California’s Department of Pesticide Regulation has released two reports on pesticide air monitoring in 2025, with gaps left by a laboratory data recall. In its Oct. 23 announcement, DPR said all detected concentrations in the valid results were below health-based screening levels and regulatory targets. That finding does not cover the periods for which measurements were voided.
The reports retain a full year of results for 38 pesticides and breakdown products, along with nine months of valid measurements for 1,3-dichloropropene, known as 1,3-D, and methyl bromide. The missing results matter because DPR uses air monitoring to evaluate pesticides’ potential effects on people and the environment and to assess whether regulatory programs are working as expected.
The 2025 Air Monitoring Network report covers testing for 40 pesticides and breakdown products across four monitoring sites. DPR reported nine compounds at concentrations that could be quantified. Its separate Study 309 report tracks 1,3-D over the long term and includes nine months of valid 2025 measurements from stations in Parlier and Delhi, in Fresno and Merced counties.
Those results describe the monitoring locations and periods represented by valid samples, not a statewide exposure finding. For 1,3-D and methyl bromide, the distinction between retained measurements and missing data is central: the reports provide partial-year coverage rather than a complete measured record for 2025.
A recall spanning two years
The California Department of Food and Agriculture’s Center for Analytical Chemistry performs independent laboratory analysis for DPR’s monitoring program. According to DPR’s Sept. 30 statement, CDFA recalled and voided results for 1,3-D and methyl bromide from Oct. 24, 2025, through June 30, 2026, affecting samples DPR began collecting in early October.
That recall spans parts of two calendar years. The roughly nine months of recalled data should not be confused with the nine months of valid 2025 measurements retained in the newly announced reports. The recall removed late-2025 results as well as results from the first half of 2026.
DPR said the problem surfaced in mid-July 2026, when the two departments identified unusual patterns in that year’s 1,3-D results. Comparing those readings with earlier monitoring results confirmed longer-running abnormalities and prompted a deeper review. CDFA notified DPR of its recall decision on Sept. 18 and provided a memorandum on Sept. 30.
Why the laboratory rejected the readings
The review traced the inaccurate readings to a degraded laboratory reference standard. Such a standard contains a precisely measured chemical concentration certified by its manufacturer and serves as a benchmark for laboratory equipment. When that benchmark deteriorates and reads lower than it should, comparisons against it can make concentrations in monitoring samples appear artificially high.
DPR reported that CDFA found degradation ranging from 18% to 96%, causing inaccurately elevated results for both chemicals. CDFA tried to develop corrections but concluded that adjusted figures would not meet the quality and confidence needed for regulatory or legal use. The agencies therefore could not simply revise the numbers and retain them as valid measurements.
In September, DPR said it would estimate 1,3-D air concentrations for the fourth quarter of 2025 and the first two quarters of 2026 using pesticide-use information and weather data. The department described modeling as a way to improve understanding of expected concentrations during the gap, while emphasizing that estimates cannot replace laboratory-measured results.
The October announcement describes retained monitoring measurements, not a completed set of modeled estimates. Its below-screening-level finding applies to the valid detected concentrations it reports. It does not resolve what concentrations occurred during the missing periods.
The recall also affected reporting beyond the two annual monitoring reports. In September, DPR identified its 2025 1,3-D Draft Annual Report as affected and said it expected revised reports after modeling. It also said it had updated fourth-quarter 2025 monitoring data to reflect the recall and released first-quarter 2026 data for the 38 unaffected pesticides.
Additional checks planned
DPR and CDFA also agreed to add laboratory quality checks, according to the September statement. One planned measure uses blind spikes: samples containing a known quantity of a chemical that DPR sends alongside routine monitoring samples without identifying them to the laboratory.
Comparing the laboratory’s answers with those known quantities would give the agencies a check on calibration and measurement reliability. A second planned measure calls for two independently sourced reference standards, allowing the laboratory to compare results against separate benchmarks and detect problems with a single standard earlier.
DPR described those safeguards as agreed next steps, not completed changes. For readers tracking the monitoring program, the released reports therefore offer two distinct records: a full year for the 38 unaffected substances and a shorter valid record for 1,3-D and methyl bromide, with modeling and additional quality controls described separately as follow-up work.
