Time-lagged Cognitive Impairment from Long-term Occupational Exhaust Exposure: A NHANES Analysis Using Distributed Lag Nonlinear Models
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Background
Occupational exposure to harmful substances such as exhaust fumes is a known risk factor for respiratory and cardiovascular diseases. However, its impact on cognitive function remains underexplored in older adults. As the population in the United States ages, understanding the influence of occupational exposures on cognitive health is critical for identifying modifiable risk factors for cognitive decline.
Methods
This study investigates the relationship between occupational exposure to exhaust fumes and cognitive performance among older adults aged 60 years and above, utilizing data from the 2011–2012 National Health and Nutrition Examination Survey (NHANES) cohort. Cognitive performance was assessed using the Consortium to Establish a Registry for Alzheimer’s Disease Word List Learning (CERAD-WL) recall test (immediate and delayed), the Animal Fluency Test (AFT), and the Digit Symbol Substitution Test (DSST). Cognitive impairment was defined as an adjusted Z-score ≤ -1.5 standard deviations (SD) (based on age- and education-corrected norms). Multivariable logistic regression analysis was conducted to evaluate the associations between occupational exposure and cognitive impairment, adjusting for demographic and health-related covariates. A Distributed Lag Nonlinear Model (DLNM) was applied to investigate the complex relationships between exposure levels and cognitive impairment.
Results
Among the 1,110 participants, 266 (24.0%) reported occupational exposure to exhaust fumes. Comparing to the unexposed group, exposed individuals were more likely to be male (81.6% vs. 41.8%), have lower education levels (31.2% vs. 24.4% with less than 12 th grade level), and exhibit higher rates of smoking (65.0% vs. 48.6%) and excessive alcohol use (15.7% vs. 7.0%). Cognitive performance was significantly poorer in the exposed group, after controlling for age, sex and education. Occupational exhaust fumes exposure was associated with substantial impairments in delayed memory (CERAD-WL Delayed recall: OR = 2.55, 95% CI: 1.61–4.05), verbal fluency (AFT: OR = 2.41, 95% CI: 1.48–3.94), and processing speed (DSST: OR = 1.95, 95% CI: 1.19–3.18). Duration of occupational exposure exhibited biphasic, time- lagged effects on cognitive function, with low levels of exposure showing minimal positive impact, and higher levels leading to significant declines in immediate and delayed recall, semantic memory, and processing speed, particularly over prolonged exposure periods (20-30 years).
Conclusion
Occupational exposure to exhaust fumes is significantly associated with poorer cognitive performance in older adults, particularly in domains of memory, semantic fluency, and processing speed. The DLNM analysis further indicates a significant time-dependent decline in cognitive scores at higher exposure levels. While further research is needed to explore the mechanisms linking occupational exposure to cognitive decline and to explore long-term brain health outcomes, these findings highlight the need for targeted interventions to reduce occupational exposures and mitigate their cognitive impacts. The current trend to use of electric vehicles may aid in reducing exposures, and populations with higher exposure risk should be monitored closely for cognitive changes.
