“Canned air,” often called a gas duster, is widely used for cleaning electronics, but the name is misleading because the can does not contain ordinary air. Instead, these containers hold a liquefied gas, typically a hydrofluorocarbon like difluoroethane or tetrafluoroethane, compressed to a liquid state. When the nozzle is pressed, this pressurized chemical releases a powerful stream of gas designed to blast away dust. Even when used correctly, the stream of gas feels noticeably cool, which hints at the much more extreme cold that can be generated.
Why Compressed Gas Cools Down
The initial cooling experienced when a can of duster is used upright stems from a fundamental physical process known as adiabatic expansion. Inside the can, the propellant exists primarily as a liquid with a layer of gas vapor above it. When the trigger is pulled, this high-pressure gas rushes out of the nozzle and rapidly expands into the lower pressure of the surrounding atmosphere.
This swift expansion forces the gas molecules to spread out and do “work” on their surroundings, which requires energy. Since this process happens too quickly for heat to be drawn from the environment, the necessary energy is taken from the gas’s own internal thermal energy, resulting in a temperature drop.
A secondary factor is the Joule-Thomson effect, which describes the temperature change of a non-ideal gas upon expansion at constant enthalpy. For the gases used in dusters, this effect further contributes to the drop in temperature as the gas leaves the high-pressure can.
Additionally, as gas is expelled, the internal pressure drops, causing the liquid propellant to boil and turn into replacement gas vapor. This phase change draws heat from the liquid and the can itself, making the entire container feel cold after continuous use. This baseline cooling is mild compared to the freezing temperatures achieved when the can is inverted.
The Dramatic Cooling Effect of Inverting the Can
Inverting the can shifts the cooling mechanism from gas expansion to a phase change involving liquid. When held upright, the valve releases the gas layer floating at the top. Turning the can upside down, however, submerges the valve into the pool of liquid propellant.
Spraying the inverted can forces the highly volatile liquid chemical to exit the nozzle directly, not just the gas vapor. Once released into the lower atmospheric pressure, the liquid propellant immediately and violently begins to evaporate, or boil. This instantaneous change of state from liquid to gas is an endothermic process, meaning it requires a significant amount of heat energy.
The liquid propellant draws this required energy from the immediate surroundings, including the nozzle, the air, and any surface it contacts. This rapid absorption of heat is known as evaporative cooling, the same principle that cools the human body via sweat. Because the propellants have a very low boiling point and high latent heat of vaporization, the cooling effect is dramatically intensified. This drops the temperature of the liquid stream and the target surface to extremely low levels, resulting in a freezing spray.
Measured Temperatures and Immediate Dangers
When the liquid propellant is sprayed from an inverted can, the temperatures reached are cryogenically cold. The stream of liquid and the surface it hits can drop rapidly to temperatures as low as -50°F to -60°F (-45°C to -51.5°C). This extreme cold is reached almost instantly due to the highly efficient evaporative cooling process.
These sub-zero temperatures pose immediate dangers to both people and equipment. Direct contact with the liquid stream can cause severe frostbite on exposed skin in a matter of seconds. Frostbite occurs when the extreme cold causes tissue to freeze, leading to cell damage and permanent injury to the skin, blood vessels, and nerves.
Exposing electronics or plastic components to such a sudden temperature drop can also cause material damage. The rapid thermal contraction may lead to the cracking of plastics, circuit boards, or solder joints. The propellant chemicals also present a hazard, carrying risks of toxicity or asphyxiation if inhaled improperly or in poorly ventilated areas.