Population at risk

Definition

The epidemiologist is concerned with a broader population group than that comprising the people obviously affected by an incident. This is the population at risk, the people who might have been exposed to the chemicals by being present in the affected area at the time of the incident. Personal characteristics may determine the extent of exposure or susceptibility to health effects. These may restrict the population at risk to a section of all the people exposed (for example, to women of reproductive age in case of a release of a teratogenic agent). In practice, however, such specific information about an incident is rarely available when it occurs. Determination of the population at risk will thus be influenced by:

· information about the nature of the incident (the media affected and the toxic properties of the chemical(s) involved);

· the estimate of spatial distribution of the contamination, depending on the dispersion of the pollutant in the environment; and

· the available demographic information (census data, address registry, etc.) and the method of identifying places of residence in the existing data sources (district identification, postal zone identifier, etc.).

The population at risk must be identified for three purposes. The first is to define the population in which the health effects due to the incident may be expected. These people may need special medical care. One can assume that the people obviously affected by an incident can be easily identified by the rescue teams, family members, neighbours or co-workers, and will be provided with medical assistance. Other affected people may show less specific symptoms and, without information about the potential exposure, may not receive optimal medical care.

Second, the population at risk must be identified to determine the target population for studies assessing the effects of the incident. The primary objective is to determine the number of people at risk in order to define the denominator for the calculation of the frequency of the symptoms that may be due to the exposure. The rates can be compared with those in the reference populations: those not affected by the incident.

The third purpose is to define the essential characteristics of the study design to be applied, and to estimate the resources, means and networks required to investigate the health impact.

Precisely defining the population at risk is not an easy task. Including too many unexposed people may diffuse the efforts of medical services and will decrease the impact estimates. If the population selected includes only some of those exposed (only those with higher doses, for example), the severity of the impact may be overestimated. If the effects are specific for a part of the population (such as people in certain occupations), the study should concentrate on that group. The specific hazard may be difficult to identify early in the investigation, however, and the restriction of the study to a predefined subset of those exposed may reduce its ability to reflect the real magnitude of the health effects.

Since the interpretation of the evaluation will depend on the composition of the study groups, the definition of the population should be clearly stated and recorded for future reference.

The most frequently used methods of determining the size of the population at risk are based on indirect methods of exposure assessment readily available soon after the incident. A useful tool is dispersion modelling, combined with data on the population density in the area of the incident. Models and the population database should ideally be prepared in advance, preferably assisted by a computer-based geographical information system (GIS) facilitating the making of estimates.

Consideration should be given to the inclusion of subjects exposed in special circumstances, such as members of the rescue teams.

Rapid appraisal

When the rescue team has little information about the size of the possibly affected population, the nature or severity of the incident, or the chemicals involved and their toxic properties, the rapid appraisal method may be helpful. This was the case after the Bhopal accident, when a team of doctors travelled in the possibly affected area meeting many people and asking each person a few basic questions (8).

In rapid appraisal, the population surveyed may not be determined beforehand and can be modified according to the information collected. Appraisal can be based on reports from affected people who make telephone enquiries for advice (to general practitioners, casualty departments, emergency services, etc.), as well as those going to physicians’ offices or casualty units for assessment. Coordination of the various sources of information on health effects would be helpful in these cases, and gives an early picture of the type and extent of problems occurring, which indicates the extent of the population at risk. Such appraisals may also be needed in a population apparently not affected by the incident, to establish reference health characteristics for the exposed group.

Rapid appraisal may be of value in the early stage of the assessment, but may be biased by the non-representative selection of the contacted persons and gives no quantitative estimate of the magnitude of the health impact of the incident. To establish the distribution and symptoms attributable to the exposure, assessment must be conducted in a pre-defined population at risk; this may exceed the framework of the rapid appraisal.

Sources of demographic information

Assuming that at least approximate information on the area affected by the chemicals released in an incident is available, the size of the population at risk must be estimated from demographic databases. For each population, there may be several sources of demographic information. The source used will depend on accessibility and the importance of its advantages and disadvantages in a given population. In each event, the possibility should be considered that non-registered people, such as commuting workers or travellers, constitute a significant part of those exposed and may influence the actual size of the population at risk.

Access to demographic data in the planning and preparedness phase

Estimating the size of the population at risk immediately after an incident requires rapid access to demographic data. This will depend on the familiarity of the emergency response team, and of the epidemiologist in particular, with the existing data sources. Thus, important tasks of the epidemiologist at the preparedness phase include:

1. identifying all relevant sources of demographic information at the smallest possible geographical scale;

2. assessing the purpose, scope and limitations of each of the available data sources;

3. exploring methods of gaming access to the information (restrictions, speed, necessary equipment, personnel);

4. identifying the format of the data provided by each of the sources;

5. defining situations in which a particular data source should be approached; and

6. specifying the information (contents, format, level of details) to be collected from each source in the case of an incident.

The purpose of this preparatory work is to identify the optimal methods of defining a population at risk and the corresponding reference group in the event of an incident. Depending on the estimated likelihood of the event, some preliminary estimates may be prepared beforehand (by using GIS, for example).

Developing the population register and follow-up

Four activities are required to develop a population register. The first is the identification of the criteria and methods for selecting the subjects (exposed and reference populations). This includes precisely defining the at-risk and reference populations, and listing the possible (practical) sampling schemes and corresponding sample frames. The feasibility and ethical aspects of following up a sample of the exposed (instead of the whole exposed population) should be considered. Sample size estimates can be calculated according to expected health outcomes.

The second task is the preparation of the questionnaire for registering the exposed population, which should ensure unique identification of the individuals. This should ideally be done in advance. The personal identification data should enable the individual data to be linked to those from the population registries (vital statistics, address registry, etc.). Possible constraints due to data protection legislation should be considered.

Third is the identification of the human and technical resources for subject registration immediately following the incident. This should include the feasibility of involving the medical and emergency services in case registration.

The fourth task is the planning of the possible involvement of various institutions in long-term follow-up studies, including the estimation of the human, technical and financial resources needed for data collection, processing and analysis.

Establishing and maintaining the subject register may require long-term organizational, technical and financial support. Its completeness - the inclusion of all subjects according to the register design, the definition of the population at risk, and the inclusion of all follow-up information about the entire population at risk -is crucial to the usefulness of the collected data. The mobility of the population is a main source of concern in long-term follow-up. When migration is related to an incident and to the exposure or health impact, the lack of full registration leads to biased estimates of the health consequences.

Box 4 summarizes important points about the population at risk.