The signs of Oxygen deficiency begin to occur at 14% to 16% Oxygen in the air with an increased rate and depth of breathing. If the Oxygen content in the air falls to 4% or less, unconsciousness can occur after only one or two breaths. The Oxygen content in the air must be between 19.5% and 22.0% for personnel to enter a confined space.

Liquid Nitrogen is colorless and odorless, is lighter than water (Density 808.29 Kg /m3), and has a boiling point of -320oF (-196oC) at atmospheric pressure. It is transported and stored as a liquid for economy and convenience. One cubic foot of liquid Nitrogen will expand to 696 cubic feet of gas at the same pressure. Rapid release of nitrogen gas into an enclosed space can displace oxygen, and therefore presents an asphyxiation hazard. This may happen with few warning symptoms, since the human carotid body is a relatively slow and a poor low-oxygen (hypoxia) sensing system. Nitrogen also dissolves in the bloodstream and body fats.

As with all cryogenic liquids, exposure to the liquid or piping can cause frostbite and cold burn. This may happen almost instantly on contact, or after a second or more, depending on the form of liquid nitrogen. Bulk liquid nitrogen causes less rapid freezing than a spray of nitrogen mist (such as is used to freeze certain skin growths in the practice of dermatology).

Oxygen sensors are sometimes used as a safety precaution when working with liquid nitrogen to alert workers of gas spills into a confined space.