Mortality risks associated with short-term exposure to ultrafine particles in London and the West Midlands

Some research has suggested that ultrafine particles (UFP), with an aerodynamic diameter less than 100 nm, might be the most harmful component of ambient particulate matter air pollution on human health. In the United Kingdom, UFP toxicity might also be affected by national vehicle fuel policies passed in 2007. We assessed associations between short-term exposure to UFP and mortality in two metropolitan areas in England.

We conducted a time-series analysis of 851,726 deaths in London and the West Midlands between 2003 and 2018. UFP was assessed as particle number concentration lagged 0-1 days. We used quasi-Poisson regression to estimate associations between particle number concentration and nonaccidental, cardiovascular, and respiratory mortality in both locations. Secondarily, we conducted an interrupted time-series analysis to evaluate potential impacts of the 2007 fuel policy.

For all mortality outcomes in both locations, risk ratios ranged from 0.991 (95% confidence interval [CI] = 0.965, 1.019) to 1.001 (0.990, 1.011) for a 10,000 particle/cm³ increase, and all 95% CIs included the null. Null results persisted when lag periods were extended to 0-5 days, linearity assumptions were relaxed, or when co-pollutants PM_2.5 and NO₂ were included. In the interrupted analysis, there was only evidence of a change in risk ratio for respiratory mortality in London (p_difference = 0.006), and risk ratios before and after the change point were null.

We found no evidence that suggested that UFP is associated with nonaccidental, cardiovascular, or respiratory death, and little evidence that the fuel policy modifies this association.