Rapid increases in production of shale gas via high volume hydraulic fracturing has resulted in a variety of environmental concerns. One of the most controversial elements of shale gas production is the lack of data regarding potential water contamination and human toxicity. This study analyzes the relative human toxicity impact of electricity produced from shale gas and coal. Using a life cycle approach, the human toxicity impact (HTI) of electricity produced from shale gas is an order of magnitude lower than electricity produced from coal, even when toxic emissions from coal mining operations are not included. Air emissions of mercury are the largest contributor to the human toxicity impact of coal electricity, whereas barium contained in produced water from hydraulic fracturing operations is the largest contributor for the HTI of electricity from shale gas. A scenario analysis indicates that the HTI of shale gas could approach the HTI of coal if containment failures occur that lead to the discharge of untreated flowback water into surface water; however, when analogous containment failures of the coal system are taken into account, the HTI of coal electricity is 2-3 orders of magnitude greater than electricity produced from shale gas. The results of the analysis can be used to identify areas for improvement and calculate relative toxicity between the two systems. Given the lack of exposure data and large inherent uncertainty in toxicity factors, the results cannot be used to infer absolute human health impacts from either system.
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