On Monday, EPA promulgated amendments to its “Exceptional Events” Rule. The rule is important, particularly in the Western states, and most particularly in connection with EPA’s latest iteration of the ozone NAAQS. EPA’s most significant revision was to eliminate the requirement that state air agencies demonstrate that, “but for” the exceptional event, the state or relevant area would have complied with the applicable NAAQS. The change is important for two reasons. First, on the merits, EPA noted that:
the “but for” criterion has often been interpreted as implying the need for a strict quantitative analysis to show a single value … of the estimated air quality impact from the event. As a result, some air agencies began using burdensome approaches to provide quantitative analyses in their exceptional events demonstrations to show that the event in question was a “but for” cause of a NAAQS exceedance or violation in the sense that without the event, the exceedance or violation would not have occurred. In many cases, the “but for” role of a single source or event is difficult to determine with certainty and it is more often the case that the impact of emissions from events and other sources cannot be separately quantified and distinguished.
I think that EPA got this exactly right. As tort professors have always known, how a burden of proof is allocated is often outcome-determinative.
Which brings me to the second reason why the change is important – at least to me. Just hearing the words “but for” causation triggers an uncontrollable wave of nostalgia. In 1996, my client, New England Telephone, was 
awarded summary judgment in a CERCLA contribution case. It was then the first – and may still be the only – case in which a defendant who admittedly sent hazardous substances to a site was awarded summary judgment on the ground that its wastes had not caused the incurrence of any response costs.
I like to think that NET prevailed due to the fine lawyering of its counsel, but I have always known in my heart of hearts that the identity of the judge may have had something to do with the result. The case was heard by Robert Keeton, distinguished judge, Harvard Law professor and – importantly – one of the authors of Prosser and Keeton on Torts.
At the summary judgment hearing, Judge Keeton did not want to hear from me, even though it was my motion. He did not really even want to hear from the plaintiffs’ counsel. Instead, he launched into an approximately 30-minute lecture on the role of causation in tort law, including, of course, a discussion of “but for” causation. When he finished the discussion from Prosser and Keeton about the so-called “Minnesota fire cases”, Judge Keeton paused, looked up, smiled broadly, and said: “I wrote that part.”
It was the best summary judgment argument I ever gave. I never said a word.