Thousands of chemicals are released into the environment yearly for agricultural, industrial and health-related needs. Sorting through which may inadvertently lead to risks to human health is a complex task. A joint study between NAU and the University of Idaho investigated whether there were any correlations between specific types of pesticides and cancer incidence; while correlation does not always mean there is a direct link to cause, their findings suggest the need for more in-depth studies for one type of pesticide in particular.
Study co-author Catherine Propper, a professor in the Department of Biological Sciences and affiliated faculty with the Southwest Health Equity Research Collaborative (SHERC); Shantel Henry, an undergraduate researcher who is now a research technician at the Pathogen and Microbiome Institute; and Indrakshi Roy, a biostatistician at the Center for Health Equity Research, collaborated on the study with University of Idaho researchers Alan Kolok, Naveen Joseph and students from their research group. The study builds on a previous project Kolok did with data from Nebraska and Idaho.
In this study, published in GeoHealth, the researchers used two large, publicly available databases from the U.S. Geological Survey and Centers for Disease Control and Prevention to evaluate whether there were correlations between usage of the 25 most commonly deployed pesticides and the rates of overall or pediatric cancer incidence. They focused their efforts on the western United States.
The results showed an association with fumigant use and cancer rate at the state and county levels, but no link across other pesticide classes like herbicide or specific insecticides. Propper noted that while the outcomes surrounding fumigant use are suggestive, more information is needed to determine whether the use of the fumigant metam is directly linked to cancer; they suggest the need for additional research that targets other steps to help find a causative link between the two. This study also showed the benefits of using these types of databases to potentially identify those that need further examination regarding the complex relationships between chemical exposure and health outcomes.
“While similar studies have evaluated risk from air pollution exposure to both particulate matter and some gases, few use this data-mining approach for understanding the associations between exposure to commonly used pesticides and diseases, like cancer, at the geographic scale used in this study,” Propper said. “Our findings demonstrate an association between exposure to this common fumigant, but this type of correlation, on its own, does not ‘prove’ that metam, or any pesticide, on its own is a carcinogen. Specific studies need to be undertaken to address whether there is a causative relationship that warrants further concern.”
Propper and Kolok were lead investigators in the project, while Henry and the students from University of Idaho collected and collated the pesticide data set and supported some of the writing and literature review. Roy and Joseph developed the statistics as part of SHERC’s Technical Assistance Group Service Center (TAG-SC); this new support group aims to facilitate the development of statistical and study design techniques for NAU faculty-led projects with links to issues surrounding health equity.
“Correlation is not causation” is a mantra for every scientist, and this research is no different. On the research side, Propper said the next steps in this work is broadening their study coverage to include the rest of the United States. These collaborations also will look at whether metam exposure, specifically, has a causative link to cancer. One way forward includes conducting a classic study that exposes cancer cell lines to these fumigants to see if there are changes or exposing normal cell lines to the chemicals and see if cancer-like changes occur in the cells.
Future research also could home in on more specific areas. Given the broad scope of this data, researchers had a difficult time predicting specific outcomes for a single state. Arizona, for example, has few counties, and many of them are geographically large and rural, which leads to the potential for over- or underestimating the data.
On the regulatory side, what’s next is much more complicated. While the Environmental Protection Agency already considers metam a probable carcinogen, this team’s data are only a single piece in the complicated puzzle that includes policymakers and both state and federal regulatory authorities.