|Assessing the Health Consequences of Major Chemical Incidents - Epidemiological Approaches, 1992 (WHO - OMS, 1992, 104 p.)|
|2. Epidemiological tools|
The exposure assessment or environment monitoring specialist will make most of the preparations performed. In an emergency, people should be allocated tasks that are related to their regular professional activities. The epidemiologist must consider the following, although other specialists may be responsible for or authorized to implement the relevant activities:
1. preparing for environmental and biological concentration measurements (sampling equipment, sampling teams, system for quality assurance and control);
2. preparing a list of external institutions and experts to supply additional sampling equipment, analytical capability and expertise;
3. securing quantitative information on the storage of chemicals in a locality;
4. identifying any sources of information on background levels of chemical contaminants in the environment and the population;
5. formulating realistic scenarios as an input to exposure models;
6. preparing a system that ensures access for the epidemiologist to exposure-related data (the description of the cause of the incident, the chemicals involved and all data produced by exposure modelling and/or measurements); and
7. identifying the means to design, print and distribute questionnaires.
The limited duration of the response phase demands that any attempt to assess the exposure is planned and the necessary personnel trained in advance. There will be no opportunity to replicate a missed or faulty measurement. Accurate and comprehensive exposure data collected in this phase are invaluable, and a recording of the progress of the incident (recording images on video and meteorological data by other means) can be used at a later stage to supplement the measurements.
Exposure monitoring provides the epidemiologist with quantitative data on exposure levels. A monitoring strategy should account for the spatial and temporal variability of the chemicals and their concentrations. As mentioned, two general approaches to exposure monitoring are used: one is based on physical and chemical measurements in the environmental media (environmental monitoring) and the other on measurements of changes in human biological systems (biological monitoring).
Environmental monitoring consists of determining a chemicals concentration in an exposure medium, such as air, water or food. When a single chemical is involved in the incident, rapid identification of the chemical and specific sampling and analysis may prevent waste of monitoring capability. Fires pose special problems because they produce many insufficiently specified chemicals.
In an incident, rescue teams may use direct readout devices for gas measurement to evaluate whether it is safe to enter the hazardous area. The results of these tests should be routinely recorded.
Routine air quality monitoring is usually bound to a fixed location. If additional monitoring sites are identified for use in case of an incident, they should be chosen with a representative population exposure in mind. Samples can be collected from different environments (both outdoor and indoor) for later analysis; they should be taken at regular intervals if continuous monitoring is not available.
A more direct method of determining individual exposure is to collect personal samples. The feasibility of personal monitoring of the primary exposure of the general population is largely determined by the duration of the exposure, access to the exposed area and practical constraints, such as the availability of monitoring devices (as chemicals often are not specified) and the organization of monitor distribution, collection and registration. Personal sampling of all exposed emergency response personnel is a realistic possibility, and should be considered whenever practicable. Items of contaminated clothing should be sent for analysis. Bioindicators (concentrations of a chemical or its metabolite in plants or animals) are potential signals of environmental contamination and can be useful for human exposure assessment.
An atmospheric emission may last for minutes or hours, but events such as chemical fires can last for days. The composition of the chemicals and their concentrations in the plume may vary with time. Timely and repeated measurements should therefore be made throughout the duration of the release.
Biological monitoring of exposure refers to cellular, biochemical or molecular measures (biomarkers) that are obtained from biological media such as human tissues, cells or fluids and are indicative of exposure to environmental contaminants (11). Its objective is to determine the internal dose or, ideally, the biologically effective dose of the chemical. These biomarkers may consist of concentrations of the parent compound or its metabolites (12). The field of biological markers is still in early stage of development. Only a few valid biological markers are available for epidemiological purposes, in terms of both assessing population exposure and contributing to the quantitative risk assessment. Nevertheless, biomarkers have a number of appealing features. Table 1 summarizes some advantages of and problems with the application of biomarkers of exposure.
Considering the potential of biological markers, every opportunity should be taken to obtain blood and urine specimens from exposed workers and members of the affected population. The personal characteristics of the sample donor, a characterization of his or her possible exposure and the time and location of the sample collection should be recorded. The choice of containers will depend on the requirements of the laboratories involved, but bottles used in hospitals may be suitable, such as EDTA tubes for blood specimens (for testing for heavy metals, pesticides and solvents, for example) and universal containers (plain bottles) for urine samples (for testing for solvents, for example). Specimens should be stored under refrigeration, as directed by the laboratory, and the analysis should be undertaken by laboratories participating in a quality control scheme.
Table 1. Advantages of and problems with biomarkers of exposure
They integrate all exposure routes.
They can be the only measure of primary exposure, in the absence of environmental data.
