What are the factors that determine when substance is toxic and unfit for human consumption?

The toxicity of a substance and its unfitness for human consumption is determined by several factors, including its chemical composition, dosage, and route of exposure. The body's metabolic response to the substance, individual susceptibility (including age, health status, and genetic factors), and the presence of other substances (which can lead to synergistic effects) also play crucial roles. Additionally, the duration of exposure and the context in which the substance is consumed (like food preparation methods) can further influence its safety.