Impact of time–activity patterns on personal exposure to black carbon

Abstract

Time–activity patterns are an important determinant of personal exposure to air pollution. This is demonstrated by measuring personal exposure of 16 participants for 7 consecutive days: 8 couples of which one person was a full-time worker and the other was a homemaker; both had a very different time–activity pattern. We used portable aethalometers to measure black carbon levels with a high temporal resolution and a PDA with GPS-logger and electronic diary. The exposure to black carbon differs between partners by up to 30%, although they live at the same location. The activity contributing most to this difference is transport: Average exposure in transport is 6445 ng m⁻³, followed by exposure during shopping (2584 ng m⁻³). Average exposure is lowest while sleeping (1153 ng m⁻³) and when doing home-based activities (1223 ng m⁻³). Full-time workers spend almost twice as much time in transport as the homemakers. As a result of the study design we measured in several different homes, shops, cars, etc. enabling a better insight in true overall exposure in those microenvironments. Other factors influencing personal exposure are: background concentrations and location of residence in an urban, suburban or rural environment.

Introduction

Personal exposure can be defined as the real exposure as it is experienced by individuals. When an individual is present at a certain place or in a certain microenvironment, he or she is exposed to the pollutant concentrations at this specific place. When an individual makes a trip from location A to location B, his personal exposure can be defined as the weighted average of concentrations present at each single location (WHO, 1999). Up till now, personal exposure is often estimated through the use of concentrations measured at fixed monitoring stations (Kaur et al., 2007, Sarnat et al., 2009). This is an approximation, as not only the ambient concentration is relevant, but also concentrations in different microenvironments (including indoors) and the whereabouts of individuals (Boudet et al., 2001, Jensen, 1999, Klepeis, 2006). Several studies have already examined the correlation between personal exposure and concentrations measured at fixed monitoring stations (Avery et al., 2010, Boudet et al., 2001, Gulliver and Briggs, 2004). This correlation shows a large spread between different studies, but overall correlation is stronger for longitudinal within-person studies, compared to cross-sectional studies (Avery et al., 2010). This indicates that differences between people and a large part of the spread within a subject can be explained by the activity pattern of the individuals and their daily environment.

Several studies are looking at the relationship between levels of exposure and health effects, but epidemiologists experience vast problems with exactly quantifying exposure. By using approximations for exposure, health effects can be wrongly assigned, or the strength of a relationship will not be sufficiently emphasized (Jerrett et al., 2005, Setton et al., 2011). Therefore researchers are looking at methods, either through direct measurements or indirect modeling, to reduce exposure misclassification (Int Panis, 2010).

We hypothesize that people, who are living at the same location, can nevertheless have a different exposure profile. The driving force for this difference will be the activity pattern and the subsequent microenvironments visited during a day. Short-term exposures may contribute significantly to daily average exposure. The aim of this study is to look at week-long exposure profiles with a high temporal resolution. Linking these data with detailed time–activity patterns will tell us what the impact is of an activity pattern on personal exposure. Two groups of people with a highly differential time–activity pattern were selected to demonstrate this.

The pollutant looked at is black carbon (BC). BC has been used as an indicator of exposure to diesel exhaust (HEI, 2010), and it has been suspected as a contributor to global warming (Highwood and Kinnersley, 2006). Several researchers have recently stressed potential short and long-term cardiovascular, respiratory and neurodegenerative health effects of BC (Baja et al., 2010, McCracken et al., 2010, Patel et al., 2010, Suglia et al., 2007). Over the last 40 years BC-concentrations have declined rapidly in Europe, although the air has still moderate to heavy BC pollution. In the last decade concentrations seem to have leveled off possibly because of increasing emissions of diesel passenger cars.