(Paul Foy/Associated Press)

Indeed, the EPA estimates that the regulation will produce $37 billion to $90 billion in health benefits by 2016 (compared with clean-up costs of about $9.6 billion). Confusingly, though, most of these estimated benefits don’t actually come from reducing mercury pollution. So where do they come from? And just how big a deal are these rules?

First, some background. Plenty of evidence suggests that mercury — a neurotoxin — inflicts quite a bit of harm on the public. A 2005 study in Environmental Health Perspectives, for instance, looked at the effects of mercury poisoning on the brains of fetuses. It found that 637,000 babies were born each year with significant amounts of mercury in their bloodstream, with about two-thirds of those kids suffering IQ loss. The authors estimated that the lost economic productivity due to decreased intelligence came to about $8.7 billion per year, with $1.3 billion of that attributable to power plants.

Oddly enough, though, when the EPA calculated benefits from its new pollution rule, mercury only played a minuscule role, mainly for technical reasons (more on that below). The vast majority of that estimated $37 billion to $90 billion in benefits comes from a reduction in premature deaths — about 11,000 per year by 2016 — caused by particulate matter, not mercury. As utilities install scrubbers at coal plants to sop up mercury, they also end up curbing other types of pollutants, especially lung-damaging particulates, and this “co-benefit” accounts for about $36 billion to $89 billion of the estimated benefits. (Indeed, former Bush administration regulatory czar Susan Dudley has criticized the rules on this front.)

Then there’s climate change. The EPA’s rule is expected to hasten the retirement of roughly one-tenth of the nation’s coal-fired power plants, mostly the oldest and dirtiest, and those plants kick up plenty of heat-trapping carbon dioxide into the atmosphere. The EPA tried to estimate the social cost of emitting carbon dioxide — trying to account for various expected global-warming impacts, like floods and agricultural damage — and estimates that the rule will help the United States avoid $360 million in climate costs by 2016. That’s only a partial picture, though, since mitigating global warming is mainly about helping future generations.

Now, that doesn’t mean the EPA isn’t cleaning up mercury, or that the mercury benefits are illusory, although they account for a mere $4 million of estimated benefits for the final rule. What it means is that the EPA had trouble calculating the full value of preventing cognitive damage to young children. Scientists are still struggling to quantify the harm wreaked by mercury. And, as Michael Livermore writes, the EPA only looked at a small subset of the population, namely kids of people who catch freshwater fish for their own consumption, and left out the hard-to-quantify impacts of a mercury-reduction in store-bought fish. What’s more, the EPA only calculated “lost wages” as a benefit, even though there are a lot of reasons beyond lost wages that we’d like to avoid neurological harm to young children.

And so, while there’s little question that the EPA’s mercury regulations are a huge deal, that still leaves plenty of room to argue over the “true” value of the new rules.

* Correction: This post originally stated that the benefits of reducing carbon-dioxide were not factored in. They were. Apologies and duly corrected.