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Dry ice, the unsung hero of the COVID-19 vaccine rollout, explained

Tom Avril, The Philadelphia Inquirer on

Published in Health & Fitness

First, carbon dioxide gas is captured as a byproduct from various industrial processes such as making ethanol. It is then compressed into a liquid inside a metal tank. When a special kind of release valve is opened, the sudden pressure drop allows the liquid to escape rapidly as a gas. The temperature plummets accordingly, so much so that the gas immediately freezes into white flakes: dry ice.

The same principle is used in air conditioning and refrigeration, as coolant fluid is compressed then allowed to expand into gaseous form, over and over in a cycle. But other substances are used for that purpose, not carbon dioxide. There is no "ice" phase, just liquid and gas. As chemistry students learn, every substance behaves differently in response to various combinations of temperature and pressure.

Members of the compressed-gas trade group produce 35,000 tons of carbon dioxide every day in the U.S. and Canada, of which 16% is made into dry ice, Gottwald said. To accommodate the added demand for shipping the vaccines, companies have boosted dry-ice production by an estimated 5% — which works out to less than 1% of the overall carbon dioxide production total. No shortages have been reported.

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Final fun fact. This has nothing to do with its usefulness in shipping vaccines, but carbon dioxide is denser than air. To demonstrate, Levis placed chunks of dry ice in a plastic bin and allowed it to start sublimating into an invisible layer of gas.

He then used a toy gun to shoot soap bubbles across the top. The bubbles were filled with air, and ordinarily would have sunk to the floor due to the weight of the thin film of soap. But in the chemist's demo, the air-filled bubbles remained aloft, floating atop the layer of denser carbon dioxide beneath.

 

"It's just like a helium balloon," he said.

Carbon dioxide also is a greenhouse gas, meaning it plays a role in climate change, but that's a subject for another day. And dry ice doesn't really contribute to the problem, since it is made from CO2 that is being produced anyway.

Back to the topic of the moment: keeping vaccines cold. A few other substances could get the job done, such as liquid nitrogen. But that would be overkill, as its temperature is negative 320 degrees.

You also could cool regular ice down far enough to keep the vaccines cold. But that would be less efficient than simply using dry ice itself, said Levis, who is senior associate dean of the Temple's College of Science and Technology. Besides, when regular ice melts, the result (surprise!) is water. The vaccine labels would come off.

So as you navigate the cumbersome process to sign up for vaccination, it makes sense to chill in at least one respect. Dry ice is on the case.

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