Every Breath, Every Bite
Chemical exposure is rarely limited to a single route. We breathe contaminated air, consume residues in food and water, and absorb substances through skin contact. Exposure assessment integrates all these pathways to estimate the total chemical burden on the body — the average daily dose (ADD) — which forms the numerator of the risk equation. Understanding which route dominates guides the most cost-effective risk reduction strategy.
Inhalation Exposure
The lungs present an enormous surface area (~70 m²) directly in contact with inspired air. Inhalation exposure depends on air concentration, breathing rate (which varies with activity level), exposure duration, and pulmonary absorption efficiency. Volatile organic compounds, particulate matter, and gases can enter the bloodstream within seconds of inhalation, making this the fastest exposure route for many occupational hazards.
Oral and Dermal Routes
Ingestion exposure includes contaminated food, water, and incidental soil/dust intake (important for children). Oral bioavailability depends on the chemical's solubility, gut metabolism, and first-pass hepatic clearance. Dermal exposure depends on skin contact area, chemical permeability (Kp), and exposure duration. Lipophilic chemicals penetrate skin more readily, while the stratum corneum limits absorption of hydrophilic substances.
From Dose to Decision
The hazard quotient (HQ = ADD/RfD) translates exposure into regulatory action. An HQ above 1 signals that exposure exceeds the reference dose — the level considered safe with built-in safety factors. For sites with multiple contaminants, the Hazard Index sums individual HQs. Risk managers use these metrics to prioritize cleanup, set emission standards, and design protective measures that reduce exposure below safe thresholds.