Discussion

We observed statistically significant reductions in total cancer mortality and incidence in Champaign County, Illinois, USA, and the zip code study areas compared with counties that were similar demographically and socioeconomically, and based on nationally standardised rates. Furthermore, cancer occurrence was lower in the study area for most cancer types, with several statistically significant reductions in cancer rates. Results were largely consistent within and across analyses, with a few exceptions. Indeed, the lower mortality rates for most cancers in the study area versus the comparison counties were in accordance with the lower cancer incidence rates observed in Champaign County and the study zip codes. The only noteworthy positive associations in the primary analyses were for prostate cancer and melanoma. Neither prostate cancer mortality nor melanoma mortality was significantly elevated. However, incidence rates for these cancers were significantly increased in Champaign County. It is not clear why elevated rates were observed, although based on chance alone, it would be expected that some cancer rates would be statistically significant in the positive and inverse directions. As indicated, we observed several statistically significant inverse associations (ie, RRs below the null value of 1.0), such as for the lung and bronchus, colorectal and breast cancers. However, only two cancer types (prostate cancer and melanoma; incidence only) were associated with statistically significant positive associations. Thus, significant associations may have been observed due to multiple comparisons—a statistical phenomenon whereby 1 out of 20 associations is statistically significant due to chance.14 ,15 In our analysis, we generated over 100 unique RRs.

Prostate cancer is the most common cancer among men in the USA, and 238 590 incident cases and 29 720 deaths were estimated to occur in 2013.16 ,17 The figures for prostate cancer represent 14.4% of all new cancer cases and 5.1% of all cancer deaths in the USA.17 Increasing age, African-American race, family history of prostate cancer, and genetic variations and mutations are established risk factors for prostate cancer, although the aetiology is largely unknown despite an extensive effort to identify causes of this malignancy.16 ,17 The role of lifestyle and dietary factors are thought to play a role in prostate cancer risk, as past research has identified obesity, physical inactivity and smoking as significant modifiable risk factors for this malignancy.18–21 Our cancer assessment is ecological in nature; thus, aside from age, sex and race, we could not model or adjust for factors such as physical activity, family history of disease or body mass index that may have influenced the results. Based on a review of the literature and statements by prominent cancer organisations,16 ,17 no manufactured gas plant by-product compound has been clearly or consistently associated with prostate cancer risk.

Perhaps the most likely reason for the elevation in prostate cancer incidence during 1991–2010 is the increased rate of prostate-specific antigen (PSA) screening in Champaign County versus the comparison counties. Indeed, based on the Behavioral Risk Factor Surveillance System (BRFSS) survey from the Centers for Disease Control and Prevention (CDC) and administered by the Illinois Department of Public Health,9 the per cent of men who underwent PSA screening was higher in Champaign County than Sangamon, Macon and Winnebago counties (as a composite percentage) during the study period. Furthermore, composite rates of PSA screening were higher in Champaign County than the counties without former gas plants. It is well established and well publicised that screening for prostate cancer results in an increase in incidence rates, whereas a proportion of prostate cancers may otherwise go undetected without screening.22–24 In a sentinel review paper on the epidemiological impact of screening on the incidence and mortality of prostate cancer in the USA, it was suggested that PSA testing was the likely cause of the dramatic increase in prostate cancer incidence during the 1990s.23 Although PSA testing is useful for early diagnosis, its value as a screening tool has been under scrutiny because the theoretical benefit on mortality is questionable.24 According to a recent study from the Prostate, Lung, Colorectal and Ovarian (PLCO) randomised screening trial, men who underwent annual prostate cancer screening with PSA testing and digital rectal examination had a 12% higher incidence rate of prostate cancer compared with men in the control group (who did not undergo PSA testing).25 However, approximately the same rate of death from prostate cancer was observed between groups, and no evidence of a mortality benefit was found in age or pretrial PSA testing strata.25 In our assessment, we observed a statistically significant increase in prostate cancer incidence in Champaign County (where a greater proportion of individuals underwent PSA testing) but no statistically significant difference in prostate cancer mortality versus the comparison counties.

