**3.1 Accidents and unintentional releases**

 UOG wells are industrial sites with heavy and moving equipment, hazardous and toxic substances, and harsh environmental conditions. As a result, accidents and malfunctions (e.g., well blowouts, explosions, failure in well integrity such as sustained casing pressure and communication of the well with other, often orphaned wells) are the cause for most documented deaths and injuries for workers at unconventional well sites [5, 6].

Although it is difficult to obtain detailed information on worker-related injuries and fatalities for UOG, the oil and gas extraction industry in general has an occupational fatality rate that is 2.5-times that of the construction industry and 7-fold higher than the industry average [5, 6]. Fatalities are primarily caused by trafficaccidents (nearly a third of all confirmed fatalities), and smaller producers tend to have a higher mortality rate than larger and multi-national companies [7]. The traffic-related occupational risk to UOG workers is not surprising considering the substantial amount of material (e.g., water, HFF chemicals and additives, proppant), equipment (e.g., pipes, compressors, work-over equipment), and waste products (e.g., flowback and produced water, used drilling mud and drill cuttings) that need to be transported to and from the well site. Drilling and fracturing a well usually involves more than 1000 truck trips, often on narrow country roads not designed for such heavy use [8]. In contrast to UOG worker fatalities, the oil and gas industry has below-average injury rates, a fact that has been attributed to underreporting [6, 7, 9]. Although most accidents and fatalities occur among oil and gas workers, they also impact nearby communities. Truck accidents, well blowouts and explosions have caused injuries and fatalities among residents (see Section 4 for details).

## **3.2 Air pollution risks**

The main sources of air pollution on UOG sites are [4]:


Workers may suffer from acute exposure to hazardous and toxic air pollutants such as hydrogen sulfide, benzene, formaldehyde and other volatile organic compounds [5]. Hydrogen sulfide arguably poses the greatest acute toxicity risk, causing irritation and central nervous system effects at concentrations as low as 100 ppm and death at around 1000 ppm [10]. Other risks arise from exposure to hydrocarbons, including aromatics such as benzene, ethylbenzene, toluene and the isomers of xylene (collectively referred to as BTEX). The health effects associated with BTEX include several types of leukemia, non-Hodgkin's lymphoma, anemia and other hematopoietic disorders, immunological effects, and reproductive and developmental effects [4, 11, 12]. While the health effects of BTEX are well documented and health-based regulatory exposure standards exist, other sources of exposure are less well characterized and not regulated. These include chemicals in HFF and volatilized components in drilling muds. A sizeable fraction of compounds used in HFF do not have Chemical Abstract Service (CAS) identifiers [13].

 In addition, workers may suffer chronic exposure to stressors such as crystalline silica, which is the main proppant used in hydraulic fracturing to hold open rock fractures and ease the flow of oil and gas to the surface. Prolonged inhalation of silica can cause silicosis and lung cancer, and it is also associated with chronic obstructive pulmonary disease, kidney disease and autoimmune diseases [14]. OSHA has issued a health alert for workers concerning exposure to silica during hydraulic fracturing [15]. Esswein et al. reports a study by the National Institute for Occupational Safety and Health (NIOSH), which collected and analyzed 111 samples of personal breathing zone data for respirable crystalline silica exposure at 11 UOG sites in five states (Colorado, Texas, North Dakota, Arkansas, and Pennsylvania) [16]. They found that 93% of samples exceeded the threshold limit value (TLV) of the American Conference of Industrial Hygienists of 0.025 mg/m3 , 76% exceeded the Recommended Exposure Limit (REL) of the National Institute of Occupational Safety and Health (NIOSH) of 0.05 mg/m3 and 51% were higher than the permissible exposure limit (PEL) by the Occupational Safety and Health Administration (OSHA) of 0.05 mg/m3 averaged over an 8 hour-day. The differences in limits reflect the health protection goal of the respective institutions and the contexts and situations in which exposures are evaluated. Much of the silica sand (also known as 'frac sand') is mined in Wisconsin and Minnesota, thereby extending the occupational health risks to workers outside of the oil and gas industry and to regions where no hydraulic fracturing takes place [5]. Esswein et al. in a separate study also identified chemical exposure risks, including benzene, at six UOG sites in Colorado and Wyoming in 2013 and again found that wearable personal breathing zone monitors provided insufficient protection and were not always worn because of malfunctions [17].

### **3.3 Risks from soil contamination**

 Soil contamination from UOG operations can occur through surface spills of HFF, chemicals, drilling muds, and other compounds used during all life cycle phases of the well [18]. Health risks in these instances are largely limited to on-site workers and occur primarily through dermal contact. Workers may also carry contaminants indoors on their clothes and boots. Soil contamination has not yet been extensively studied in the UOG literature.

*Human Health Risks of Unconventional Oil and Gas Development Using Hydraulic Fracturing DOI: http://dx.doi.org/10.5772/intechopen.82479* 
