GIS Exposure Assessment


A forty-year-old mother of three has been diagnosed with breast cancer. She has no family history of the disease. She breastfed her children, who were all born before her thirtieth birthday. She eats only organic produce; she uses all-natural products. One risk factor, though, has been her lifelong residency on Cape Cod. As a child, she lived in the small town of Harwich, near a cranberry bog. She now lives on a cul-de-sac in Barnstable, on land that was once agricultural. Could her breast cancer have been triggered by the pesticides that were aerially sprayed over the bog during the 1960s, or by the pesticides that may have lingered in the soil of her front yard for decades now?

To understand this woman’s risk—and that of thousands of other women on Cape Cod—requires investigation through time and across geographies. As part of its Cape Cod Breast Cancer and Environment Study, Silent Spring Institute created a geographic information system (GIS) to capture a complexity of data. Working with Applied Geographics, Inc., the researchers expanded on the extensive geographic data resources of MassGIS and the Cape Cod Commission to develop the study’s GIS, the first to integrate current and historical information on the Cape environment for use in health research.

This GIS has since become a central data management and analysis tool in the Cape Cod Study, as it has tracked the exposures of 2,100 women to multiple environmental pollutants through four decades. But this advancement has taken some innovation. Standard GIS tools are better suited to their original applications—such as urban planning—than they are to health research. To incorporate the Cape Cod GIS more effectively into their work, the Silent Spring researchers have developed custom tools and devised new ways to use standard tools. Although the Institute developed these tools specifically for the Cape Cod Study, their potential use extends beyond the study to a range of environmental health research applications.

The primary exposures of interest in the Cape Cod Study relate to three overlapping sets of chemicals: endocrine disruptors, mammary carcinogens, and pesticides. The study team used GIS methods to assess exposures to these compounds from wide-area pesticide use, such as on agricultural land and forests, and drinking water infiltrated by wastewater or affected by residential, commercial, or industrial land use.

For several exposures of interest in the Cape Cod Study, the distance and direction from the exposure source—such as a cranberry bog or area sprayed for tree pests—to a study residence are key factors. For these variables, Silent Spring Institute researchers developed the Spatial Proximity Tool, a GIS-based program that calculates distance, direction, and other geographically based information while maintaining links to the database containing attributes of the exposure source.

Institute scientists refined the Spatial Proximity Tool to incorporate results of computer modeling for aerial spraying of pesticides on cranberry bogs and for tree pests. The tool can also identify environmental characteristics that lie between the exposure source and a participant’s residence—for example, forest vegetation that reduces pesticide drift from agricultural land—and provide information about the size of the environmental characteristic.

In other areas where pesticides were likely to have been used on the Cape—including agricultural lands other than cranberry bogs, wetlands, rights-of-way, and golf courses—pesticides were primarily applied from the ground. For these exposures, the researchers used the GIS to calculate acres of pesticide use within a specified distance of each residence to represent the intensity of the exposure.

Because some agricultural land becomes residential, resulting in residents’ higher exposure to persistent pesticides, the study team evaluated separately whether women in the study lived directly on agricultural land. The researchers included the category of wetlands as a surrogate for areas treated with pesticides to control mosquitoes. Rights-of-way calculations captured the exposures from herbicides sprayed along power lines and railroads.