Which of the following is a common precautionary statement regarding hazards to humans?

The statement "Harmful if swallowed" is a common precautionary warning that identifies a specific hazard to human health. It alerts individuals to the risk of ingestion, emphasizing the need for caution when handling substances that pose toxicity if ingested. This type of labeling is crucial for ensuring safety and informing users about potential dangers associated with a product, thereby allowing them to take appropriate precautions to avoid harm.

In contrast, options like "Non-toxic" may imply safety but do not specifically address hazards to humans. “Always safe for pets” is a misleading statement, as it does not pertain to human hazards and may falsely suggest that a product can be safely consumed or used around pets without risk. “Environmental friendly” focuses on ecological impact rather than direct human health risks. Thus, the chosen statement effectively communicates a particular danger, which is why it is the correct answer.