An estimate of the exposure-response relationship between traffic-related ultrafine particles and systolic blood pressure

Abstract

Exposure to traffic-related air pollution (TRAP) is associated with adverse health outcomes, including hypertension and elevated blood pressure; however, the nature of the exposure-response functions for these associations is not well established. In a previous three-exposure, three-period crossover trial we showed that using portable HEPA filters to reduce indoor infiltration of TRAP was significantly effective in preventing short-term average increases in systolic blood pressure (SBP). We measured particle number concentrations (PNC), black carbon (BC) concentrations, and room temperatures continuously during the trial, and we measured participant blood pressures every 10 minutes. We then examined the relationship between SBP and PNC. A total of 1321 SBPs arising from 76 participants were analyzed. The average age of the participants was 60 years, 79% were female, 77% were Asian, and 17% were White. None had serious health conditions. The average SBP was 120.0 mmHg and the average particle number concentration (PNC) was 13,000 particles/cm3, ranging from 860 to 99,000 particles/cm3. A linear mixed model was used to fit a curvilinear relationship between SBP and PNC. The model included BC concentrations and room temperatures as covariates, as well as a random intercept to account for the interdependence of multiple SBPs recorded from each participant. The resulting adjusted model, yielding statistically significant regression coefficients (p < 0.001), can be written as: SBP = 117.54 + 1.22 (PNC/5000) - 0.059 (PNC/5000)2, which implies that SBP increases by 1.22 mmHg for every 5000 particles/cm3 increase in PNC but tapers off at higher levels of PNC. For example, at 5000 particles/cm3 the average SBP is expected to be 118.7 mmHg, but it increases to 121.0 mmHg at 15,000 particles/cm3 and to 124.5 mmHg at 30,000 particles/cm3. The results of the present study suggest an exposure-response function for SBP and PNC.