EXPOSURE AND HEALTH DATA ON HEAVY METALS

Dr. Isabelle Romieu

Exposure to heavy metals is wide spread and a serious threat to the health of women and children. Sources of exposure and health outcomes need to be monitored in order to design, implement and evaluate prevention and control activities. However, traditional surveillance approaches used in public health practice are difficult to apply to heavy metals poisoning because adverse health effects related to heavy metal exposure may not be clinically diagnosed, except at very high exposure levels, and are not usually listed as reportable diseases.

To evaluate the amplitude of an environmental problem such as ambient heavy metals, it is important to define three keys elements: (1) the potential sources of exposure: (2) the indicators for the evaluation of health effects and environmental exposure: (3) the population at risk.

Exposure to heavy metals occurs through different sources and pathways, but databases on potential exposure sources are often not available. When sources are not well identified, data on industrial use of heavy metals can provide information on potential sources. For example, high lead exposure is likely to occur close to lead smelters. Lead is used in many countries for different industrial purposes such as an additive to gasoline, paints pigments, solder, leaded glaze, and battery manufacturing. The resulting pathway of exposure will be through air, food, water, dust, soil, and housewares. Arsenic occurs naturally in a variety of sulfide ores and is used for different industrial productions such as insecticides and herbicides, and semiconductors. It is produced as a by-product in the smelting of copper and lead ores. It may be released into the environment by natural weathering processes or by anthropogenic activities. Exposure pathways include air, water, soil and food (for organic arsenic). Inorganic mercury is naturally emitted in the environment, but is also produced by anthropogenic activities such as mining, smelters, combustion of fossil fuel and refining gold. It is also used in the electrical industry. The major pathway of exposure is through mercury vapor inhalation. Organic mercury is used in different pesticides and its main pathway of exposure is through consumption of fish and other seafood given the high concentration of monomethyl mercury in the marine food chain.

Different biomarkers can be used to assess exposure to heavy metals. For lead, venous and capillary blood, umbilical cord blood, plasma, urine, teeth, bone and hair have been used. The most commonly used is blood lead and it reflects primarily recent exposure. Plasma lead represents the diffusible fraction, which has the greatest toxicology significance but is difficult to measure accurately. Teeth and bone lead reflect cumulative exposure but cannot routinely be used for surveillance. Arsenic has been measured in blood, urine, hair and nails. The biomarkers most commonly used to monitor arsenic exposure are arsenic and its metabolites (monomethylarsonic acid (MMA) and demethylarsonic acid (DMA) in urine that reflects the intake of inorganic arsenic. Mercury has been measured in blood, plasma, urine and hair. The mercury concentration in plasma and urine mainly reflects the exposure to inorganic mercury, while the concentration in erythrocytes mirrors the exposure of organic mercury. When exposure in constant over time mercury in blood and urine may reflect recent as well as longer-term exposure. In general, caution must be taken in the analysis of hair because of potential for contamination.

Although questionnaires may provide some information on important sources of exposure, environmental samples are necessary to confirm the sources. Prior knowledge of local sources and pathway of exposure will help focus the environmental sampling. For lead, environmental sampling could include; water, dust, soil, paint, air, ceramic, food, and specific local sources of exposure such as kohl, and herbal medicine. For arsenic, environmental sampling could include; water, soil, dust, air, and food. And sampling for mercury; air, water and food (especially fish). However, the analysis of environmental samples requires a high-qualified laboratory and standards are often difficult to obtain. Recently, the development of new technologies has allowed on-site blood lead and environmental lead analysis. Utilization required well-trained technicians and careful standardization; however, the pieces of equipment permit a rapid evaluation of lead contamination.

Several strategies can be used to select the population to be studied depending on the objectives, the timeline and the resources. If the objective is to evaluate the magnitude and sources of the problem, the following sampling methods can be used:

If the objective is to monitor changes in exposure or health data overtime, it is necessary to collect comparable data at different points in time. For example, in the United States, monitoring of the impact of lead control programs has been done using population-based surveys. In Mexico, monitoring has been conducted using umbilical cord lead levels collected in random samples of maternity clinics.

Finally, if the objective is to investigate a cluster of cases of severe intoxication, a case-control study design can be used to determine major sources of exposure.

For all sampling designs, information gathering should include the use of a questionnaire to collect general information on the participants as well as information on local potential sources of exposure and the collection of biological samples. Interpretation of data should consider the type of sampling used and the non-response rate.