Summary
A prospective cohort study of 12,235 adults in England linked more days above WHO daily NO₂ guideline values with faster subsequent decline in memory and verbal fluency. The observational findings come from a medRxiv preprint.
A prospective cohort analysis posted as a medRxiv preprint found that adults in England with more days of nitrogen dioxide (NO₂) pollution above World Health Organization (WHO) daily guideline values had faster subsequent decline in memory and verbal fluency. The association remained after accounting for annual mean air-pollution exposure. The researchers found no corresponding association between days above the daily guidelines for fine particulate matter (PM₂.₅) or PM₁₀ and cognitive decline.
The finding focuses on the frequency of high-pollution days, rather than only the annual average. Because this was an observational study, it identifies an association; it cannot establish that NO₂ exceedance days caused the cognitive changes.
How the study measured exposure and cognitive change
Researchers analyzed waves 1–10 of the English Longitudinal Study of Ageing, covering 2002–03 to 2021–23. The analysis included 12,235 community-dwelling adults aged 50 or older at baseline. Exposure was estimated using participants’ residences: the researchers counted days in the calendar year before baseline when estimated NO₂, PM₂.₅ or PM₁₀ concentrations exceeded the relevant WHO daily guideline value.
The team assessed episodic memory and verbal fluency at baseline and modeled changes over time. Its estimate of 14-year decline corresponds to the 75th percentile of participants’ follow-up duration. The analysis compared cognitive decline for people at the 75th percentile of NO₂ exceedance days with those at the median, while adjusting for annual mean pollutant exposure.
NO₂ exceedance days and the results
The higher number of NO₂ exceedance days was associated with faster modeled decline in both measures. Compared with the median, the 75th percentile of NO₂ exceedance days corresponded to a difference of −0.09 standard deviations in 14-year memory decline (95% confidence interval, −0.17 to −0.01; p = 0.02). For verbal fluency, the difference was −0.10 standard deviations (95% confidence interval, −0.19 to 0.00; p = 0.04).
Exceedance days for the three pollutants were not associated with baseline memory or verbal fluency after adjustment for annual mean exposure. The study also found no association between PM₂.₅ or PM₁₀ exceedance days and subsequent cognitive decline.
The results suggest that counting recurrent high-NO₂ days could add information to assessments based on annual averages alone. The authors say this may help identify areas with repeated high exposure and the local emission sources contributing to it. Exposure was estimated from residence-linked environmental data, and the preprint’s cohort design means the findings should be interpreted as an association rather than a demonstrated causal effect.