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The Environmental Protection Agency has proposed tightening the standard for smog. The goal is cleaner air and improved human health. But due to a quirk in atmospheric chemistry, the move would likely increase global warming.

NPR's Richard Harris explains.

RICHARD HARRIS: Smog isn't just ugly, EPA official Lydia Wegman says ozone smog kills. So, setting a new lower limit for ground-level ozone is a good thing.

Ms. LYDIA WEGMAN (EPA Official): There are very significant human health benefits that can be achieved.

HARRIS: Less smog means fewer asthma attacks, fewer kids in the hospital and fewer days of lost school.

Ms. WEGMAN: And we also believe that we can reduce the risk of early death for people with heart and lung disease.

HARRIS: But it turns out that fixing this problem is likely to make another one worse. That would be global warming. That's because the way many states and localities will reduce smog is by cracking down on the chemicals that produce ozone. And those include nitrogen oxides, or NOx.

Professor JASON WEST (University of North Carolina): The problem with reducing NOx is that by reducing NOx, you also, through atmospheric chemistry, increase the concentrations of methane in the atmosphere.

HARRIS: Jason West is at the University of North Carolina. He says methane gas is much more potent than even carbon dioxide at heating up the atmosphere.

Prof. WEST: By reducing NOx, the net effect on climate is that you make global warming worse.

HARRIS: In fact, you could make warming a lot worse if you got rid of all NOx and a related sulfur compound, that action alone would be enough to increase the Earth's temperature by two degrees Celsius and that's into the danger zone for the climate, according to many scientists and governments. U.S. pollution control laws are moving us gradually in that direction.

Prof. WEST: Europe's in very much the same situation we are. They're trying to control their own ozone as well. So, if they reduce their NOx and we reduce our NOx, suddenly we're talking about big numbers.

HARRIS: So, what do you do? Leave NOx in the atmosphere? Atmospheric Scientist Drew Shindell, at NASA's Goddard Institute in New York City, cringes at the thought.

Mr. DREW SHINDELL (NASA Goddard Institute): I would have trouble with the ethics of ever saying something like people should be choking in regions where these are emitted because of the sake of global climate.

HARRIS: Shindell argues that what's needed is a more holistic approach to the intimately linked problems of climate change and air quality.

Mr. SHINDELL: I think we need to have policies which try to take into account both air quality and climate change. And traditionally, these have been entirely separate.

HARRIS: And in the case of smog, there's actually a fairly easy way to deal with this. Jason West says it's true that reducing NOx will put more methane into the air, but it's not too hard to prevent emissions of methane from sources, like coal mines or leaky gas pipelines.

Prof. WEST: "Reducing methane emissions looks like a pretty promising way in that we know that we can get some of that methane reduced even at a cost savings. And we know that it's good for climate and good for air quality.

HARRIS: Unfortunately, unlike smog, which is a local problem and can be addressed with local measures, methane is a global pollutant. So, the usual way of attacking air pollution with local efforts simply won't do the trick.

Prof. WEST: If Los Angeles, for example, wanted to control its own methane, it would find that it would really have no effect.

HARRIS: So, eventually, dealing with local air pollution will require global action. Lydia Wegman at the EPA says her agency is starting to move in that direction.

Ms. WEGMAN: We definitely think that integrating air quality and climate concerns is the best way to go.

HARRIS: And she says that idea is being considered in the next review of smog standards, which is now getting under way.

Richard Harris, NPR News.

INSKEEP: And since these are intricately linked problems, we have a couple of linked reports here. Tomorrow, we'll hear about what it would take to control the potent warming gas, methane.

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