Helium atoms are so tiny they can leak through solid glass and metal.

Helium is an inert noble gas with an atomic radius of only about 31 picometers. Because it exists as single atoms rather than molecules, it is much smaller than the diatomic nitrogen and oxygen that make up most of our air. This extreme smallness allows helium to pass through the atomic lattice of solids in a process called molecular diffusion.In laboratory settings, researchers often use borosilicate glass or quartz for experiments. However, studies have shown that helium can permeate these materials at measurable rates, especially at higher temperatures. This phenomenon was documented as early as the mid-20th century by researchers at institutions like the Massachusetts Institute of Technology (MIT) who were developing vacuum systems.The rate of leakage depends on the material's permeability constant. For example, standard latex is highly porous to helium, causing balloons to sink within a day. