7.6 Time-activity data and personal monitoring

This object introduces the concept of time-activity data, which are used to assess individuals’ exposure based on their daily movements between home, work and elsewhere. In exposure studies, individuals are often represented on the map as a point location by geo-coding either their place of residence or place of work. In reality, individuals are highly mobile and travel between work, home and many other locations over the course of a week. Their exposure to an environmental hazard is dependent on pollutant concentrations at all of these places and not simply the pollutant concentration at home or in work. With time-activity data, individuals can be represented spatially through multiple point locations or lines. This object considers how time-activity data can be incorporated within a GIS, an important task if exposure is to be assessed in a realistic way.

As well as indicating how long individuals spend in different locations, time-activity data are important in determining how long individuals spend in different micro-environments. This is important because air pollution levels at a given location may be very different indoors compared to outdoors. Similarly, the inhaled dose of a pollutant may be different for a pedestrian than a motorist.

Improving geo-referencing in exposure studies

There are several methods that may be used to geo-reference individuals more realistically rather than using single point locations. One approach to recording daily movements among a population at risk is to issue a sample of individuals with GPS units (Elgethun et al , 2003). Potentially, the movement pathways of individuals recorded using GPS can be related to map layers depicting pollutant concentrations or source locations. Another approach is to record time-activity patterns among a sample of individuals either through a questionnaire survey or by using diaries (Hertl et al, 2001; Yoo Min Park, 2020). These diaries or questionnaires record the time spent in different locations and micro-environments. The time spent at each location can be multiplied by the pollutant concentration for that location, so giving an overall cumulative exposure estimate.

Diaries or questionnaires only provide a measure of current time-activity patterns. Identifying activity patterns earlier in an individual’s life – when dangerously high exposure may have occurred – is almost impossible. Furthermore, time-activity patterns vary greatly even within the same age cohort and so it can be difficult to generalize from sample surveys of time activity to the population in general.

As a result, improving geo-referencing of population groups has proved more difficult than representing the movement of individuals spatially. In some countries, census data record the daily movement of the whole population. For example, the UK census records both place of work and place of residence and thus origin-destination statistics can be generated. This information is used to generate origin-destination data that describe flows of commuters between census output areas and workplace zones (or other reporting units). In the US , the Census Transportation Planning Package (CTPP) produced by the Bureau of the Census also records commuting patterns. These data have not yet been used in environmental exposure studies. Why might this be? (When you have thought about this, click on ‘show’ below for some possible reasons)

Answer

Aside from short-term daily movements of population, in the longer term, individuals may change jobs or move home. In many case-control studies, cases are geo-coded using the patient’s address at the time of diagnosis. In such circumstances, current residence or place of work will not reflect their past history of environmental exposure. In more recent surveys, a residential history is recorded, rather than current address only (e.g. Bellander et al, 2001). This means that cumulative exposure over an individual’s life-time can be estimated from pollutant concentrations at all of their previous addresses.

There are more general problems in geo-coding addresses, such as incomplete post (zip) codes and the variation in environmental exposure that may occur within the same unit post code boundary.

Personal Monitoring

Another means of assessing an individual’s exposure as they move around their environment is to use personal monitoring. Personal monitoring is concerned with the assessment of environmental exposure for individuals who may be at risk. Instead of monitoring ambient pollution levels in the general environment, personal monitoring devices measure pollutant levels in or immediately around particular individuals. Three types of exposure can be measured using personal monitoring: external , internal and target exposures.

External exposure is concerned with pollutant levels in the air immediately surrounding an individual or in the food and water that (s)he consumes. In practice, air-borne external pollutants, particularly nitrogen dioxide, are the most widely monitored. Portable monitoring systems for air-borne pollutants include active devices, where air is pumped into a chamber with a sensor, and passive devices, where air gradually diffuses through to a detector. Passive devices are cheaper but provide less information about how exposure has changed over time. Typically, passive devices measure aggregate exposure over a week or longer, whilst active devices take multiple exposure measurements over much shorter periods.

Internal exposure is used to establish the dose of a pollutant absorbed by a particular individual. An example of internal exposure assessment would be the taking of blood samples to estimate internal exposure to lead. Urine and hair samples are also used to assess internal exposure (particularly to arsenic) with hair samples being especially useful since the approximate timing of exposure episodes can be estimated.

Target exposure is concerned with the pollutant dose absorbed by a specific organ within the body, such as heavy metal concentrations within the liver. Personal monitoring is potentially highly intrusive and internal and target exposure monitoring are both subject to strict ethical approval procedures.

Closely allied to personal monitoring is micro-media or micro-environmental monitoring . This entails monitoring pollutant levels at multiple points in the immediate environment of individuals at risk. For example, nitrogen dioxide sensors might be placed in different parts of the home, office and car of an individual at risk and used to assess exposure.

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