At a glance
- These National Exposure Report (NER) tables represent the first NHANES cycle data collected since the COVID-19 pandemic began.
- NHANES August 2021–August 2023 data collection resumed after the COVID-19–related pause with a redesigned participant sampling approach, updated questionnaires, and revised data collection procedures and protocols.
- Changes in oversampling, multimode screening, subgroup sample sizes, and the introduction of new phlebotomy weights affect the calculation, precision, and interpretation of biomonitoring-based NER concentration measurements; phlebotomy weights must be used for analyses involving blood-based analytes.
Background
After NHANES suspended field operations in March 2020 due to the COVID-19 pandemic, data collection resumed in August 2021 with a new 2-year cycle. The NHANES August 2021 – August 2023 nationally representative data files reflect an updated sample design, modified questionnaires, and revised examination procedures. These files represent the first NHANES data released since the pandemic began.
The pandemic had several major impacts on NHANES operations. The 2019–2020 cycle ended early in March 2020, and those partial data were combined with 2017–2018 and released as NHANES 2017 – March 2020 Pre-Pandemic Data. When data collection resumed in August 2021, NHANES implemented a redesigned participant sampling approach intended to reduce in-person contact between interviewers and participants while still producing reliable national estimates. Additional evaluations of weighting methods for August 2021–August 2023 were conducted to assess data quality.
Comprehensive documentation describing sample design changes, weighting methodology, nonresponse bias assessments, quality evaluations, new phlebotomy weights, and analytic considerations is available in the NCHS analytic guidance and accompanying technical reports here: Brief Overview of Sample Design, Nonresponse Bias Assessment, and Analytic Guidelines for NHANES August 2021-August 2023
Impact on the National Exposure Report
Data produced by NHANES are the foundation for the National Exposure Report (NER) data tables. Because the August 2021–August 2023 cycle used a new sample design and introduced methodological changes—including revised oversampling procedures, multimode household screening, and new phlebotomy weights—these updates influence how biomonitoring data are produced and how concentration measurements are derived and interpreted.
Key considerations for the National Exposure Report include:
- Subgroup sample sizes differ from previous cycles due to the removal of race/Hispanic origin oversampling. As a result, some demographic estimates in the NER may have reduced statistical precision compared with past releases.
- Phlebotomy weights were introduced to address differential participation in blood draws. These weights should be used for all NER tables that include laboratory-measured analytes.
- Because a 16-month gap exists between NHANES 2017–March 2020 and August 2021–August 2023, direct comparison of cycles should be approached cautiously; short-term pandemic impacts may influence exposure levels.
- Despite these changes, the August 2021–August 2023 data remain nationally representative, and standard analytic procedures continue to support valid comparisons in the National Exposure Report.
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