Melanoma was also associated with a statistically significant positive association (incidence only). Cancer incidence data for melanoma were not available at the zip code level; thus, it is uncertain if melanoma occurrence was higher in the areas directly circumscribing the abandoned plant. Approximately 21.3 out of 100 000 men and women are diagnosed annually with melanoma, and it is about 20 times more common in Caucasians than in African-Americans.17 The major risk factor for melanoma is exposure to ultraviolet (UV) rays (sunlight is the primary source of UV rays), particularly among persons with fair skin. Other risk factors include having a large number of moles, having one or more first-degree relatives who have had melanoma and being immunosuppressed.16 It has been hypothesised that malignant melanoma may occur as a result of exposure to occupational or environmental chemicals (eg, vinyl chloride, arsenic, polychlorinated biphenyls, petrochemicals, pesticides), particularly because malignancy can develop in cutaneous areas that have not been exposed to sunlight.26 However, the epidemiological evidence relating chemical exposures to melanoma risk is inconsistent. Thus, it is unclear whether the positive incidence rate ratio in Champaign County is the result of an artifactual finding from multiple comparisons, or has been confounded by sun exposure or other factors, such as immunosuppression. Moreover, general health concerns that the local population may have about living next to a former gas manufacturing site may lead to surveillance bias due to increased screening. Furthermore, a statistically significant deficit of melanoma was observed in Champaign County based on SIR analyses, and no statistically significant associations for melanoma were found based on comparisons with counties that did not have a former gas plant. Finally, oesophageal cancer incidence was elevated when Champaign County was compared with the non-gas plant counties in our sensitivity analysis of cancer incidence, but this observation was not substantiated by the main analyses, the sensitivity analysis of cancer mortality or by the SIR analysis.

Alternatively, an unknown manufactured gas product compound may have produced the slight elevations in prostate cancer and/or melanoma incidence in the main analyses. This scenario is unlikely, however, given the plausible explanations listed above and because reduced rates were observed for cancers with known environmental or chemical relationships. That is, no ‘indicator’ cancer types with established environmental or chemical aetiology were observed in excess. For example, IARC has classified ‘coke production’ as carcinogenic to humans (group 1) for lung cancer because of exposure to PAHs in the industry (although associations from the occupational studies are somewhat tenuous).27–29 In a quantitative review of occupational exposures to PAHs, lung cancer and bladder cancer risk was elevated significantly among workers in the coal gasification industry.30 However, in our assessment, we observed statistically significant reduced mortality and incidence rates of 15% and 17%, respectively, for lung and bronchus cancers, and no associations for urinary system and bladder cancers. Moreover, our analyses were based on rates of cancer at the community level, not among workers likely exposed to much higher concentrations of possible chemical exposures.

It is unclear as to why there was a preponderance of inverse associations in Champaign County and the study zip codes versus the comparison counties. A priori, we developed a systematic protocol for identifying comparable counties. We identified counties based on residential status (urban/rural) and similar demographic and socioeconomic characteristics, and our analyses were adjusted for age, sex and race (at the county level). Based on the 2000 census information, there were approximately 7% fewer ‘ever’ smokers in Champaign County versus the comparison counties. This may explain, in part, the observation of lower cancer rates, particularly for lung and bronchus, in Champaign County. In addition, the prevalence of other potentially important factors, such as alcohol consumption or obesity, in these counties may have confounded the observed associations. However, relatively similar patterns of associations were observed in sensitivity analyses comparing Champaign County and the study zip codes with counties that did not have a former gas plant, and based on SIR analyses using nationally representative cancer data from the SEER programme.

