Oil and natural-gas companies are spending tens of millions of dollars to reduce pollution from their wells in Northern Colorado, in part to comply with emission regulations but also because of their own concerns over air quality. Also, they've found it's good business.

Wells hydraulically fractured in the Niobrara shale formation emit a toxic brew that includes methane, toluene and benzene, according to the Colorado Department of Public Health and Environment.

Thanks to the oil boom, these sources emitted 15,975 tons of pollutants last year, up from about 9,372 tons of pollutants in 2008, according to department estimates.

Companies have tried to reduce that pollution.

In just the Wattenberg field, an area that covers Weld County, they have spent more than $40 million on some 3,000 emissions-control devices since tighter regulations went into effect in 2005, said Tisha Conoly Schuller, president of the Colorado Oil and Gas Association.

Despite those efforts, the industry has seen its share of criticism from residents concerned about emissions.

"Emissions are very complicated and it's understandable that people, community members, don't understand what the industry is doing