This paper presented a review of the available scientific evidence on environmental health impacts of UNGD, specifically shale gas, tight gas, and CSG. While some environmental health research has been conducted with regard to UNGD, it is clear that there is a lack of highly relevant evidence of direct health outcomes caused by the activities of UNGD. However, it should be noted that absence of evidence does not mean evidence of absence. The literature reviewed in this paper, as well as a persistent and substantial public response, continues to suggest concern.
The research reported here is dominated by traditional environmental health issues such as health-related impacts from air and water quality. There are still many unknowns such as vector-borne disease, the hazard potential of environmental pollutants, soil quality, noise and light pollution, traffic, and risk perception. More research is also needed into the impacts that these risk factors and exposures might have on human health outcomes. The research is also dominated by studies on short-term impacts, with some risk assessments on long-term impacts or discussions alluding to long-term impacts. This may be due to the rapid pace of development in recent years; however, it is imperative that longitudinal studies are put in place so that potential long-term impacts, or lack thereof, can be assessed.
With interest in researching UNGD and health rapidly increasing, it is inevitable that more studies will be published in the near future in peer-reviewed literature and in publically accessible media. While we attempted to do this review as exhaustively and comprehensively as possible, it is recognised that this review will have had shortcomings in terms of our search strategy, which may not have identified all relevant, and perhaps available, literature. Additionally, there may be more data in domains that cannot be searched so, regrettably, data from these sources were not included here. It is imperative for government and industry bodies to make the process as transparent and open as possible so that data can be accessed, which can help to add more studies to the ‘highly relevant’ category.
When considering UNGD on a more global scale, the majority of the reviewed research comes from the USA, with some also from Australia, and limited research being reported in the English-language databases from outside these two countries where UNGD is ongoing, e.g., Europe. The majority of research reviewed here focusses on shale gas, with some research on tight gas and even less research on CSG. This may be due to differences in the length and pace of development in various regions, as well as differences in emission profiles for the various types of gas. There can be little doubt that as many additional countries develop their natural gas resources, the hydrogeology and other environmental settings will vary from one area to another. Therefore, it is important that similar research is conducted in other regions to determine if environmental health hazards and impacts are similar, or if there are additional environmental health concerns that should be considered.
In reviewing available studies for strength of evidence, there are very few, if any, methodologically rigorous studies that have examined the potential cause-and-effect of UNGD in the construct of hazard analyses, linked to exposure pathways and the actual health outcomes. In fact, our review shows that most of the peer-reviewed research was ‘not very relevant’ in this context. Most of the ‘highly relevant’ studies cannot be described as scientifically rigorous, due to methodological limitations, such as measurement and selection bias, as well as potential confounding.
Overall, our review identified that adverse health impacts were most often alluded to only in the context of UNGD or perhaps attributed to these activities as a principle of precaution. Regarding the grey literature, most of the studies were in the ‘relevant’ to ‘not very relevant’ categories, with the majority of the reports considered ‘relevant’.
Overall, there was very limited systematically gathered, scientific evidence of health effects directly caused by UNGD activity. Notably, this review identified only seven studies as ‘highly relevant’, demonstrating the lack of research on direct health impacts associated with UNGD. More importantly, while evidence of the environmental cause of adverse health impact was lacking, several scholars and experts voiced concerns about the potential for adverse health outcomes. These concerns were based on credible evidence of detrimental environmental impact and strongly suggest that the lack of evidence of health impact does not dismiss claims of health impact. The available evidence, or lack thereof, is not sufficient cause to rule in or rule out significant or specific, future, or cumulative health impacts of UNGD activities.
It is probable that the lack of evidence on direct causal links between environmental hazards and health outcomes is a result of the rapid expansion of this industry in a short period of time — leaving evidence-based research activities with very little time to respond. Additionally, there is the potential for environmental health outcomes with longer latencies for which effects may not yet be seen.
While some authors are adamant about the potential health harm, it remains difficult to credibly assess the extent of the risk posed to the public, and implications for government agencies and the resource companies, while this gap in scientific knowledge remains. Future work needs to be focussed on research that includes baseline monitoring and prospective studies to summarise, diagnose, and predict what environmental health impacts of UNGD might be.