In the Colorado mountains, a spike in air pollution has been linked to a boom in oil and gas drilling. About 800 miles away on the plains of north Texas, there's a drilling boom, too, but some air pollution levels have declined. Opponents of drilling point to Colorado and say it's dangerous. Companies point to Texas and say drilling is safe.
The answer appears to be that drilling can be safe or it can be dangerous. Industry practices, enforcement, geography and even snow cover can minimize or magnify air pollution problems.
"It's like a vehicle. Some cars drip oil," said Russell Schnell, deputy director of the federal Earth System Research Laboratory in Boulder, Colo. "You have wells that are absolutely tight. And you have other places where a valve gives out, and you have huge leaks."
The good news, nearly all sides agree, is that the technology exists to control methane gas leaks and other air pollution associated with drilling. The bad news is that the industry is booming so rapidly that some companies and some regulators can't seem to get ahead of the problems, which could ultimately cost billions of dollars to remedy.
The worries about what drilling does to the air are both global and local, with scientists concerned about the effects on climate change as well as the possible health consequences from breathing smog, soot and other pollutants.
Hydraulic fracturing, or fracking, has made it possible to tap into deep reserves of oil and gas but has also raised concerns about pollution. The industry and many federal and state officials say the practice is safe when done properly, but environmental groups and some scientists say there hasn't been enough research.
Some environmentalists say if leaks and pollution can be minimized, the boom has benefits, since gas burns much cleaner than coal, emitting half the carbon dioxide.
Al Gore told The Associated Press that it's "not irresponsible" to look at gas as a short-term substitute for coal-fired electricity. But Gore added that the main component of gas, methane, is a more potent heat-trapping greenhouse gas than CO2. That means that if large quantities leak, the advantage over coal disappears, the former vice president said.
In Colorado, the National Oceanic and Atmospheric Administration estimated that 4 percent of methane was leaking from wells, far more than previously estimated, and that people who live near production areas may be exposed to worrisome levels of benzene and other toxic compounds present in oil and gas.
Across the industry, the technology for stopping leaks can be as simple as fixing seals and gaskets, or it can involve hundreds of millions of dollars of new construction.
"I think it's totally fixable," Schnell said. "At least the bigger companies, they are really on top of this."
Gore added that when companies capture leaking methane, they end up with more to sell. "So there's an economic incentive to capture it and stop the leaking," he said.
Another major source of worry is the industry's practice of burning off, or flaring, natural gas that comes out of the ground as a byproduct of oil drilling. Over the past five years, the U.S. has increased the amount of flared and wasted gas more than any other nation, though Russia still burns off far more than any other country.
In some places, energy companies haven't invested in the infrastructure needed to capture and process the gas because the oil is more valuable.
In the Bakken Shale oil fields of North Dakota, for example, about 30 percent of the natural gas is flared off because there aren't enough pipelines yet to carry it away. The amount of gas wasted in the state is estimated at up to $100 million a year. And officials in North Dakota said last month that the situation there might not be completely solved until the end of the decade.