They can assess the effectiveness of protective equipment.
They allow for the influence of physical exercise on the inhalation of chemicals.
They may not be chemical-specific.
They are sensitive to collection and storage methods.
Analysis method may not be available.
Sampling strategy must account for the toxicokinetics and toxicodynamics of the chemical.
Sampling and analysis strategies may have been validated only for occupational exposure.
The chemical to be determined may have a substantial background level.
It can be difficult to interpret the results of biological monitoring when the background levels of the biomarkers for the population involved have not been studied previously.
In addition to personal samples from the members of the population at risk, emergency response personnel can be requested routinely to deliver a biological sample after termination of their work shifts. In the event of a controlled evacuation, the collection of biological specimens may be easier if evacuees are accommodated in centralized facilities.
A quality assurance and quality control programme should be instituted whenever environmental and/or biological samples are collected. The topics to be covered are:
· collection (the choice of containers and the possibility of their contamination, type of additives, qualification of personnel, registration of collection time);
· storage and transport;
· the qualifications of laboratory personnel;
· the validity of sampling and analysis techniques for the particular circumstances; and
· duplicate sample collection.
Special attention is needed in cases where more than one sampling and analytical technique has been applied by more than one provider (own personnel, industry, etc.).
Questionnaires and interviews
Questionnaires and structured interviews are used to obtain information from individuals on exposures, factors that may modify their exposure and dose, and/or health effects. Questions related to exposure assessment should therefore be an integral part of any questionnaire to be filled out by the potentially exposed subjects. For the purpose of exposure assessments, the topics covered should at least include:
· activities and whereabouts or locations during the period under study;
· physical activity;
· measures that alter exposure, such as staying indoors or outdoors;
· observations of exposure (sight, smell, taste, etc.); and
· consumption patterns in case of food or drinking-water contamination.
In most emergencies, specially hired staff conduct the interviews. The use of computerized questionnaires and the direct entry of the responses will speed up data processing and the analysis and presentation of the results. Several computer packages can be used for this purpose, including EpiInfo (13).
The strength of the modelling approach is that it can make predictions of exposure available to the emergency services within minutes of reporting of the incident. The disadvantage is that models can only give approximate estimates of dense gas concentration. At present, they do not take account of topography and the built environment.
Exposure modelling must include at least the three phases of source description, chemical dispersion and exposure.
The source description should describe the chemical involved and its physical properties, the release rate and/or total quantity released, and the initial source geometry. Very often the source term (the quantity released) is unknown during the response phase. For example, in the Seveso incident (see Annex), determining which toxic compound was of concern took ten days, and the quantity released is still a matter of dispute (14).
The identity of the chemical and its release rate have to be obtained from the company involved in the incident or after special sampling at the chemical plant. The dispersion of the released chemical in ambient air is determined by the speed and direction of the wind, the buoyancy of the plume, atmospheric stability, the roughness of the terrain (including obstacles) and the physical state of the released material. Modelling exposure involves linking peoples location and exposure-relevant behaviour to the concentrations modelled in the previous step. This requires additional data, usually obtained with questionnaires, such as time-activity patterns, sheltering, protective equipment, etc.
Assessing primary exposure immediately after the incident is crucial for the follow-up evaluation. The follow-up phase includes a retrospective assessment of initial exposure and the assessment of continuing exposure risk.
Possibilities for retrospective assessment of initial exposure include:
· analysis of all collected environmental samples (using methods with a lower detection limit and/or higher precision, and analysis for additional chemicals), including opportunities for further in-depth investigation of environmental samples such as soil, vegetation or water, which can be used as indirect indicators for human exposure;
· biological monitoring of human beings, in which possible continuing exposure should be considered; and
· post-hoc modelling, in which the estimates of exposure can be further refined because:
- the quantity released can be estimated more precisely;
- environmental measurements can be used as an additional input;
- the boundaries of affected vegetation can give an indication of the amount of chemical deposited; and
- more sophisticated models can be applied.
In the follow-up phase, banks of biological specimens can give an accurate estimate of exposure. Once samples have been obtained, it may be advisable not to use all the collected material for analysis.
The storage of biological specimens in banks may have a number of advantages (9,15). For example, it permits the analysis of selected samples, such as those from the subjects who will enter a follow-up study, rather than the examination of all specimens, which is usually very costly. New analytical techniques and theories of toxicodynamic mechanisms may develop that may be applied to specimens in existing banks. Finally, specimens stored in banks provide data on the exposure related to the incident.
Box 7 summarizes important points related to exposure assessment.
Box 7. Exposure assessment - summary
· Appropriate exposure data are crucial for studies on the health impact of chemical incidents.