Carbon load in airway macrophages as a biomarker of exposure to particulate air pollution; a longitudinal study of an international Panel.

Background: Carbon load in airway macrophages (AM) has been proposed as an internal marker to assess long-term exposure to combustion-derived pollutant particles. However, it is not known how this biomarker is affected by changes in exposure. We studied the clearance kinetics of black carbon (BC) in AM, obtained by sputum induction, in a one-year panel study.

Methods: AM BC was measured 8 times with 6 weeks intervals in healthy young subjects: 15 long-term residents in Leuven, Belgium (BE, mean annual PM 20-30 μg/m) and 30 newcomers having arrived recently (< 3 weeks) in Leuven from highly polluted cities (mean annual PM > 50 μg/m) in low and middle-income countries (LMIC, n = 15), or from low to moderately polluted cities in high-income countries (HIC, n = 15). The median and 90th percentile values of AM BC were quantified by image analysis of 25 macrophages per sputum sample; the carbonaceous nature of the black inclusions in AM was verified by Femtosecond Pulsed Laser Microscopy in 30 macrophages. We used a Bayesian hierarchical single-exponential decay model to describe the evolution of AM BC.

Results: In the LMIC group, the mean (95% credible interval) initial quantity (R) of median AM BC [1.122 (0.750-1.509) μm] was higher than in the HIC group [0.387 (0.168-0.613) μm] and BE group [0.275 (0.147-0.404) μm]. Median AM BC content decreased in the LMIC group (decay constant 0.013 μm/day), but remained stable over one year in the other two groups. In the LMIC group, clearance half-lives of 53 (30-99) and 116 (63-231) days, were calculated for median and 90th percentile AM BC, respectively.

Conclusions: In this real-life study of an international panel of healthy young subjects, we demonstrated that carbon load in airway macrophages obtained by induced sputum reflects past long-term exposure to particulate air pollution. Values of AM BC do not change over one year when exposure remains stable, but AM BC decreases upon moving from high to moderate exposure, with average half-lives of 53 and 116 days depending on the carbon load.