Environmental Science, Chemistry, and Toxicology
Fate and transport
Physical-chemical properties decide where a contaminant goes. Vapor pressure drives volatilization; water solubility drives dissolution; the organic-carbon partition coefficient (Koc) predicts sorption to soil; the Henry's law constant (approximately vapor pressure divided by solubility) predicts stripping and vapor intrusion. Density splits non-aqueous phase liquids: LNAPLs such as gasoline (about 0.75 g/cm³) float on the water table, while DNAPLs such as TCE (1.46) and PCE (1.62 g/cm³) sink and pool on low-permeability layers. Retardation in groundwater follows R = 1 + (ρb/θ)Kd, where Kd = Koc × foc.
Toxicology
Toxicology is dose-response, not exposure alone. LD50 and LC50 quantify acute lethality; NOAEL and LOAEL anchor chronic toxicity. The reference dose equals the NOAEL divided by uncertainty factors (typically 10× interspecies, 10× intraspecies, plus database and modifying factors) and applies to noncancer effects; the cancer slope factor converts dose to excess lifetime risk. Latency, target-organ specificity, metabolic bioactivation, and interactions (synergistic, antagonistic, potentiative) modify the response.
Units and traps
Convert ppm to mg/m³ as mg/m³ = ppm × molecular weight ÷ 24.45 at 25 °C and 1 atm. Common traps: confusing Henry's law constant with vapor pressure; applying the slope factor to noncancer endpoints; treating a literature Kd as site-independent when it scales with soil organic carbon; and forgetting that an IDLH is an acute escape criterion, not a chronic limit.
Worked example. Toluene (MW 92) at 50 ppm = 50 × 92 ÷ 24.45 ≈ 188 mg/m³. A lifetime dose of 0.002 mg/kg-day multiplied by a slope factor of 0.1 (mg/kg-day)⁻¹ gives an excess cancer risk of 2 × 10⁻⁴.