On Sept. 2, the Obama White House pulled the plug on its own 2010 proposal to impose new and tougher ground-level ozone standards for the United States, deferring a final decision until 2013.

The reaction from environmental advocates was almost uniformly negative. Frances Beinecke, president of the Natural Resources Defense Council, complained that the "White House is siding with corporate polluters over the American people."

Less temperate advocates were, um, less restrained. How did it come to this? President Obama was supposed to be the anti-Bush, a friend to the environment. Now some environmentalists seem ready to give up on him.

Should they? No. Not because of the ozone decision, anyway.

Not all environmental rules are created equal. Some make obvious good sense, some don't. Some are no-brainers, some aren't.

The deferred ozone rules don't, and they aren't. Not yet.

Ozone, an unstable molecule of three oxygen atoms, is a bit schizophrenic. In the upper atmosphere, it's good, hovering in a layer that protects us from the sun's harmful ultraviolet rays.

Near the ground, it's bad, causing respiratory damage to breathers of air, especially the young, the old, and asthmatics like me. I'm glad I live in Minnesota, where ozone levels are low.

And its effects go far beyond asthma attacks. Ground-level ozone kills people. The science on this question is not controversial.

Current rules, from 2008, limit ground-level ozone concentrations to 75 parts per billion. The proposed rules would have reduced the limit to 70 or, perhaps, 60 ppb. What looks like a small change would prevent somewhere between 2,000 and 12,000 premature deaths. Every year.

That's why EPA's science advisors recommended the tighter limit.

Look carefully at the EPA's comparison of the costs and benefits of this change, though, and doubts emerge.

Under the authority of a Clinton-era executive order, the EPA compares the monetary benefits and costs of major environmental proposals. Though not part of the formal rulemaking process, this information is released "to inform the public."

It can also sway political decisions.

According to the EPA, limiting ozone concentrations to 60 parts per billion would cost between $52 and $90 billion annually. The benefits, due to improved human health and avoided deaths, fall between $35 and $100 billion annually.

The evidence is murky: There's a good chance that the costs exceed the benefits. (The numbers for the 70 ppb limit are smaller, but the ranges still overlap significantly.)

The ozone proposal was never a slam-dunk. From an economic perspective, given what we know today, it's a tough call.

For comparison, consider the Cross-State Air Pollution Rule, which limits industrial emissions of sulfur dioxide and nitrogen oxides in the eastern United States. The rule was finalized on July 6 of this year.

According to the EPA's estimates, the annual 2014 benefits due to CSAPR will be $120 to $280 billion. The estimated annual costs: $2.4 billion. Nothing is murky here. The benefits exceed the costs by at least a factor of 50.

This is technically called a no-brainer. The rule, which will prevent up to 34,000 premature deaths per year, was issued with little fanfare. The science, and EPA's supporting benefit-cost analysis, all but silenced potential critics.

Advocates who condemn the president's ozone decision have missed a crucial point. Their political muscle is best used to push for policy changes that are clear winners, of which there are plenty.

Some have argued that Obama's decision was a mistake because spending tens of billions to reduce ozone would stimulate the economy. Yes, it would.

But those same billions, spent on clear winners like green energy, would also stimulate the economy. And there, on top of it, we'd get environmental benefits far exceeding the costs.

The Clean Air Act as a whole has saved countless thousands of lives since its passage in 1970. In another EPA study, issued this April and subjected to vigorous review, annual benefits due to the act's regulations were estimated at $2 trillion and annual costs at $65 billion.

The Clean Air Act pays back $30 for every dollar invested. This doesn't mean, though, that every rule proposed under its authority makes economic sense.

Not so long ago, benefit-cost analysis was viewed by many on the left as nothing more than a right-wing tool for killing environmental regulations. It hasn't turned out that way.

Often, compelling evidence that benefits dramatically exceed costs has lent crucial support to tighter environmental controls.

Environmental advocates happily reap the rewards of such studies when the results support a cleaner environment. A certain restraint is in order when the economic numbers, and the policy decisions, go the other way.

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Jay Coggins is an associate professor in the Department of Applied Economics at the University of Minnesota. The views expressed here do not necessarily reflect the position of the university.