Definition of toxic:
Able to poison or harm an organism.
This is the common definition for toxic, other definitions can be discussed in the article
Like bioaccumulation and persistence, tocicity is an important criterion to determine the environmental impact of a substance. According to OSPAR criteria a substance should be considered toxic if
- exposure to marine concentrations below 1 mg/l of the substance can cause acute toxicity to marine organisms.
- long term exposure to marine concentrations below 0,1 mg/l can have an impact on marine organisms.
- or if the substance is carcinogenic or can cause endocrine disruptive effects to mammals. 
- ↑ OSPAR CONVENTION 2005, Cut-Off Values for the Selection Criteria of the OSPAR Dynamic Selection and Prioritisation Mechanism for Hazardous Substances