Toxic apportionment: a causation and risk contribution model.

• Author: Boston, Gerald W.

1. Introduction and Summary

   1. The Basic Idea

      In a typical toxic tort case,(1) the plaintiff or the plaintiffs property suffers some harm as a result of exposure to one or more sources of a toxic substance.(2) For example, if the plaintiff has been exposed to asbestos-containing materials(3) and has a history of cigarette smoking, and after years of exposure to both develops a cancer of the lungs, the question arises in tort law as to which of the exposures caused the cancer. However, under the current state of medical knowledge, the question of which toxic substance, if either, actually caused the cancer cannot be answered definitively.(4) If the exposed person sues asbestos manufacturers for products liability, he will have to attempt to prove that the exposure to asbestos-containing materials was either the but-for cause(5) of or a substantial causal factor in the cancer.(6) The trial of such a case will no doubt produce conflicting expert testimony, with the plaintiffs experts stressing the well-established causal relationship between certain cancers and asbestos exposure by inhalation,(7) and the defendant's experts stressing with equally well-documented data the causal relationship between smoking and lung cancer.(8) If the duration and extent of the exposures are significant enough that epidemiological data would support a plausible argument that each is a cause of the plaintiffs cancer, then the data do not warrant concluding that only the defendant's product or only the plaintiff's smoking should be regarded as the cause, thereby requiring either the defendant or the plaintiff to bear the entire loss.(9)

      One method of avoiding an all-or-nothing outcome in such a case is to apply principles of apportionment: the plaintiffs cancer would be apportioned among the parties on the basis of the comparative cancer risks each created through its conduct. Nevertheless, this risk contribution model would not predicate liability based solely on risk creation, as the plaintiff must sustain actual harm and those risks created also must be "a' cause of that actual harm. Apportionment using a risk contribution model also avoids the rubric of "fault" that results when courts apportion liability under comparative negligence or fault-based schemes,(10) and it yields fairer and more efficient results than those achieved under some of the alternative causation models employed by courts over the last few decades.(11) For example, some courts hold that so long as a plaintiff demonstrates that asbestos exposure is a substantial factor in bringing about the cancer, the defendant is liable for the entire loss, whether or not there may be other causal factors.(12) In our smoking and asbestos exposure example, courts until recently were reluctant to find a reasonable basis on which to predicate an apportionment of the harm, and, as a result, our plaintiff would recover all of his damages despite the obvious relevance of a history of smoking.(13)

      If the plaintiffs asbestos exposure and his smoking are both shown to be causal factors in the plaintiffs lung cancer, then the loss is necessarily capable of apportionment on the basis of the relative risks demonstrated for each kind of toxic exposure. From the perspective of individualized justice, the model produces a fairer result than nonapportionment. Because notions of individualized justice place emphasis on individual accountability, if a party's own conduct creates a kind of risk known to result in the harm complained of, then that party should bear a portion of the loss. Moreover, by apportioning the harm among the respective risk contributors, each pays no more than the portion of the harm fairly attributable to it. Thus, so long as individual responsibility is a valued goal of tort law, courts should apportion losses to reflect the contribution of each responsible party based on the risks each created.

      Requiring the defendant to bear the entire loss is inefficient because it eliminates an incentive for persons to provide for their own health and safety. At the other extreme, having the plaintiff bear the entire loss is also inefficient because defendants then lack the incentive to avoid creating toxic-related risks. Apportionment of harm should yield the optimal measure of deterrence: if the defendants are liable only for a portion of the total harm, consistent with the risks they created, it will avoid both the underdeterrence and overdeterrence that result from placing the entire loss on either the plaintiffs or defendants. Moreover, the plaintiffs have an incentive to engage in safer lifestyles, an incentive that is missing if the defendants bear the entire loss.

