Association between short-term exposure to air pollution and ischemic stroke onset: a time-stratified case-crossover analysis using a distributed lag nonlinear model in Shenzhen, China

Abstract

Background: Stroke, especially ischemic stroke (IS), has been a severe public health problem around the world. However, the association between air pollution and ischemic stroke remains ambiguous.

Methods: A total of 63, 997 IS cases aged 18 years or above in Shenzhen were collected from 2008 to 2014. We used the time-stratified case-crossover design combining with distributed lag nonlinear model (DLNM) to estimate the association between air pollution and IS onset. Furthermore, this study explored the variability across gender and age groups.

Results: The cumulative exposure-response curves were J-shaped for SO₂, NO₂ and PM₁₀, and V-shaped for O₃, and crossed over the relative risk (RR) of one. The 99th, 50th (median) and 1st percentiles of concentration (μg/m³) respectively were 37.86, 10.06, 3.71 for SO₂, 116.26, 41.29, 18.51 for NO₂, 145.94, 48.29, 16.14 for PM₁₀, and 111.57, 49.82, 16.00 for O₃. Extreme high-SO₂, high-NO₂, high-PM₁₀, high-O₃, and low-O₃ concentration increased the risk of IS, with the maximum RR values and 95% CIs: 1.50(1.22, 1.84) (99th vs median) at 0-12 lag days, 1.37(1.13, 1.67) (99th vs median) at 0-10 lag days, 1.26(1.04, 1.53) (99th vs median) at 0-12 lag days, 1.25(1.04, 1.49) (99th vs median) at 0-14 lag days, and 1.29(1.03, 1.61) (1st vs median) at 0-14 lag days, respectively. The statistically significant minimal RR value and 95% CI was 0.79(0.66,0.94) at 0-10 lag days for extreme low-PM₁₀. The elderly aged over 65 years were susceptible to extreme pollution conditions. Difference from the vulnerability of males to extreme high-SO₂, high-NO₂ and low-O₃, females were vulnerable to extreme high-PM₁₀ and high-O₃. Comparing with the elderly, adults aged 18-64 year were immune to extreme low-NO₂ and low-PM₁₀. However, no association between CO and IS onset was found.

Conclusions: SO₂, NO₂, PM₁₀ and O₃ exerted non-linear and delayed influence on IS, and such influence varied with gender and age. These findings may have significant public health implications for the prevention of IS.

Keywords: Air pollution; Case-crossover design; Distributed lag nonlinear model; Ischemic stroke; Quasi-Poisson regression.

Fig. 1

Time-series results regarding the association of ischemic stroke onset with air pollution indicators and meteorological factors in Shenzhen from 2008 to 2014

Fig. 2

Summary of cumulative exposure-response curves on ischemic stroke for air pollution factors (SO₂, NO₂, PM₁₀ and O₃) for total cases at lag0–14 using two-pollutant model in Shenzhen, 2008–2014

Fig. 3

Summary of single day lag-response curves on ischemic stroke for air pollution factors (SO₂, NO₂, PM₁₀ and O₃) for total cases at different lags using two-pollutant model in Shenzhen, 2008–2014. The extreme-high influence was estimated by the RR of ischemic stroke by comparing the 99th percentile of daily air pollution value to the median value, whereas the extreme-low influence was estimated by comparing the 1st percentile of daily air pollution value to the median value

Fig. 4

Summary of cumulative exposure-response curves on ischemic stroke for air pollution factors (SO₂, NO₂, PM₁₀ and O₃) for subgroups at lag0–14 using two-pollutant model in Shenzhen, 2008–2014. Male and female were subgroups according to gender. The elderly and adult were subgroups according to age (adult: 18–64 years; the elderly: ≥ 65 years)