## 3 An Experiment in Houston
As discussed in the previous section, this aspect can be represented as the \(E_{i}\) for the individual's space-time path that is made up of instances \(I\) (\(x_{i}\), \(y_{i}\), \(t_{i}\), \(E_{i}\)). Figure 4 is a visualization of the dynamic individual-environment interaction along the space-time path of the volunteer; particularly, his real-time AQI along the space-time path is visualized together with his position in the space-time cube. The volunteer's real-time personal O\({}_{3}\) exposure (_i.e._, O\({}_{3}\) concentration in immediate surroundings), real-time AQIs, and real-time O\({}_{3}\) intake rates are plotted together in Figure 5. Note that the three measures were plotted to be compatible along the y-axis for visualization purpose. As AQIs are directly related to O\({}_{3}\) concentration, the variation of the volunteer's real-time personal AQIs is similar to that of the concentration of O\({}_{3}\). However, there may be a big difference between the pollution concentration level (_i.e._, concentration of O\({}_{3}\)) and the pollutant intake level (_i.e._, O\({}_{3}\) intake rate), as shown in Figure 5 for the mid-afternoon on 27 December. The nature of individual activities during this time is the major cause for this difference. Given a pollutant concentration level, the different types of activities required different levels of air intake, leading to different pollutant intake levels. This clearly shows the importance of understanding the dynamics of individual-environment interaction along a space-time path, which is the \(E_{i}\) in the qua-tuples of \(I\) (\(x_{i},y_{i},t_{i}\), \(E_{i}\)). Figure 5 shows that the volunteer's O\({}_{3}\) intake rates were higher and more unstable during the daytime. The volunteer visited different places and conducted different types of daytime activities, including walking, running, and staying. These activities changed his breathing rate and resulted in increased and  
Figure 4: The volunteer’s space-time path with personal AQIs in Houston, Texas on 27 and 28 December 2010.  
changing levels of O\({}_{3}\) intake. During the nighttime, the volunteer stayed indoor with low-level physical activities, leading to low and stable O\({}_{3}\) intake rates. A total of six peaks of O\({}_{3}\) intake rate (_i.e._, intake rates \(>10^{-6}\) L/min) can be identified from Figure 5. Two peaks were occurred in the afternoon of 27 December, one peak in the morning of 28 December, one peak in the noon of December 28, and two peaks in the afternoon of December 28. By cross-referencing GPS trajectory data, the six peaks can be registered to specific positions in the space-time cube. Figure 6 highlights these corresponding six nodes as the high O\({}_{3}\) intake danger zones along the volunteer's space-time path. These zones signify the dangerous health risk segments in the individual's space-time behavior during the study period. Note that some of these danger zones fall onto the space-time segments where AQIs were not good, but others fall onto AQI safe segments along the volunteer's space-time path. The air quality at zone 1 and 2 was moderate (AQI color: yellow); the air quality at zone 3, 5, and 6 was good (AQI color: green); the air quality at zone 4 was unhealthy for sensitive groups (USG) (AQI color: orange). This manifests that even if the air quality is good, strenuous exercise may lead to inhalation of excessive air pollutants, and therefore exposure an individual to dangerous level of pollutant intake. On the other hand, when air pollutant concentration is relatively high and AQI is not good, maintaining inactive or less active physical activities so a minimum amount of pollutant entering into one's body may be an effective measure for alleviating the adverse health effects of air pollution. Figure 5: The volunteer’s real-time air pollution exposure, intake, and AQI values during the 27–28 December 2010 data period in Houston, Texas. (**a**) O\({}_{3}\) intake rate (\(10^{-8}\) L/min); (**b**) AQI; (**c**) measured O\({}_{3}\) concentration (ppb).