Background

Although short-term exposure to ambient particulate matter has increasingly been linked with cardiovascular diseases, it is not quite clear how physical characteristics of particles, such as particle size may be responsible for the association. This study aimed at investigating whether daily changes in number or mass concentrations of accurately size-segregated particles in the range of 3 nm–10 μm are associated with daily cardiovascular emergency room visits in Beijing, China.

Methods

Cardiovascular emergency room visit counts, particle size distribution data, and meteorological data were collected from Mar. 2004 to Dec. 2006. Particle size distribution data was used to calculate particle number concentration in different size fractions, which were then converted to particle mass concentration assuming spherical particles. We applied a time-series analysis approach. We evaluated lagged associations between cardiovascular emergency room visits and particulate number and mass concentration using distributed lag non-linear models up to lag 10. We calculated percentage changes of cardiovascular emergency room visits, together with 95% confidence intervals (CI), in association with an interquartile range (IQR, difference between the third and first quartile) increase of 11-day or 2-day moving average number or mass concentration of particulate matter within each size fraction, assuming linear effects. We put interaction terms between season and 11-day or 2-day average particulate concentration in the models to estimate the modification of the particle effects by season.

Results

We observed delayed associations between number concentration of ultrafine particles and cardiovascular emergency room visits, mainly from lag 4 to lag 10, mostly contributed by 10–30 nm and 30–50 nm particles. An IQR (9040 cm⁻³) increase in 11-day average number concentration of ultrafine particles was associated with a 7.2% (1.1–13.7%) increase in total, and a 7.9% (0.5–15.9%) increase in severe cardiovascular emergency room visits. The delayed effects of particulate mass concentration were small. Regarding immediate effects, 2-day average number concentration of Aitken mode (30–100 nm) particles had strongest effects. An IQR (2269 cm⁻³) increase in 2-day average number concentration of 30–50 nm particles led to a 2.4% (−1.5–6.5%) increase in total, and a 1.7% (−2.9–6.5%) increase in severe cardiovascular emergency room visits. The immediate effects of mass concentration came mainly from 1000–2500 nm particles. An IQR (11.7 μg m⁻³) increase in 2-day average mass concentration of 1000–2500 nm particles led to an around 2.4% (0.4–4.4%) increase in total, and a 1.7% (−0.8–4.2%) increase in severe cardiovascular emergency room visits. The lagged effect curves of number and mass concentrations of 100–300 nm particles or 300–1000 nm particles were quite similar, indicating that using particulate number or mass concentrations seemed not to affect the cardiovascular effect (of particles within one size fraction). The effects of number concentration of ultrafine particles, sub-micrometer particles (3–1000 nm) and 10–30 nm particles were substantially higher in winter comparing with in summer.

Conclusions

Elevated concentration levels of sub-micrometer particles were associated with increased cardiovascular morbidity. Ultrafine particles showed delayed effects, while accumulation mode (100–1000 nm) particles showed immediate effects. Using number or mass concentrations did not affect the particle effects.