Question

when scientists think a chemical might be risky, what rule says we need to check if its safe before we use it more?
eutrophic principle
precautionary principle

Explanation:

The Precautionary Principle states that when an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause - effect relationships are not fully established scientifically. It is used to determine if a potentially risky chemical should be checked for safety before more use. The Eutrophic Principle is related to the over - enrichment of water bodies with nutrients and not relevant here.

Answer:

Precautionary Principle