Both seals and heavy human smokers have high levels of carbon monoxide in their blood streams. While humans acquire it from burning tobacco, researchers think seals' blood carbon monoxide levels are connected to their deep dives. One study found that elephant seals' blood is around 10% carbon monoxide, which researchers attribute to the animals holding their breaths for about 75% of their lives. Exhaling is the only way for an animal to clear carbon monoxide from its body