Illustration of a shale-gas electricity system (left), including hydraulic fracturing operations and electricity generation. At right, sketch of a coal electricity system, including coal mining and electricity generation. Source: Lu Chen, Shelie A. Miller and Brian R. Ellis, "Comparative human toxicity impact of electricity produced from shale gas and coal," in Environmental Science & Technology.

New research suggests that lifetime toxic chemical releases are much higher with coal-generated electricity than those from electricity generated with natural gas obtained by fracking.

Researchers from the University of Michigan (U-M) have conducted a comparative analysis of the harmful health effects of electricity produced by both shale gas and coal and found that the lifetime toxic chemical releases were 10 to 100 times greater from coal than shale gas.

They looked at the amount of toxic chemicals released into the air, soil and water during both the resource extraction and electricity generation phases of both technologies. The results suggest that if the U.S. energy market continues to shift from coal to natural gas the overall toxicity burden will decrease.

“This analysis does not imply that concerns associated with shale gas production are unfounded, only that the overall toxic load of coal is definitely greater,” Shelie Miller, an environmental engineer and an associate professor at the U-M School for Environment and Sustainability's Center for Sustainable Systems, said in a statement. “And while the study doesn't address this directly, we should be pursuing renewables more aggressively if we really want to decrease the human toxicity burden of our energy system.”

Recently, the combination of horizontal drilling and hydraulic fracturing or fracking has helped unlock vast stores of natural gas in shale formations. Increased shale-gas production has created a boom in some parts of the U.S. but has also led to concerns over potential contamination of drinking water and possible human health impacts related to hydraulic fracturing.

In the study, the researchers conducted a lifecycle impact assessment in Pennsylvania for 23 natural gas plants and 13 coal-fired power plants as well as data from 2,900 hydraulically fractured wells.

The researchers estimated the toxicity associated with air pollutants emitted during power generation in the coal system, as well as toxic chemical releases during the coal-mining process from acid-mine drainage and coal-ash impoundment. They also analyzed air pollutants for the coal system including particulate matter, mercury, volatile organic compounds, nitrogen oxides and sulfur oxides.

For the fracking system, the researched estimated the toxicity of the fracturing fluid chemicals used to crack rock and release natural gas, as well as the wastewater associated with shale-gas extraction and also looked at air pollutants emitted during both shale-gas extraction and electricity generation.

For both systems, particulate matter released into the air from power plants during electricity generation was the dominant toxicity contributor and outweighed chemical releases that may occur during extraction, while the harmful air emissions from coal-fired power plants were much worse than those from cleaner-burning natural gas plants.

“We looked at the total mass of emissions released per unit of electricity generated throughout the lifetime of both systems, and the overall toxic load is much greater for coal,” Miller said. “Emissions of particulate matter pumped into the air every single day by coal-fired power plants have greater potential human health impacts than any of the other chemicals we examined.”

Miller added that while the results provide a comparison of relative toxicity between the two systems, large uncertainties related to the chemicals used in fracking and a lack of data precluded a full-blown risk assessment.