Short-term effects of ultrafine particles on stroke events: an assessment using four different exposure metrics

BACKGROUND: The effects of different ultrafine particle (UFP) metrics on strokes are unclear. This case-crossover study investigated the association between short-term exposure to four size-segregated UFP metrics and stroke occurrence. METHODS: From 2006 to 2020, we included 19,518 stroke cases from the University Hospital Augsburg, Germany, a less polluted area. Meanwhile, daily averages of four UFP metrics, including particle number (PNC), mass (PMC), length (PLC), and surface area (PSC) concentrations, were collected from fixed monitoring sites in Augsburg. Conditional logistic regression was employed to assess the association between UFP metrics and stroke risk. Potential individual vulnerability and effect modification were examined using the stratified and interaction analyses. RESULTS: Elevated risk of stroke events was largely similar across all four UFP metrics. The odds ratios (95 % confidence intervals) of strokes for each interquartile range increase in lag 0-6 days of UFPs were 4.76 % (1.06; 8.60) for PNC, 3.99 % (0.93; 7.13) for PMC, 4.52 % (1.11; 8.05) for PLC, and 4.14 % (1.00; 7.38) for PSC. Stable associations with strokes were mainly found for the size fractions of 10-100 nm and 30-100 nm. The cumulative effects of UFP were more pronounced for ischemic strokes and minor strokes with a lower severity. Cold spells might exaggerate the effects of UFPs. CONCLUSION: UFP metrics like particle length and surface area concentration, aside from particle number, may provide valuable insights into particle properties relevant to stroke risk. Expanding real-time, size-segregated monitoring of UFPs represents an effective strategy to mitigate the health impacts of traffic-related air pollution.