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|date=March 2011}}</ref> A 2011 study for the city of Fort Worth, Texas, examining air quality around natural gas sites "did not reveal any significant health threats."<ref name=fortworthtexas>{{cite report | url=http://fortworthtexas.gov/uploadedFiles/Gas_Wells/AirQualityStudy_final.pdf | title=City of Fort Worth: Natural Gas Air Quality Study | date=July 13, 2011|format = PDF | publisher = Fort Worth, Texas| author = Eastern Research Group, Sage Environmental Consulting| accessdate = 2012-05-07}}</ref><ref>{{cite news | url=http://www.star-telegram.com/2011/07/14/3222750/no-significant-health-threats.html | title=Study: No 'significant health threats' from natural gas sites in Fort Worth | newspaper=Fort Worth Star-Telegram | date=July 15, 2011}}</ref> In DISH, Texas, elevated levels of disulphides, benzene, xylenes and naphthalene have been detected in the air, emitted from the compressor stations.<ref name="DISH TX SciAm">{{cite news |url=http://www.scientificamerican.com/article.cfm?id=shale-gas-and-hydraulic-fracturing|title=Natural gas cracked out of shale deposits may mean the U.S. has a stable supply for a century—but at what cost to the environment and human health? |author=Biello, David |date=30 March 2010 |publisher=Scientific American |accessdate=23 March 2012}}</ref> People living near shale gas drilling sites often "complain of headaches, diarrhea, nosebleeds, dizziness, blackouts, muscle spasms, and other problems."<ref name="DISH TX EHP">{{cite journal |url=http://dx.doi.org/10.1289/ehp.119-a348|title=Blind Rush? Shale Gas Boom Proceeds Amid Human Health Questions|author=Schmidt, Charles |date=1 August 2011 |journal=Environmental Health Perspectives |volume=119 |pages=a348-a353|accessdate=23 March 2012|doi=10.1289/ehp.119-a348}}</ref> Cause-and-effect relationships have not been established.<ref name="DISH TX EHP" />  In Garfield County, Colorado, another area with a high concentration of drilling rigs, volatile organic compound emissions increased 30% between 2004 and 2006; during the same period there was a rash of health complaints from local residents. Epidemiological studies that might confirm or rule out any connection between these complaints and fracking are virtually non-existent.<ref name="HeatOnGas"/> In 2012, researchers from the Colorado School of Public Health showed that air pollution caused by fracking may contribute to "acute and chronic health problems" for those living near drilling sites.<ref>{{cite web |url= http://attheforefront.ucdenver.edu/?p=2546 |title= Study shows air emissions near fracking sites may have serious health impacts |date= 19 March 2012 |work= @theForefront |publisher= Colorado School of Public Health |accessdate= 25 April 2012 }}</ref>