The “triggered” concept is a vaguely-defined notion that refers to an immediate exposure that makes the heart attack occur on a particular day, as opposed to the long-term exposures or conditions that make someone have a heart attack they would not have otherwise had, or have it a decade earlier than they otherwise would have. As for what exactly a trigger means, it could mean having the impending heart attack a day sooner than it would have otherwise have happened, or perhaps a week, or maybe even a few hours or a month. There is little reason to believe that the authors had a precise (i.e., scientifically useful) definition in mind, and there is no doubt that the hodgepodge of sources they used to create their estimate all used different definitions, most of which were probably not well defined either. Obviously there is a rather important difference between use of cocaine or another powerful stimulant that causes a heart attack in a generally healthy forty-five-year-old and air pollution that causes ailing ninety-year-olds to die a week earlier than they would have.
These observations should cause you to ask something that none of the reporters or other commentators I read thought to ask: What does this trigger concept really mean? If they are talking about events or deaths that occur a few days earlier than they otherwise would, it is bad but the impact is limited. If they are talking about events that would not have otherwise occurred, or would have occurred decades later then we are back in the realm of what we usually mean by causes of a disease. But if it was what I guess, some random mix of these, then how do the results make any sense at all?
I know that the literature on air pollution and fatal disease events is confused and controversial. I doubt anyone would seriously claim we know the effects within a factor of two. Yet the authors of the new paper estimated the effects to two significant figures – i.e., numbers like exposure to traffic triggering 7.4% of heart attacks. So they are claiming to not merely know it is around 7% – already way more precise than is possible to estimate – but that it is more than 7.3% but not as high as 7.5%. And yet no one asked, how could you possibly know that? Once again, the faith that non-experts seem to have in the abilities of epidemiology would be charming if it were not for the credulity.
Oh, and you will be excused for not noticing this – unlike the reporters and commentators who also did not notice it – because I was vague in my list of factors they studied: The study conspicuously left out cigarette smoking, even though it would probably top the list if it were actually possible to do this study legitimately. I am guessing that the reason is that no one presumes to be able to estimate the “triggering” effect of smoking apart from the serious long-term damage that causes heart attacks that would not have otherwise happened. But, then again, air pollution involves long-term exposure for most who are exposed. In theory it is possible to sort out trigger effects from long-term exposure effects of anything, but perhaps someone should have asked “how can you do that, and how good are the estimates really?”
There are some more technical criticisms that can be made, such as the exposures not really being comparable to each other. Also, for various reasons it is folly to try to divvy up disease cases to their individual causes for a disease with a long list of interacting causes. But this level of understanding is not necessary for anyone who just thinks to ask “wait a minute; how could they possibly estimate that?”
Finally anyone who actually reads the abstract could discover that the authors separated “sexual activity” into its own category even though there was an aggregate “physical exertion” category. (Maybe they just don’t do it right.) If they had included sexual activity in it, the physical exertion category would have been at the top of the list they made (recall UN35 about how you can always play this game). But then the headlines would have had to read that physical exertion is an unrecognized public health hazard that we need to do something about. If the authors had actually admitted to this conclusion, though, they probably would not have generated headlines or gotten published in The Lancet. I am sure they thought about it for minutes before deciding to go for the headlines rather than taking on tough and complicated questions about exposures that have both costs and benefits.
Of course, air pollution is caused by activities that have benefits too, but why worry about that. It was undoubtedly more fun to engage in an ad hoc arithmetic exercise, which the authors either did not understand was or did not care was nonsense, and claim to have shown something of “considerable public health relevance” without addressing the larger worldly context. A running-joke observation made at the academic epidemiology meetings notes that your relative risk for a heart attack in the hour after having sex is about 50 compared to the average hour of your life, but most people who know this realize that a 50-fold increase is really not of such considerable importance in that context.