What pressure and extreme cold actually do

A compressed-gas cylinder stores its contents at high pressure in a fixed volume. If the valve, cap, or fitting is damaged, or if the cylinder is dropped, dragged by its valve, or otherwise stressed, that stored pressure can turn the cylinder itself into a projectile or release its contents suddenly. This is a mechanical hazard first, independent of whatever gas is inside; a cylinder of an inert gas is still a pressure hazard.

Cryogens add a second mechanism: extreme cold combined with a large liquid-to-gas expansion. Liquid nitrogen boils at about -196 degrees C (-321 degrees F); dry ice (solid carbon dioxide, CO2) sublimes directly from solid to gas at about -78 degrees C (-109 degrees F), skipping the liquid phase entirely. Either substance boiling or subliming in a closed or poorly ventilated room adds a large volume of gas to the air and displaces the oxygen already there.

Neither LN2 nor dry ice is toxic in the way a chemical fume is; the danger is that they can silently make a room's air unbreathable by displacement, and CO2 from dry ice can independently reach dangerous concentrations in an enclosed space even before the oxygen fraction alone would trigger a low-oxygen alarm, because CO2 is colorless, odorless, and heavier than air and can pool in low, poorly ventilated spaces.

The same expansion that displaces room air can also rupture a sealed container. Liquid nitrogen trapped inside a sealed vial expands many times its liquid volume as it warms from liquid back to gas; if that expansion has nowhere to go, the container can fail explosively. Never store a cryogen, and never leave trapped cryogenic liquid, in a sealed airtight container above its boiling point.

OSHA defines an oxygen-deficient atmosphere as less than 19.5% oxygen by volume. That definition is not a facility alarm or pre-alarm setting. Monitor setpoints, alarm response, calibration status, and re-entry criteria come from the current approved site procedure. Before a cryogen transfer, confirm the required monitor is present, functioning, in date, and verified under that procedure; if it is missing, failed, overdue, unavailable, or unverified, do not proceed.

Treat both a pressurized cylinder and a boiling or subliming cryogen as sources of stored energy that fail without warning signs a person can feel, which is why physical securement and instrumentation, not judgment alone, are the controls that catch the failure.