Methodology for mapping pesticide use and related health concerns for people living or working in California’s Ventura County


Pesticide use and land use data

EWG determined the types, quantities and locations of pesticides used in Ventura County, Calif., by drawing on data from 2015 to 2020 provided by the Ventura County Agricultural Commissioner’s office through a public records request.

Pesticide use record-keeping and reporting in California is required by law under the California Pesticide Use Reporting Program, established in 1990, and is reported to the agricultural commissioner’s office every month.

Pesticide use data are reported based on township sections of approximately one square mile, and sections contain agricultural and nonagricultural land. Crop maps were overlaid with land sections to identify the locations of pesticide application in each township more accurately. Specifically, every crop field whose majority land area fell within a section was assigned the pesticide use characteristics reported for that section. Although application areas within a section are likely not uniform, and pesticide use may vary from one field to the next, the data did not allow for exact application areas within a section to be used.  

To identify land and field ownership, data were taken from and placed into the land section data.

Residential and school data

The locations of residences were collected from 2020 land ownership parcel data acquired from the county tax assessor. All residential parcel boundaries located within 2.5 miles of a crop field with pesticide use (straight-line distance measured from the center of each residential parcel to the center of each crop field) were selected and are included on the map.  

To make them easier to visualize, groups of residences were combined into larger groups based on the hazard score calculated for each residence, described in more detail below. Residences were grouped together if their boundaries touched and they fell within the same percentile rank for hazard scores, represented by three classes from hazardous, or some level of concern, to most hazardous.

Public school locations were gathered from the California Department of Education 2019-2020 schools map. Public school location data were crossed with parcel boundaries and subset to identify K-12 schools within 2.5 miles of a cropland field.  

Pesticide toxicity scores

To assign specific types of toxicological impacts to individual pesticides, EWG scientists evaluated published assessments by authoritative health agencies and toxicity information in peer-reviewed literature about the pesticides used in Ventura County.

EWG included the following chronic and acute health effects: cancer, harm to the brain, hormone disruption, thyroid toxicity, harm to developing children, harm to the reproductive system, and harm to the immune system, including asthma and respiratory irritation.

Data gathered from available sources were scored on the basis of four components:

  • The severity of the health outcome, where chronic health effects like cancer were assigned greater weight than shorter-term effects, such as respiratory irritation.
  • The strength of the association, where chemicals known to cause harm and affect a specific health outcome, were given greater weight than possible or suspected toxicants.
  • The data reliability, where comprehensive assessments published by authoritative health agencies were assigned greater weight than screening data, such as single studies performed in cells.
  • The dose at which the health effect occurred, if available, where chemicals toxic at lower doses were assigned greater weight, compared to chemicals that are harmful at higher doses.

Since some pesticides have multiple health effects, such as neurotoxicity and carcinogenicity, the total toxicity score for the pesticide was calculated by adding together the scores from each data source.

EWG used the following data sources to evaluate toxicity and calculate toxicity scores:



Endocrine disruption

Thyroid toxicity

Developmental and reproductive toxicity

Respiratory toxicity


Defining the levels of concern, or hazard scores, for crop areas

EWG scientists calculated level of concern, or hazard score, for each township section by multiplying the total pounds of each pesticide applied in that area from 2015 to 2020 by each pesticide’s toxicity score, to give a value known as the toxicity-adjusted pounds. The toxicity-adjusted pounds were then summed together for all pesticides applied in a section.

Scores were divided into percentiles to color-code fields based on a scale from hazardous or some level of concern, to most hazardous. Yellow-colored fields represent the 50th percentile and lower of calculated scores. Orange-colored fields include the 51st to 75th percentile. And fields colored red, indicating the highest scores or most hazardous fields, have scores or between the 76th and 100th percentiles.

Calculation of hazard scores for residential areas

The hazard scores for residential areas and schools are weighted according to the distance from a cropland area where pesticides were used. EWG conducted hazard score scaling for the distance from a pesticide-applied field of up to 2.5 miles, and scaling was calculated proportionate to the distance. Because published peer-reviewed epidemiological studies have found associations between pesticide use and adverse health effects in people within 2.5 miles of pesticide use, we included residences up to that distance away.

Residential locations closer to fields, corresponding to a shorter distance from pesticide use, have higher hazard scores, compared to residential locations farther away. For example, if a field where a specific mix of pesticides was sprayed has a hazard score of 100, the hazard score for the residential areas within a quarter-mile is 90, the hazard score for areas within one mile is 60, and the hazard score is zero at the margin of 2.5 miles.

All distance-weighted hazard scores from nearby cropland fields were summed for individual residences or schools.

Residences were then grouped and colored by score based on the same percentiles and coloring as described for the fields.