|Assessing the Health Consequences of Major Chemical Incidents - Epidemiological Approaches, 1992 (WHO - OMS, 1992, 104 p.)|
|2. Epidemiological tools|
Exposure assessment within the framework of major chemical incidents has three goals:
· to contribute to quantitative assessment of the health risks to the exposed population;
· to provide exposure estimates for epidemiological studies; and
· to aid in the evaluation of the effectiveness of interventions taken to restrict the emission and dispersion of the contaminants, and to reduce human exposure.
These goals require similar but not identical exposure assessment strategies. This section emphasizes exposure assessment for epidemiological purposes. The limited usefulness of studies on the health consequences of chemical incidents is often due to the lack of appropriate exposure data (9). A substantial investment in exposure assessment is required to ensure better effectiveness of epidemiological follow-up. Present practice shows that the response to a chemical incident must often proceed without a knowledge of the nature and toxicity or even the chemical constituents of the exposure. This can result from a number of factors. Some can be related to delays in characterization of the pollutants or to limitations in analytical methodology and equipment. Another may be the limited knowledge of the possible toxic properties of the chemicals involved. In such a case, the observed health effects determine the epidemiological approach.
Before making any exposure assessment, it is essential to determine whether a chemical incident is taking place (see Box 5). After confirmation, the first need is to identify the source, if it is not obvious. Contrary to appearances, this may be a difficult task (as in the case of toxic oil syndrome in Spain). Tracing a source in densely industrialized areas poses a particular problem. A notification system that requires all incidents to be immediately reported to an appointed agency may be of benefit.
Box 5. Event in Rotterdam
In September 1994, the emergency response services in Rotterdam were notified of a leaking railway wagon. The wagons label identified the contents as tetramethyl lead (TML); there was some confusion on the consignment note. The incident caused considerable disturbance, because TML is a highly toxic and volatile chemical for which no direct reading exposure assessment methods were available. After about 1½ hours, the leaking chemical was identified as red wine, which was also the cargo of the two adjacent wagons. The leaking wagon had received a wrong label.
After confirmation, the chemicals involved need to be identified. Here, too, the notification system can be valuable. In some instances, such as releases of a mixture of chemicals, identification can be difficult but should be considered. Fires pose an even more complicated problem, because combustion products also need to be considered, gases can be emitted at high temperature and aerosols with a complex composition may be formed (see Box 6). The feasibility of the assessment of exposure to substances relevant from the health point of view should be considered as well.
The spatial and temporal distribution of the chemical in the environmental medium needs to be determined. Concentrations at various locations and times may depend on the identity and phase (solid, liquid, gas) of the released chemicals. Finally, individual subjects exposure can be determined by assigning a value to some indicator of exposure. The final two steps of exposure assessment are the main topic of this chapter.
Box 6. Chemical fire in Basle
In the Schweizerhalle incident (see Annex) over a thousand tonnes of agrochemicals went up in flames, covering a part of the city of Basle with an evil-smelling plume. During the fire, the identities of the burning chemicals were unknown. Even now, the exact composition of the plume to which the population was exposed remains unknown.
The choice of methods for the exposure assessment must take account of a continuum, from the release of the chemical from its source to the health or nuisance effects it may cause in the population at risk (Fig. 1). At each part of the continuum, different indicators can be identified. These vary between estimates of visible emission or concentrations in media on the one hand to the dose received at a target organ on the other. Their applicability is determined by the validity of the indicators (10), as well as the availability or feasibility of the measurement methods. Conditions after the incident may impose special constraints on method selection.
Almost every observational epidemiological study has to cope with a discrepancy between the ideal (desired) and the attainable exposure data. This discrepancy is even greater in studies of the health consequences of major chemical incidents. The often short duration of the primary exposure, the resultant time constraints and the poor qualitative definition of the toxic chemicals in the acute phase of the incident are particular problems.
More sophisticated indicators usually require more extensive and/or more complex measurement. In most chemical incidents, the biologically effective dose of the toxic chemical cannot be determined. In the few instances where biological monitoring is possible, an internal dose estimate may be feasible. In others, even a clear distinction between exposed and unexposed individuals may be problematic. In the absence of any other measures, however, a surrogate may be used, such as the presence of an individual in a certain exposure zone.
Fig. 1. Source-effect continuum
Different exposure categories
Three different groups of subject may be exposed in a chemical incident:
· emergency response personnel (fire-fighters, rescue workers, police, ambulance personnel);
· personnel from the facility where the incident occurred, including drivers in the case of a transportation incident; and
· the general population, resident and transient.
It is crucial to recognize that these groups may have qualitatively and quantitatively different exposures. For example, soaked clothing may be an additional exposure route and medium for fire-fighters; thus, dermal contact is likely to be more important for them than for the general population. Second, certain sampling techniques may be feasible for one group and impossible for the others.
Primary and secondary exposure
In an incident, a chemical is usually released in one medium: air, water, soil or food. Exposure to this contaminated vehicle is defined as primary exposure. At the incident site, additional exposure media may be involved, such as run-off water used to extinguish a fire. Examples of secondary exposure include ground contamination (exposure via direct contact or the food chain), permeation of drinking-water piping by soil pollutants, food contamination after releases into the atmosphere or deposition in drinking-water or irrigation water. A complicating factor in assessing the exposure caused by the incident can be pre-existing background pollution. Distinguishing this pollution from contamination due to the incident may be difficult.
Exposure assessment methodology
There are three broad categories of exposure assessment: monitoring, questionnaires and interviews, and modelling.
Environmental monitoring estimates the concentrations of the chemical in the environment, biological monitoring estimates the concentrations in exposed humans. Monitoring instruments are used in both cases.
Questionnaires and interviews estimate activity patterns and the location of people during the exposure. A tentative classification of subjects into exposure categories may result from this information. Acute symptoms may give an indication of the level of exposure.
With exposure models, the concentrations of the chemical in the exposure medium can be estimated without monitoring data. Any modelling attempt should include at least source description, dispersion modelling and exposure modelling. In addition, the observation of vegetation change and the health of animals is a very important indirect method of exposure assessment. Ad hoc inquiries on the health and behaviour of domestic animals may be advisable. In Seveso (see Annex), the first signals of potential risk to human health were the deaths of animals and vegetation.
Often, more than one exposure assessment approach is applied. Even scarce monitoring results may be very useful to verify and adjust model predictions of concentrations. Alternatively, personal monitoring results from a few subjects, combined with questionnaire data, can be used as a basis for the exposure estimates for the population.