Environmental pathways, water and air
Environmental health follows a source through an exposure pathway to a person or population. A hazardous substance in soil does not by itself establish a harmful absorbed dose. Ask how it reaches air, water, food or skin, how often exposure occurs, who is susceptible and which measurement represents that pathway. Spatial clustering can generate a hypothesis, but a map alone cannot distinguish a source from a correlated social or infrastructure pattern.
Air pollution includes primary emissions and secondary pollutants formed through atmospheric reactions. Nitrogen oxides and volatile organic compounds can contribute to photochemical ozone under suitable sunlight and other conditions. A temperature inversion can suppress mixing and allow pollutants to accumulate. Particle size influences deposition, but all fine particles should not be described as passing directly into the bloodstream. Respiratory, inflammatory and cardiovascular pathways can contribute to harm without that simplistic assumption.
Indoor hazards can differ from outdoor averages. Radon progeny can deliver ionizing radiation to respiratory tissues; ventilation, building characteristics and local geology influence exposure. Combustion appliances can create carbon-monoxide hazards even without a recognizable odor. Formaldehyde and other emissions depend on materials, sources and conditions. A single compliant spot measurement does not establish that every exposure period or person is protected.
Water safety uses multiple barriers. Source protection, appropriate clarification, filtration, disinfection and distribution-system management address different hazards. A conventional surface-water treatment train is an example, not the universal design of every plant. Disinfection conditions must be validated for the actual water, organism and process. Reducing chemical byproducts must not casually sacrifice microbiological control; the complete risk-management plan matters.
Interpret an indicator organism, chemical measurement or sensor result with sampling and analytical limitations. A sample below a detection limit is not proof of absolute absence. Investigate changes, vulnerable points and operational records, then select source control or another appropriate barrier. This chapter teaches exposure reasoning and does not replace current Chinese water, indoor-air, emission or occupational numerical standards. Actual limits, averaging periods and statutory duties require the currently applicable text.