Air Quality and Environmental Health Risk
Air Quality and Environmental Health Risk
This domain tests how air pollutants form, are measured, are regulated and are translated into health risk. Expect criteria pollutant standards, dispersion basics, indoor air and inhalation risk calculations.
Pollutants and chemistry
Criteria pollutants: PM2.5, PM10, ozone, NO2, SO2, CO and lead. Ozone is not emitted directly; it forms when volatile organic compounds and nitrogen oxides react in sunlight, so it peaks downwind in the afternoon. PM2.5 includes primary particles plus secondary sulphate, nitrate and organic aerosol. Health effects: PM2.5 - cardiovascular and respiratory mortality; ozone - airway irritation, reduced lung function, asthma exacerbation; CO - carboxyhaemoglobin and ischaemic heart disease; lead - neurodevelopmental effects in children; radon - lung cancer.
Standards and management
NAAQS are concentration limits: primary protect public health with an adequate margin of safety; secondary protect public welfare. Nonattainment triggers state plans and offset requirements for new sources. Hazardous air pollutants are controlled with technology-based MACT standards. Permitting runs from minor source review to Title V operating permits. Dispersion depends on stack height, exit velocity, stability class and mixing height; a low mixing height under an inversion concentrates ground-level concentrations.
Indoor air is often the dominant exposure: radon (action level 4 pCi/L), environmental tobacco smoke, combustion CO and NO2, formaldehyde and other VOCs, dampness and mould. Ventilation and source control are the primary interventions.
Risk calculations
Inhalation cancer risk = concentration (µg/m³) × inhalation unit risk (per µg/m³). Non-cancer hazard quotient = exposure concentration / reference concentration; a quotient above 1 signals the need for review, not proven harm.
Worked example
Ambient benzene is 1.5 µg/m³; the inhalation unit risk is 7.8 × 10⁻⁶ per µg/m³. Lifetime excess cancer risk = 1.5 × 7.8 × 10⁻⁶ = 1.2 × 10⁻⁵, about 12 per million. To reach a 1 × 10⁻⁶ target the concentration must fall to 1 × 10⁻⁶ / 7.8 × 10⁻⁶ ≈ 0.13 µg/m³. Reasoning: unit risk is already a lifetime, per-unit-concentration value, so no averaging time or body weight is applied - the common error is plugging it into the oral dose equation.
Traps that cost marks
- Describing ozone as a directly emitted pollutant.
- Mixing primary and secondary standards.
- Confusing the reference concentration (non-cancer) with unit risk (cancer).
- Unit conversion: mg/m³ = ppm × molecular weight / 24.45 at 25 °C.
- Assuming a tall stack always helps; downwash and inversions can reverse it.