**1. Introduction**

Advances in technologies that allow directional drilling coupled with highvolume hydraulic fracturing have made large unconventional oil and gas deposits accessible in the United States. The Energy Information Agency (EIA) estimates that in 2017 approximately 60% of U.S. dry natural gas production came from shale resources [1]. Similarly, oil production from tight oil formations rose from a negligible fraction in 2000 to 50% of total crude oil production in 2017 [2]. This growth has brought oil and gas production and related infrastructure closer to towns and communities in more than 20 states, with more than 15 active shale plays, and

*Exploitation of Unconventional Oil and Gas Resources - Hydraulic Fracturing...* 

raised concerns about the risks to public health from chemical and nonchemical stressors associated with unconventional oil and gas production (UOG) [3]. This chapter provides a summary of these risks by taking a life cycle approach to characterizing the sources and types of health stressors and their likely exposure pathways. While UOG shares a number of processes with conventional oil and gas production, it differs in several important aspects, noticeably the use of directional (horizontal drilling) and large-volume hydraulic fracturing to stimulate the flow of natural gas or oil to the wellhead. These differences are particularly important as they pose additional, and to date not exhaustively regulated health, risks. There is also still greater uncertainty compared with conventional oil and gas production regarding the lack of information about the content of hydraulic fracturing fluids (HFF) and their health effects.
