Agency for Toxic Substances and Disease Registry
Case Studies in Environmental Medicine (CSEM) 

Pediatric Environmental Health
The Exposure-Disease Model

To better conceptualize "exposure" and the steps necessary to effect disease, the exposure-disease model (Figure 1) is often used.

No matter how toxic, no chemical can harm a person (a child or an adult) unless exposure occurs. After a sufficient level of exposure (dose) to the chemical, with subsequent biologic uptake and target organ contact, biologic change can occur, which might lead to disease (Figure 1). This process is the same for everyone, although some toxicants might be more hazardous to a child than to an adult or vice-versa. Special consideration of a child's exposure and consumption patterns combined with critical periods of target organ development is necessary to assess a child's risk from a particular toxicant exposure. This is discussed further in the Age-Dependent Toxicokinetic Changes section.

Application of the preceding case study to the exposure-disease model follows:

• Environmental contamination (potential exposure): Elemental mercury, whether spilled or tracked on the carpet from contaminated work boots (or both), volatized at room temperature in the child's room or aerosolized by vacuuming the carpet. Vapors accumulate near the floor where children play and breathe.
• Biologic uptake (exposure): In this case, exposure occurs primarily though the respiratory system via inhalation. The respiratory rate is considerably higher in a child than in an adult. In the case study, the 2½-year-old child's respiratory rate is 30 breaths/minute. In adults, it is about 16 breaths/minute. Indicators of exposure in this case include increased urinary mercury.
• Target organ contact: Target organs might include the skin, central nervous system (CNS), peripheral nervous system (PNS), renal system, and respiratory system.
• Biologic change: In this case study, biologic changes include CNS changes (e.g., irritability), dermal changes (e.g., erythema of palms, soles, and face; with the characteristic edema and desquamation of the skin of the hands and feet), ocular changes (e.g., photophobia), and PNS changes (e.g., limb weakness and tongue tremor).
• Clinical disease: acrodynia.

Note: This condition is rare (see discussion under Diagnosis section). Not all children exposed to mercury vapors will have acrodynia.

The exposure-disease model (Figure 1) depicts the relationship between an environmental contaminant and an adverse health effect. The model predicts that the harm caused by a contaminant depends on its toxicity, route of exposure, and host factors. (For chemical properties, personal risk, biologic fate, and other information about mercury poisoning, see Case Studies in Environmental Medicine: Mercury Toxicity [ATSDR 1992].)

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