Final answer:
The World Health Organization (WHO) defines marine pollutants as substances that enter the marine environment and cause harm to living organisms. Naturally-occurring elements like uranium or radon are not considered pollutants by the WHO.
Step-by-step explanation:
According to the World Health Organization (WHO), marine pollutants are substances that enter the marine environment and cause harm to living organisms. They can be of both natural and human origin. However, hazardous naturally-occurring elements like uranium (U) or radon (Rn) are not considered pollutants by the WHO. While these elements may have negative effects on human health, they are not considered pollutants because they are naturally present in some geological formations and are not introduced into the environment through human activities.