowledged in his deposition that while these symptoms are symptoms of asthma, ‘they’re not specifically asthma worsening.’” Dr. Freeman had himself acknowledged that “[p]eople who have worsening of asthma all have trouble breathing or will feel shortness of breath and wheezing, by definition that’s asthma. But not everybody who wheezes or has shortness of breath, et cetera, has asthma.” The dissenting opinion is unpersuaded by the district court’s criticism of Dr. Freeman’s reports and the Lawrence Study. 64 It points out that Dr. Freeman attempted to address the Study’s limitations and emphasizes that the Study found a “linear relationship” between exposure and asthma. 65 The Lawrence Study, however, did not measure its participants’ severity of asthma. The linear relationship that it found was between a non-asthmatic worker’s level of chemical exposure and their relative risk of developing asthma, defined as self-reported wheezing or a self-reported medical diagnosis. The inferential step that Dr. Freeman failed to establish, in the district court’s judgment, was how that trend related to DuFour’s alleged injury of pre-existing asthma worsening. According to the Lawrence Study, the more exposed a non-asthmatic worker was, the higher their risk of reporting either wheezing or an asthma _____________________ 64 Post, at 29-30. 65 Post, at 29. 17 Case: 23-60361 Document: 78-1 Page: 18 Date Filed: 10/27/2026 No. 23-60361 diagnosis. The Study did not conclude that the more exposed a worker with pre-existing was, the more severe their asthma became. Put differently, if one were to picture a graph with X and Y axes, the X-axis in the Lawrence Study is level of exposure and the Y-axis is relative risk of developing asthma. The Y-axis is not, as the dissenting opinion characterizes it, the severity of asthma a worker was expected to develop. The dissenting opinion also cites the reported p-value, or statistical significance, of the Lawrence Study to support its strength. 66 But as the Federal Judicial Center’s Reference Manual on Scientific Evidence makes clear, the fact “[t]hat a study’s results are statistically significant says nothing about the magnitude of any association (i.e., the relative risk or odds ratio) or about the biological, clinical, or public health importance of the finding. . . . A large study may find a statistically significant relationship that is quite modest in magnitude.” 67 Statistical significance is only a useful metric insofar as the underlying results of the Study capture a meaningful relationship between relevant variables. The Study’s statistical significance alone, therefore, cannot make up for the other limitations that the district court recognized. Most notable among these are its failure to study workers who, like DuFour, already had asthma, and the fact that when self-reported wheezing was excluded from the definition of asthma, the results were “largely attenuated.” Although the _____________________ 66 Post, at 31. 67 Fed. Jud. Ctr., Reference Manual on Scientific Evidence 930 (4th ed. 2025). The dissenting opinion highlights criticism of this latest version of the Reference Manual by some state Attorneys General. Post, at 30. This quotation is very similar to language in the uncontroversial third edition of the Reference Manual, and neither versions’ discussion of expert witnesses and statistical methods has been criticized. See Fed. Jud. Ctr., Reference Manual on Scientific Evidence 573 (3rd ed. 2011). 18 Case: 23-60361 Document: 78-1 Page: 19 Date Filed: 10/27/2026 No. 23-60361 Lawrence Study found a relative risk of greater than 2.0, a common benchmark for reliability under Daubert, 68 for some categories of workers, Dr. Freeman did not attempt to demonstrate to the district court that DuFour would have been in one of those categories, even if he had not been excluded from the Study for his pre-existing asthma. Therefore, it was not an abuse of discretion for the district court to conclude that DuFour “was not on common footing with any participant in that Study.” Ultimately, the district court reasoned that “Dr. Freeman has not provided a reasonable basis for using respiratory symptoms alone as a proxy for chronic asthma in opining as to causation in DuFour’s case.” “His leap from data showing an increased chance of respiratory symptoms based on duration and frequency of exposure to a conclusion ‘that exposures to oil spill-associated irritants during [DuFour’s] response work were a substantial factor in causing his asthma to worsen during his employment and to persist afterward,’” “constitutes ipse dixit testimony and leaves too great of an analytical gap between the data and his causation opinion.” The district court did not abuse its discretion in concluding that “DuFour has not carried his burden of establishing the reliability of Dr. Freeman’s opinions or testimony concerning legal causation in this case, whether it be it general, specific, or otherwise.” The district court also excluded the testimony of Dr. French, one of DuFour’s treating physicians. Dr. French did not provide a written report. The district court explained that, “When a party does not provide an expert report from a treating physician who will testify at trial, the doctor’s _____________________ 68 See Merrell Dow Pharms., Inc. v. Havner, 953 S.W.2d 706, 716 (Tex. 1997) (collecting cases requiri