In matters regarding pollution, President Barack Obama often finds himself with an impossible choice. Should he protect public health, as environmentalists demand, or preserve jobs, as congressional Republicans urge? Both concerns are valid, and both are important tasks for the president. But sometimes, he has to give one or the other priority.

Last week, Obama decided against a new Environmental Protection Agency proposal to reduce smog, which contributes to such illnesses as asthma and heart disease. The decision on ground-level ozone drew a chorus of condemnation from such groups as the Sierra Club and the American Lung Association, which said it will cost lives and raise health care costs.

That prediction is no doubt accurate. But every public policy decision involves trade-offs, and this one was trickier than most.

It would have been one of the most expensive environmental measures ever — with an annual price tag as high as $90 billion. The expected benefits could be even higher, but they also could be as low as $13 billion per year, according to the EPA.

Hundreds of counties would have found themselves in violation of the new standards. Electric utilities and other businesses would have had to lay out funds for upgraded pollution-control equipment. Most would have complied, but some would have closed down.

On a strict cost-benefit basis, the proposal comes close to a tossup. Other expenditures could yield a much bigger payoff.

That’s not the only reason the White House rejected the change. Cass Sunstein, head of the president’s Office of Information and Regulatory Affairs, noted in a letter to the EPA that the agency is required to revisit the smog standard in 2013, by which time better scientific data will be available.

It made little sense, he indicated, to impose a new rule that might be overhauled in just two years — particularly when the president has promised that federal regulations “must promote predictability and reduce uncertainty.” Besides, said Sunstein, the EPA has taken other anti-pollution steps that will have the side effect of reducing smog.

Even if the ozone rule were clearly worth the eventual cost, presidents also have to think about the immediate future. At the moment, Obama is right to focus on the short-term health of the economy, which would suffer from new mandates that put a burden on the private sector.

If the economy remains stalled, the funds needed to pay for environmental improvements will be hard to come by. A strong economy, by contrast, will make such investments more affordable — and far more appealing to the electorate.

Sometimes the difference between a good idea and a bad one is a simple matter of timing. Exercise, for instance, fosters good health. But someone in bed with the flu is well-advised to put off a strenuous run or spin class until after the malady passes.

Over the coming decades, Americans might very well be better off with tighter rules on ozone. But right now, as Obama wisely understands, it’s more important to nurse the economy back to health.