Date of Degree
Access restricted until 02/23/2018
PhD (Doctor of Philosophy)
Arsenic and atrazine are two water contaminants of high public health concern in Iowa. Arsenic is released into drinking water from soil and atrazine is the most heavily used herbicide. My hypothesis was that people in Iowa have high risks of certain cancers from exposure to arsenic and atrazine via drinking water. This study was performed to examine this hypothesis with three study aims: 1) to quantify the contamination of arsenic and atrazine in Iowa drinking water, 2) to analyze patterns of major cancers in Iowa and the US, and 3) to evaluate the association between arsenic exposure from drinking water and prostate cancer.
First, I investigated the occurrence of arsenic and atrazine in drinking water from Iowa private wells and public water systems over several decades. Percentages of detection and violation of regulations were compared over region, season, and water source. Factors affecting detection and concentration of arsenic and atrazine, and correlations among atrazine and its degradation products, were analyzed using a mixed effects model. I found that detection and concentration of atrazine in drinking water decreased over time (all samples were below the drinking water standard of 3 ppb). However, the percentage of arsenic detections and concentrations higher than the drinking water standard (10 ppb) increased over time in the public water systems. Therefore, I focused on arsenic as a water contaminants of concern for the further study.
Second, the patterns of trends in cancers related to arsenic and atrazine in Iowa were analyzed and compared to the US. Cancer is the second leading cause of death in the USA. However, cancer rates vary by different regions. In this study, the cancer trends in Iowa were investigated and compared to 8 other states for white individuals aged over 20. Temporal trends in age-standardized cancer rates were evaluated using joinpoint regression analysis by gender. Results of analysis indicate that overall cancer incidence and mortality were lower in Iowa than the US, and different trends of major cancers were found between Iowa and the US. However, prostate cancer was the most frequent type of cancer in men in both Iowa and the US and I focused on prostate cancer for further study.
Lastly, based on what I found from previous studies, I conducted an ecologic study to evaluate the association of prostate cancer and arsenic in drinking water in Iowa, where exposure levels are low, but duration of exposure can be long. Spatial Poisson regressions were conducted to estimate the risk ratios of prostate cancer by tertiles of arsenic level at a county level, adjusted for demographic and risk factors. The county averages of water arsenic levels varied from 1.08 to 18.59 ppb across the counties, with three counties above the 10 ppb limit. Based on the tertiles of their arsenic levels, counties were divided into three groups: low (1.08-2.06 ppb), medium (2.07-2.98 ppb), and high (2.99-18.59 ppb). Spatial Poisson regression model analysis showed that the risk ratios of prostate cancer were 1.16 (95% CI, 1.10-1.23) and 1.28 (95% CI, 1.21-1.34) in the medium and high groups (p-trend < 0.001), compared to the low group after adjusting for risk factors. The results were similar when analyses were restricted to aggressive prostate cancers. These data show a significant dose-dependent association between low-level arsenic exposure and prostate cancer. These findings need to be confirmed from more in-depth individual studies.
Overall, the results showed that 1) the detections of arsenic increased in Iowa drinking water in the last decade, 2) there were disparities of trends in major cancers between Iowa and the rest of US, and 3) the risk of prostate cancer increased with arsenic exposure via drinking water. This study approach allows for the identification of possible health issues caused by water contaminants.
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