On August 21,1986, a cloud of carbon dioxide gas was released from Lake Nyos [Cameroon, Africa]. Because carbon dioxide is more dense than air it hugged the ground and flowed down valleys. The cloud traveled as far as 15 miles (25 km) from the lake. It was moving fast enough to flatten vegetation, including a few trees. 1,700 deaths were caused by suffocation. 845 people were hospitalized.That's got to be a terrible way to die.
Carbon sequestration, unless out in the deep ocean, still carries with it the possibility of leakage and, therefore, of harming a great many people. It seems to me to have killed at least as many people as Three Mile Island + Chernobyl combined. I'm surprised environmentalists aren't more concerned about this type of environmental disaster.