Scientific Principles and Their Application
Exposure, dose, and risk
Scientific principles questions are applied, not trivia. Start with the four-step risk assessment: hazard identification, dose–response assessment, exposure assessment, and risk characterization. LD50 and LC50 are median lethal dose and concentration - lower means more toxic - and NOAEL and LOAEL feed reference doses (RfD = NOAEL divided by uncertainty factors). Non-cancer risk is a hazard quotient (intake ÷ RfD); cancer risk is intake times a slope factor, with acceptable risk usually framed as 10⁻⁶ to 10⁻⁴ over a 70-year lifetime.
Fate and transport
Chemical properties drive behavior: vapor pressure and Henry's law constant for volatilization; Koc and Kow for sorption and bioaccumulation; water solubility for leaching; half-life for persistence. Dense non-aqueous phase liquids sink; LNAPLs float. A vapor density greater than air (28.97) means pooling in sumps and basements - a classic ignition scenario. In groundwater, Darcy's law (Q = K·i·A) gives flow, and the retardation factor shows contaminants moving slower than water.
Traps
Mixing units is the most common error. ppm is volume/volume and mg/m³ is mass/volume; conversion requires molecular weight. Candidates confuse flash point with autoignition temperature, or treat hydraulic conductivity as permeability. Remember that a substance can be harmful without being flammable or corrosive.
Worked example
Convert 25 ppm benzene (MW 78) to mg/m³ at 25 °C: 25 × 78 ÷ 24.45 ≈ 80 mg/m³. Use 22.4 as the molar volume at 0 °C.