      In addition to requiring an analysis of the relative contribution of risk, this model demands proof of causation. In our smoking and asbestos exposure example, each party's proofs must be sufficient to make out a submissible issue on causation - that the toxic exposure was "a" cause of the harm, applying the tests for causation that have evolved in toxics cases. This Article develops the issue of precisely what kinds of proof would be sufficient for this purpose in part VI, below.(14)

      The risk contribution model is not based on proportional recovery for risk alone, as has been advocated by some(15) and criticized by others.(16) Instead, it combines some of the benefits sought by those advocating proportional recovery for risk with the elements of causation and actual harm traditional to the toxic tort setting. The model represents a synthesis of ex ante risk and ex post harm. Moreover, it does not treat toxic exposure as an independent harm, as Professor Wright advocates.(17) Instead, this model requires proof of actual harm above and beyond exposure itself. Rather than looking to economic theory as the basis of apportiorment, it looks to science as the underlying basis for apportionment. By examining toxicology and epidemiology, along with whatever particularistic evidence may be available, as the basis for determining disease causes in humans, the model achieves both fairness and deterrence goals and will simplify trials in some cases. In the case involving the plaintiff exposed to asbestos who also smokes, the experts themselves, as seen below, examine the scientific literature to answer the question of what caused the plaintiffs lung cancer and of how much of a risk each source of toxic exposure created toward that resultant harm. Tort law should also he willing to base an apportionment of that harm on the same considerations that medical science applies to these questions.

      In part II, the Article considers the existing black-letter rules and comments governing apportionment set forth in the Restatement (Second) of Torts, identifies the strengths and weaknesses of those rules, and assesses their compatibility with the model suggested here. Part III develops the risk contribution and causation model by focusing on one recent decision involving a plaintiffs asbestos exposure and smoking history. Part IV considers multiple-defendant, real property cases in which each defendant contributes to the risk of the resultant harm and develops how apportionment functions in that setting. Part V expands on the multiple-defendant apportionment issues by discussing the risk factors developed in litigation under the Comprehensive Environmental Response, Compensation, and Liability Act.(18) Thus, both personal injury and real property torts are considered. Part VI examines the sciences of toxicology and epidemiology, the two major sources of data relevant to risk assessment in toxic tort cases. Part VII is devoted to examining and identifying who are the risk contributors and explores in greater detail the causation threshold that must be crossed as a prerequisite to application of toxic apportionment. Parts VIII and IX consider some of the impediments to and rationales for the adoption of a risk contribution and causation model of toxic apportionment.

   2. The Context

      It is useful to see where the risk contribution model fits within the bigger picture of tort law. Much of the litigation involving apportionment has arisen in the context of joint torts, in which courts hold multiple actors liable for a loss caused to the plaintiff. Initially courts limited joint torts to cases of vicarious liability for concerted action, holding each defendant in a common scheme jointly and severally responsible for the entire harm.(19) No effort has ever been undertaken in these concert of action cases to allocate or apportion the harm among the respective actors based on the degree of culpability each one bore or on the causal contribution of each to the harm. Gradually courts expanded the concept of joint torts to incorporate situations where each of two or more persons acted independently but their actions concurred or combined to produce a single injury.(20) In these cases, the courts often speak of "single, indivisible" injuries, thereby implying that there exists no basis for determining how much of the harm can be attributed to each of the actors.(21) For example, where two negligent drivers collide, killing a third person, the courts have consistently regarded the harm as indivisible and have held both drivers jointly and severally liable for death damages.(22) In contrast, where two actors each inflict a separate injury on the plaintiff, albeit concurrently, there has been a willingness to regard the harm as two separate injuries and hold each actor responsible only for the harm it has caused.(23)

      Difficulties arise when there is a reasonable basis for making some division among various causes even though the injury is singular. Once that division is made, the prerequisite for joint and several liability - single, indivisible injury - no longer exists, but proportionate liability is measured only by the harm each actor caused. Debate centers around whether such a division is purely arbitrary, or whether it is predicated on...