Alveolar macrophages carbon load: a marker of exposure?

In the editorial accompanying our article reporting higher alveolar macrophage carbon in adults who cycled to work in London, ACKERMANN-LIEBRICH [1] speculated that ‘‘very short, high peaks of exposure contribute more to total load of inhaled pollutants and, thus, we ought to control not only the mean pollutant levels but avoid higher peaks.’’ In our initial analyses, we found no association between airway macrophage carbon and highest single exposure black carbon concentration (ng?m) [2]. Since we had not assessed the effect of multiple peaks, we performed an additional analysis using the number of peaks of black carbon .10,000 ng?m over 24 h as an exposure variable. Peaks of black carbon .10,000 ng?m in each subject were identified from visual inspection of the time/exposure graph of the portable aethalometer output (Magee Scientific AE51, Berkeley, CA, USA). We found a significant association between airway macrophage carbon and the number of peaks of black carbon (r50.33, p,0.01, n527; fig. 1). As expected, the number of peaks of monitored black carbon was higher in cyclists compared with non-cyclists (mean¡SEM 6.14¡1.0 (n514) versus 2.46¡0.7 (n513); p,0.001, unpaired t-test). We therefore agree with ACKERMANNLIEBRICH [1] that short-term exposure to high concentrations of freshly generated diesel soot should be considered when developing air pollution exposure-reduction policies.