Final answer:
Lead poisoning can cause a blue line at the gingival margin, known as the Burton line, and it accumulates in tissues leading to various health issues.
Step-by-step explanation:
The agent that causes poisoning in humans and can result in a blue line at the gingival margin is lead. When toxic heavy metals like lead get into the body, they can accumulate in tissues and may lead to a variety of health issues, including the distinctive blue line on the gums known as the Burton line, which is a classic symptom of chronic lead poisoning (also known as plumbism). This condition can also cause damage to brain cells, affect muscular coordination, and even lead to developmental disorders in children such as attention deficit disorder.