*The conceptual chain distinguishes a substance's potential harm, actual contact, and the likelihood of harm occurring.*
Hazard refers to the ability of a substance or process to cause harm. Exposure describes how much, for how long, and through which route a person comes into contact with it. Risk concerns the likelihood and severity of harm occurring under specific conditions. The three concepts are related but not interchangeable. Finding a hazardous substance is a starting point for assessment, not a complete risk conclusion.
For example, the concentration of the same substance in the original liquid, in the aerosol, and inside the human body are not the same thing. The amount released by a device must go through inhalation, deposition, and metabolism before it forms exposure within the body. Using only the name of an ingredient to infer that a person will inevitably develop a certain disease skips necessary steps; the absence of immediate discomfort also cannot prove that long-term repeated exposure is safe.
Suppose an experiment detects a substance with hazardous properties. The next step is to determine how much of it is released each time and whether the test conditions resemble real-world conditions. If one extracts only the substance's name, one cannot judge the magnitude of the risk. Conversely, a single very low measurement does not mean that other substances and other conditions are the same. Risk assessment is a chain of gradually accumulating evidence; it should neither be skipped nor completed on the strength of a single detection term.
When reading research, one can organize information by "substance—route—dose—time—outcome." If any link is missing, the limits of the conclusion should be stated. E-cigarettes involve many ingredients and use behaviors and cannot have all their effects summarized by a single indicator. [CDC Health Effects information](https://www.cdc.gov/tobacco/e-cigarettes/health-effects.html)
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