In tort law, an activity that (1) is not of common usage, and (2) creates a foreseeable and highly significant risk of physical harm even when reasonable care is exercised by all actors. See Restatement (Third) of Torts § 20(b) (2009). A person who is found by a court to have carried on an abnormally dangerous activity will be subject to strict liability for physical harm resulting from that activity. See id. § 20(a) (2009).
ORIGIN OF THE RULE
Courts have often identified blasting (the controlled use of explosives to break down or remove rocks) as the paradigm of an abnormally dangerous activity because of its inherent dangers, and they applied strict liability in cases where blasting resulted in physical harm. The victims of physical harm resulting from blasting were often totally innocent and uninvolved in the activity, while the persons conducting the blasting were doing so for their own financial benefit and were well-aware of the risks. Courts therefore took the position that defendants should be held strictly liable for any harm caused by projected debris. See Restatement (Third) of Torts § 20, cmt.(e) (2009).
In Rylands v. Fletcher, an English case from 1868, the opinion read that "[a] person who for his own purposes brings on his lands and collects and keeps there anything likely to do mischief if it escapes, must keep it in at his peril." American courts often cite this case as providing the origin of the rule on abnormally dangerous activities. In US jurisdictions, courts have never required that the activity take place on the defendant's land. However, they retained the requirement of "unnatural use" in the form of "not of common usage", meaning an activity that is unreasonable or inappropriate in light of the circumstances. See Restatement (Third) of Torts § 20, cmt.(d) (2009).
Common usage: an activity that is in common usage is not abnormally dangerous, even if it involves a great amount of risk. An activity will be deemed of common usage if a significant part of the community engages in it. For example, driving cars is in common usage, but storing toxic chemicals is not. The rationale is that there is no need for strict liability when many people impose a risk on each other, because of the principle of reciprocity: all of them enjoy the benefits and share the risks of the activity. On the other hand, if a dangerous activity is unusual, the persons engaging in it impose a risk on others who do not benefit, hence the need for strict liability. See Restatement (Third) of Torts § 20, cmt.(j) (2009).
Highly significant risk of physical harm: the term "physical harm" generally includes both bodily harm and property damage. A risk of physical harm is deemed "highly significant" if any of the following is true:
- the likelihood of physical harm is unusually high, even though the severity of that harm is ordinary. For example, using explosives in a residential neighborhood is very likely to result in physical harm, but the harm may be limited, in most cases, to moderate property damage.
- the likelihood of physical harm is relatively low, but the severity of the potential harm is very high.
In any case, courts must evaluate the likelihood of physical harm and the potential severity of that harm in the context in which the activity took place. See Restatement (Third) of Torts § 20, cmt.(g) (2009).
Reasonable care: an activity that is generally safe when all participants exercise reasonable care is not an abnormally dangerous activity. To be deemed abnormally dangerous, any activity must present a highly significant risk of physical harm even when all participants are reasonably careful. Most ordinary activities can be made generally safe through the exercise of reasonable care, and thus fall under the rule of negligence liability (as opposed to strict liability). See Restatement (Third) of Torts § 20, cmt.(h) (2009).
Foreseeability: most defendants who are found to have engaged in abnormally dangerous activities actually knew about the risky nature of their actions. However, actual knowledge of the risks is not necessary for an activity to be found abnormally dangerous. It is sufficient that the defendant should have known / had reason to know (often because the risky nature of the activity is of common knowledge). On the other hand, if the defendant sincerely and reasonably believed that the activity did not create a highly significant risk of physical harm, the risky nature of the activity was not foreseeable and there is no justification for imposing strict liability. See Restatement (Third) of Torts § 20, cmt.(i) (2009).