This magnification of a page from the DTSC approved "Risk Assessment Methodology" shows two components of the "suburban residential" standard, each highlighted.
The column to the right includes a backyard garden in its calculations because it's focusing on the risk of consuming a poison.
The one to the left, which skips the garden, focuses on exposures such as skin contact.
The two columns are supposed to be calculated together to come up with the final suburban residential standard.
However, in 2013 the DTSC let Boeing use screening levels for poisons that cited only the column to the left and that omitted the garden. That resulted in "risk-based
screening levels" that were far less stringent, often by orders of magnitude. They are the benchmarks Boeing has used in declaring what pollution must be removed from
its land and what is safe to leave behind.

Substance

Chromium 6, a potent carcinogen
Arsenic
Lead
Mercury

Risk-Based Screening Level That
Includes a Garden
milligrams per kilogram of dirt

Risk-Based Screening Level that Omits
a Garden (Boeing Option)
milligrams per kilogram of dirt

How Much Less Stringent
Is Boeing’s Option?

.00194

1.29

665

.0000992

.0658

663

6.9

80

12

.0504

16.8

333