Complex sentence: Cobalt naturally occurs in animals in minute amounts and forms part of the structure of Vitamin B12 without which animals cannot live however when levels increase beyond a safe limit cobalt is highly toxic and can cause death.
Split into simpler sentences: Cobalt naturally occurs in animals in minute amounts and forms part of the structure of Vitamin B12 without which animals cannot live. However, when levels increase beyond a safe limit, cobalt is highly toxic and can cause death.