^A Census tracts with no land area are excluded, leaving 9,107 of California's 9,129 census tracts.^1
B At least 25% of the residential parcels were built before 1978. The American Academy of Pediatrics recommends blood lead testing for children ages 12 to 24 months living in communities where at least 25% of the housing stock was built before 1960. To acknowledge the risk of lead-based paint in houses built between 1960 and 1978, the criterion was applied using 1978 instead of 1960. A census tract met this criterion if at least 25% of its residential housing, based on Digital Map Product's parcel data from October 2022, was built before 1978. Residential parcels with a missing year built were included as pre-1978 parcels in these calculations to be protective.^{2, 3, 4
C} Census tract is within 1.7 miles of a current or historic lead emitting facility. A list of sites from the US EPA Toxic Release Inventory that emitted lead since 1988 (extracted on July 27, 2023) was mapped and a 1.7-mile buffer was drawn. The 1.7-mile buffer was chosen in accordance with literature on the lead contamination from two major emitters, Exide and Quemetco.^{5, 6, 7
D} Census tract is within 1,000 feet of a state highway. A layer for the California State Highway Network from a December 31, 2017 extraction from the Transportation System Network database maintained by the California Department of Transportation (Caltrans) was used with a 1,000-foot buffer. The 1,000-foot buffer was determined based on a California Air Resources Board Technical Advisory about air pollution around freeways.^{8, 9
E} Census tract is within the city of a known current or historic smelter. A list was compiled of the location and activities of iron and steel plants, metal foundries, lead smelters, storage battery manufacturing plants, scrap metal plants, mines that may have mined lead along with zinc, iron, or copper, metal rolling, stamping and metal powder producers, brass and copper smelters, and babbitt and solder manufacturers in California. Some of the texts used were rare and required special handling. Many of the locations were not specific (only the name of the city or town was given) and in two instances, references were only found in older newspapers. Due to the lack of an address and site size for most sites, all census tracts within a city listed as having one of these facilities are included. Because census tracts can cross city boundaries, a census tract was considered to be within a city with a smelter if the majority of the census tract’s land area was in that city.
^F Census tract is within 1 km of an airport using leaded avgas. Lead continues to be used in avgas for small-craft airplanes. A list of 183 airports where leaded fuel is recorded as being used in the Airport Data and Information Portal from the Federal Aviation Administration (extracted on March 21, 2021) was mapped and a 1 km buffer was drawn. The same extract was performed on July 13, 2023 to identify any new airports and the two lists were then combined. Airports that were on the 2021 list but not the 2023 list were kept in the analysis as there could still be legacy soil contamination even if there is no longer air contamination. An article by Miranda found lead soil contamination up to 1 km away from airports where planes use avgas.^{10, 11, 12, 13
G} Census tract is within 1 km of a railroad. Trains carrying coal are often uncovered, allowing coal dust to travel into the areas surrounding the tracks. Coal has historically and is currently being transported in this manner in California. The layer of railroads in California from Caltrans was extracted three times and merged; the layers were last modified on October 31, 2013, January 27, 2020, and June 20, 2023. Railroads that were on any of the layers were kept in the analysis as there could be soil contamination from whenever the railroad was in use. Only railroads marked as moving freight were included (i.e. passenger-only railroads are excluded.) A study by Li found lead-contaminated dust up to 1 km away from railroad tracks.^{14, 15, 16, 17
H} Census tract is within 1,000 feet of a speedway. While leaded fuel for on-road vehicles was banned in the 1990s, the ban did not cover race car fuel, which continued to be used into the 2000s. The United States Environmental Protection Agency noted in their 2006 report on sources of lead that populations living in the vicinity of racetracks were at an increased risk of lead exposure. A list of speedways in California was extracted from a racing website on April 11, 2019. A 1,000-foot buffer was used with the assumption that on-road vehicles on highways and race cars on speedways will emit lead particles in a similar manner.^{18, 19
I} Census tract is served by a water district with at least one known leaded user service line or fitting. Water service lines and fittings that contain lead pose a risk of drinking water contamination. A list of water service areas with at least one known leaded user service line or fitting in calendar year 2019 was extracted from the California Water Board's Lead Service Line Replacement Inventory Status database (updated February 3, 2021). The list was revised down based on replacement/identification reports completed in 2022, such that only those water services areas that still had known leaded user service lines or fittings remained. The water service areas were joined to California Water Resources Control Board's Drinking Water Service Area Boundaries layer updated on May 23, 2023.​^{20​, 21, 22}

References:

¹ United States Census Bureau. 2020 TIGER/Line Shapefiles tl_2020_06_tract.
² American Academy of Pediatrics Council on Environmental Health. Prevention of Childhood Lead Toxicity. Pediatrics. 2016; 138(1): e20161493.
³ Digital Map Products. Digital Map Products Parcel Data – Bulk Data. July 13, 2020.
⁴ American Academy of Pediatrics Council on Environmental Health. Errata, Prevention of Childhood Lead Toxicity. Pediatrics. 2017; 140(2): e20171490
⁵ United States Environmental Protection Agency. TRI Customized Search: run on July 27, 2023: https://www.epa.gov/enviro/tri-customized-search
⁶ Removal Action Plan (Cleanup Plan): Offsite Properties within the Exide Preliminary Investigation Area. URS Corporation Americas for California Environmental Protection Agency Department of Toxic Substances Control. 2017.
⁷ DTSC Orders Battery Recycler Quemetco to Expand Soil Sampling, Clean Up Areas Near City of Industry Facility (News Release). California Environmental Protection Agency Department of Toxic Substances Control. 2018.
⁸ Strategies to Reduce Air Pollution Exposure Near High-Volume Roadways (Technical Advisory). California Environmental Protection Agency Air Resources Board Research Division. 2017.
⁹ State Highway Network (SHN) and Postmile System. California Department of Transportation. December 31, ​2017.
¹⁰ Federal Aviation Administration, Airport Data and Information Portal Advanced Search. March 24, 2021.
¹¹ Federal Aviation Administration. Airport Data and Information Portal Advanced Facility Search. July 13, 2023.
¹² California Department of Transportation. Airport Runways. February 16, 2022.
¹³ Miranda, ML, Anthopolos R, Hastings, D. A Geospatial Analysis of the Effects of Aviation Gasoline on Childhood Blood Lead Levels. Environmental Health Perspectives. 2011; 119 (10).
¹⁴ California Department of Transportation. California Rail Network. October 31, 2013.
¹⁵ California Department of Transportation. California Rail Network. January 27, 2020.

¹⁶ California Department of Transportation. California Rail Network. June 20, 2023.

¹⁷ Spatial Characteristics of Heavy Metals in Street Dust of Coal Railway Transportation Hubs: A Case Study in Yuanping, China. Li, Dongyue and Liao, Yilan. 12, 2018, International Journal of Environmental Research and Public Health, Vol. 15.

¹⁸ Air Quality Criteria for Lead, Volume I of II. United States Environmental Protection Agency. 2006.
¹⁹ Racing Tracks Found. RacingIn.com. April 11, 2019.

²⁰ Division of Drinking Water California Water Boards, Lead Service Line Replacement Inventory Status. February 3, 2021.

²¹ Division of Drinking Water California Water Resources Control Board. California Drinking Water Service Area Boundaries. May 23, 2023.

²²  Division of Drinking Water California Water Boards. 2022 Updated Replacement Plans. July 12, 2023.​​
​