Most of the literature on manufactured gas plants focuses on the environmental and ecological impacts of the gas process residues and waste products. As such, considerable literature exists on the methodological, toxicological, elemental chemistry and extracting techniques involving remediation and compound evaluations from abandoned sites.31–34 While potentially hazardous compounds may have been produced as part of the gas manufacturing process, the extent and level to which compound residues persist at the sites is unclear. In certain cases, potential hazards may be overestimated as samples in some studies have been consistent with background levels or below the assumed level.1 ,35 As mentioned previously, the literature on direct or indirect human health risks from an epidemiological standpoint, is sparse. DeHate et al1 investigated soil vapour intrusion at 10 commercial buildings and 26 single family and multifamily residential properties overlying and/or adjacent to three former manufactured gas plant sites. Soil vapour samples and indoor/outdoor air were analysed for VOCs, and comparative risks were evaluated based on maximum and mean concentrations for BTEX relative to background levels. All hazard indices were less than one or were comparable to mean and maximum background levels, and there was no evidence of manufactured gas plant-related soil vapour intrusion from any of the 36 sites. Based on these findings, the authors reported that no increased public health risks were associated with occupied residential or commercial properties overlying or surrounding former manufactured gas plant facilities.1 Occupational epidemiological studies involving postulated gas plant exposures, such as PAHs,27 ,30 BTEX36–38 and coal tar,30 ,39–41 are extensive but none have evaluated potential disease outcomes resulting from residing in a community that includes a former manufactured gas plant site.

Our historical cancer assessment has limitations that are commonplace with analysing population-level data. Prime among the limitations is that we did not have individual-level information on lifestyle, dietary, medical or occupational factors. In addition, we did not have personal information regarding potential exposures (eg, to soil, groundwater or air) from the manufactured gas process. We were, however, able to adjust cancer estimates for age, sex and race. Because of the complete, systematic and statewide registry in Illinois, data for persons diagnosed with cancer were assembled in an unbiased fashion. Identification and reporting of cancer cases in the ISCR is mandated by state law. The population sizes (ie, the denominator for estimating cancer rates) for the study periods were based on the 1990 and 2000 census information. If there was considerable in-migration or out-migration of the population over time, the estimated RRs may have been affected. The gas plant ceased operations in 1953, and although immediate release of potential compounds may have diminished at that time, concern about the persistence of compounds over time in the vicinity has been raised. Our analyses were conducted on the basis of publically available aggregate-level data. We began our analytical period at the earliest time point possible that enabled us to merge cancer data and census data. Given our analytical periods, there is sufficient latency to observe a carcinogenic effect, if one exists. However, we were not able to capture cases that occurred in earlier time periods, with possible greater exposures. It may be possible that our analyses are not sensitive enough to identify small effects of plant exposures because of the nature of residential exposure classification. Despite these limitations, we found no clear or consistent evidence of an increase in cancer occurrence among residents in a community circumscribing a former manufactured gas plant.

The validity of our results is enhanced by the utilisation of three comparison populations: (1) counties very well matched demographically that had former gas plants, (2) comparison counties without former gas plants and (3) nationally representative cancer data from the SEER programme. These three types of analyses serve as complementary comparisons to examine the consistency of findings across different population metrics. We implemented an objective methodological approach to identify counties in the state of Illinois that were the most closely matched (demographically) to the analytical zones. By using this approach, we were able to account for some prominent confounding factors at the aggregate level. However, the most closely matched comparison areas also had former gas plants. Therefore, we conducted a second analysis by matching demographic factors that was restricted to counties without former gas plants. Finally, we conducted analyses using nationally standardised rates of cancer. Despite the variation in analytical approaches, results were consistent between techniques.

We conducted a community cancer assessment for the purpose of appraising the public health regarding the occurrence of cancer among residents in a community with a former manufactured gas plant. Although this study did not include individual-level information, rates of total cancer and most cancer sites in the Champaign County area and zip codes circumscribing the abandoned facility were lower versus similar comparison areas, and based on nationally standardised rates of cancer. The primary exception is for prostate cancer, although there may be relevant explanations for the higher rates aside from potential exposure emanating from the former manufactured gas plant site, such as an incidence spike due to higher PSA testing rates in Champaign County, a statistical artifact based on multiple comparisons, or confounding by unmeasured factors. Furthermore, a review of the literature did not reveal any known relation between the potential gas plant compounds and prostate cancer risk. Interpretation of results from our analyses should be made in the context of the many limitations of ecological-based study designs. However, the results from this retrospective cancer mortality and incidence assessment do not support an increase in cancer occurrence in communities surrounding a former manufactured gas plant in Champaign, Illinois, USA.