In the present investigation, the most prominent effect on the CC16 concentration was a drop > 2.5 μg/L between baseline concentrations at 7:00 am and those measured at 2:30 pm. As CC16 serum concentrations in normal healthy subjects, on average, range from 10 to 15 μg/L, a fall of this magnitude reflects a substantial reduction in the baseline CC16 concentrations, an effect that may be in the same range as, or even greater than, any air pollution-induced effect on circulating CC16 levels. Therefore, the time-dependent diurnal variation must be considered when CC16 is used as a biomarker for air pollution exposure. The present study also confirmed the presence of a major interindividual difference. Of importance, however, was that the time-dependent diurnal change in CC16 was similar regardless of the individual and the individual baseline concentrations. This made it possible to develop a mathematic model consisting of a second-degree polynomial to determine the fall in serum CC16 across a day, given a known baseline morning concentration. Furthermore, it does necessitate that serum collected for field study use has to be controlled for sampling time as well as compared to an individual baseline serum sample drawn on the same day. Given these precautions, CC16 may be employed as a biomarker for ozone exposure in field studies. An alteration from the calculated CC16 value at a certain time point, using the mathematic model, can thus be interpreted as an air pollution-induced effect on CC16.