Association Between 2,4-dichlorophenoxyacetic Acid and Cognitive Impairment in Older Adults: a Cross-sectional Study from NHANES 2001-2002 and 2011-2014

Background: 2,4-Dichlorophenoxyacetic acid (2,4-D) is reported to be the most widely used herbicide in home and garden environments, rendering it commonly encountered in daily life. Despite being ubiquitous, there is a scarcity of studies that have comprehensively assessed the relationship between 2,4-D exposure and cognition using multiple models.

Objective: To explore the association between 2,4-D exposure and cognition among older American people.

Methods: This was a cross-sectional study that included 3 cycles of data from the National Health and Nutrition Examination Survey. Generalized linear models (GLMs), restricted cubic spline (RCS) regression, and generalized additive models (GAMs) were used to assess the relationship between exposure to 2,4-D and cognitive performance by the Consortium to Establish a Registry for Alzheimer's Disease (CERAD) word learning sub-test, Digit Symbol Substitution Test (DSST), and Animal Fluency Test (AFT).

Results: A total of 1364 older U.S. adults (60+ years) were included in the study. The GLMs revealed a negative association between median high levels (0.315-0.566 μg/L) of 2,4-D and cognitive impairment on the DSST and AFT, with multivariate-adjusted ORs of 0.403 (95% CI: 0.208-0.781, P = 0.009) and 0.396 (95% CI: 0.159-0.986, P = 0.047); the RCS regression and GAMs revealed a "U" shaped curve, the left part of which is consistent with the result of the GLMs.

Impact Statement: There is a U-shaped relationship between human urinary 2,4-D concentrations and cognitive impairment in older U.S. adults, especially in males, so controlling 2,4-D exposure within an appropriate range is particularly important for cognitive function.