2,4-D and NHANES: Sources of exposure and identification of data gaps

The herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) has been widely used for many decades. Occupational determinants of 2,4-D exposure are linked to processes related to its application. However, in the general population, the sources, pathways, and routes of exposure are less well-studied. Should exposure reductions be desired, this type of information will be essential. We examined urinary 2,4-D data in conjunction with National Health and Nutrition Examination Survey (NHANES) variables, as well as other publicly reported data. The goal was to identify signals for sources of exposure to the herbicide. Urinary 2,4-D measurement data were available from 1999 to 2014, with the exception of survey years 2005–2006. The geometric mean urinary 2,4-D concentrations for the total population were near the limits of detection (LOD) and ranged from 0.1 to 0.4 µg/L. Depending on the survey year, the percent of the population with urinary 2,4-D levels above the LOD ranged from 26.7 to 75.6%. There were no consistent patterns for urinary concentrations according to race/ethnicity, age, or gender. Higher median urinary 2,4-D levels were observed among those who reported recent use of a lawn or garden product. Working in agriculture was suggestive of an occupational influence in this cross section of the population. A generally consistent inverse relationship between fasting time and urinary 2,4-D levels was found although US databases on food and drinking water did not demonstrate widespread detection of 2,4-D. Biomonitoring data from studies such as NHANES preclude us from definitively ascertaining source contributions for exposures because the sources are likely to be diverse, with exposure occurring via several pathways. The extent to which diet or drinking water contributes to the overall 2,4-D exposure in nonoccupational populations is not well-established. Resources are needed for more studies that pair environmental- and biomonitoring-based exposure data from various regions and over multiple seasons.