The primary health dangers of carbon dioxide

Asphyxiation. Releasing any gas in a confined or unventilated area can lower the concentration of oxygen to a level that is immediately dangerous to life or health (see the first link under Further Info below). Be very careful entering a truck or room where solid or gaseous CO2 is stored. Likewise, some businesses and laboratories store Dry Ice in large top-loading ice chests; be very careful sticking your head down to retrieve a block from the bottom.

Concentrations greater than 10% in air. Remember those chemical reactions we saw above? These are chemical equilibria, which means that the relative amounts of products and reactants depend on their concentrations. Large changes in carbon dioxide or bicarbonate concentration can lead to kidney damage, coma, or even death! For a great discussion of this in detail see this article in the Merck Manual.

Frostbite. Solid carbon dioxide (Dry Ice) is always -78 oC (-109 0F) at regular atmospheric pressure regardless of the air temperature. Handling this material for more than a second or two without proper gloves can cause serious blisters or worse. Carbon dioxide gas released from a compressed gas cylinder (such as a fire extinguisher) poses a similar danger. Avoid putting any bodily part directly in the path of discharge and be careful when touching any metal parts that the gas flows through.

Pressure Explosion. Carbon dioxide has a vapor pressure of 830 psi at 20 °C. In other words, if one places solid carbon dioxide in a closed container at room temperature, the carbon dioxide will eventually convert to liquid and the pressure above that liquid will be 830 psi (approximately 56 times normal atmospheric pressure). The pressure will always be 830 psi as long as some liquid carbon dioxide is present in the closed container, and at higher temperatures the pressure will be even greater. While such pressures are no problem for pressure-tested gas cylinders or fire extinguishers, ordinary containers (soda bottles, paint thinner cans, Thermos containers etc.) cannot handle such pressures and will explode and create shrapnel if solid carbon dioxide is sealed inside them.

Putting solid carbon dioxide into any sort of closed container (glass, plastic, metal etc.) is exceedingly dangerous and is likely to result in severe personal injury or death.