## 3 An Experiment in Houston  
On 27 and 28 December 2010, an adult male volunteer traveled in Houston, Texas and collected two essential data sets and some supplementary data. The first data set was real-time air pollution level data, particularly ozone (O\({}_{3}\)) concentration level. A BW GasAlert Extreme single gas detector (O\({}_{3}\)) with 10 ppb increments was used as the air pollutant sampler. This sampler was carried by the data collector to record pollutant concentration data at his immediate surroundings. The second data set was individual space-time trajectory data. A Garmin eTrex Vista H handheld GPS navigator was used to record travel data (_i.e._, travel trajectories, travel speeds, and stops). Both instruments were set to collect data at a 10-s interval. The volunteer also maintained a travel diary describing the location and nature of activities during the data collection period. The volunteer's space-time travel data was processed using ArcScene in ArcGIS software package. Figure 3 is an ArcScene generated 3D map showing the individual's space-time path as laid on top of a Houston base map. The vertical dimension (z) represents the two-day travel time; the horizontal dimensions (x, y) represent the space of Houston, Texas. The real-time O\({}_{3}\) concentration data for the individual's immediate surroundings was downloaded from the portable sensors. The real-time AQI values along the individual's space-time path were calculated following Equation (1). The real-time personal O\({}_{3}\) intake rates were computed using Equation (2) by considering the real-time ambient O\({}_{3}\) concentration data, individual physical activities (as recorded in travel diary), and physical activity-related air intake rate (as listed in Table 1). When the real-time personal O\({}_{3}\) intake volume during a unit time was higher than EPA air quality standards, the person was determined to be in an air pollution danger zone. The recorded O\({}_{3}\) concentration values ranged from 20 ppb to 80 ppb on 27 and 28 December 2010. During the afternoon of 27 December, the measured O\({}_{3}\) concentration values varied a lot. By connecting to the GPS data and the travel diary, it was found that O\({}_{3}\) concentration was mostly at medium level when the volunteer traveled on major roads (e.g., Interstate Highway 10, Interstate Highway 45, and W Sam Houston Parkway S); O\({}_{3}\) concentration was low when the volunteer traveled on minor roads or stayed indoors (e.g., shopping and rest). This indicates that O\({}_{3}\) concentration level is closely related to traffic emissions in urban Houston. During the night of 27 December and the morning of 28 December, the O\({}_{3}\) concentration level remained low and stable. The volunteer stayed inside mostly during this time period except for a 40-min outdoor morning exercise, which included jogging and walking. When the volunteer traveled through the city of Houston along highways during midday on 28 December, the O\({}_{3}\) concentration was recorded as being relatively high. It was low in the afternoon of 28 December, during which time the volunteer traveled short distances while conducting a variety of activities, including staying, walking, and running. Using the recorded O\({}_{3}\) concentration data, real-time AQI values were calculated for the volunteer throughout the study period. Following the U.S. AQI standard (see Table 1), different colors are used to visualize the volunteer's real-time personal AQIs. In Figure 4, green indicates good air quality (AQI range: 0-50), yellow indicates moderate air quality (AQI range: 51-100), and orange indicates that the air quality is unhealthy for sensitive groups (USG) (AQI range: 101-150). The continuous AQI  
Figure 3: The space-time path of the volunteer in Houston, Texas on 27 and 28 December 2010.  
values and its color scheme is an important individual property that is uniquely associated with the volunteer's space-time trajectory.