![]() 14 – 17 Pollutant levels can be much higher indoors than outdoors for those with predominant indoor sources (eg, radon and secondhand smoke) and vice versa for those mainly generated outdoors (eg, ozone). 1 For instance, traffic-generated air pollutants such as ultrafine particles and volatile organic compounds can be up to ten times higher inside a vehicle than ambient outdoors. ![]() Air pollutant concentrations can vary significantly by space and time. 6, 8 However, challenges exist in adequately classifying individuals’ time location patterns in air pollution epidemiological studies. Recent technological advances and consumer demand for location-aware technologies have made commercially available GPS receivers smaller, more accurate, energy efficient, user friendly, and more accessible to researchers. This multipath problem leads to errors in the distance calculation in GPS. 6 GPS proximity to high steel-frame structures in urban areas may impact positional accuracy given they may reflect satellite signals and cause them to bounce many times before reaching a GPS device. Differential-corrected GPS have been incorporated in certain GPS devices to improve spatial accuracy and minimize bias by calibrating a given location by the measured location of a base station relative to its true location. Horizontal DOP (HDOP) indicates the quality of a horizontal position (latitude and longitude) based on satellite geometry low HDOP value indicates widely-dispersed satellites and potentially higher positional accuracy. Certain GPS devices output diagnostic information for recorded locations such as the number of available satellites and dilution of precision (DOP) which measure satellite signal quality and positional accuracy. Three factors determine the spatial accuracy of the GPS data: the availability and spatial distribution of satellites, the surrounding environment, and the GPS chip and antenna which varies across individual manufacturers. The use of GPS for human tracking presents an enormous opportunity for improving our understanding of the space-time activities of individuals and how they influence environmental exposure and health outcomes. 4 – 11 The GPS determines geographic location (ie, latitude, longitude and altitude) and time with four or more earth-orbiting satellites. 4 Recently, new techniques have been used to improve the traditional methods that collect time-location data, including video-taping children’s micro-activities (eg, hand-to-mouth behaviors) for a short period of time and tracking people’s time-location patterns or commuting behaviors using portable global positioning system (GPS) technology with or without corresponding traditional diary information. 1 Conventional time-location data are collected by recall interviews or diaries, 2, 3 but these are limited by accuracy of recall, reliability, reproducibility, and compliance. Such time-location information can be linked with corresponding pollutant concentration data to improve estimates of personal exposures. The knowledge of where individuals spend their time is an essential component of human exposure assessment and environmental epidemiology.
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