Our pesticides map was built using data from the California Department of Pesticide Regulation. The state requires growers and licensed pesticide applicators to report detailed information every time a pesticide is used in a commercial context. This includes what chemicals were used, how much was applied and where they were applied.

The app contains information on about 22 million applications from 2003 to 2012. This accounts for about 1.5 billion pounds of pesticides.

We focused on pesticides applied to crops, but they also are used in other contexts, such as eradicating termites from a building. Some of the data was missing or had inaccurate information about where the pesticides were applied and couldn’t be mapped.

We also included data the state department collected from Dow AgroSciences on a specific chemical, 1,3-Dichloropropene, or 1,3-D. This data shows where growers have exceeded the original health limits of 90,250 adjusted pounds per year that the state created following concerns about how much of the chemical, which it considers a carcinogen, lingered in the air.

These safety limits are not measured in raw pounds. The number of adjusted pounds reflects credits for growers who use 1,3-D with tarps that reduce the amount of the chemical that escapes from the field, and penalties for growers if they use it at certain times of the year when the weather makes it more likely the chemical will drift, for example. As a result, the adjusted pounds listed for 1,3-D overages may not match the raw pounds of 1,3-D from the state’s pesticide reporting data.

The state tracks the location of pesticide applications using Public Land Survey System grids, often called townships. Each application occurred somewhere inside one of those grids, which is usually a 1-square-mile area. Some grids have irregular shapes and sizes. For the state's regulation of 1,3-D, it used larger grids that are usually 6 miles by 6 miles in size. (In our story, we sometimes refer to townships as “communities,” a term that’s likely to be more familiar for many people.)

This map includes four layers: chemicals of public health concern, all fumigants, all chemicals and 1,3-D. The chemicals of public health concern were identified by the California Department of Public Health in a recent report about pesticide use near schools. The report’s authors considered the chemicals to be of concern because they fall into at least one of six categories: chemicals that may cause cancer, chemicals that may cause developmental issues, neurotoxins, air pollutants, fumigants and chemicals considered risky by the state for other reasons.

Fumigants are one kind of chemical of concern identified in the Department of Public Health. “All chemicals” includes every pesticide application we could map, many of which are not considered harmful to public health.

Fumigants, such as chloropicrin and metam sodium, are some of the most heavily used pesticides in the state and can be applied at rates of 200 pounds or more per acre. They are gases that are prone to drift away from the field and can affect workers and people living nearby.

It’s important to remember that living or working near places where pesticides are used does not necessarily mean you have been exposed. But looking at the pounds of pesticides applied can be a measure of potential risk. In a recent report on exposure to toxic materials, the California Office of Environmental Health Hazard Assessment said: “Pesticide use, especially use of volatile chemicals that can easily become airborne, can serve as an indicator of potential exposure. Similarly, unintended environmental damage from the use of pesticides may increase in areas with greater use.”

The health office has more information in its report about why it used pounds of pesticides as an indicator of exposure.

If you would like more recent information about pesticide use near you, contact your county agricultural commissioner.

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Jaena Rae Cabrera is a web producer for The Center for Investigative Reporting. Prior to joining CIR, she was web producer for Renaissance Journalism, a San Francisco-based nonprofit specializing in providing training, technical assistance, consultation and grants to journalists for media innovations that strengthen communities. She is also a page designer for the San Francisco Public Press. Jaena received a master’s degree in library and information science from Syracuse University. She graduated from San Francisco State University with a bachelor's in journalism.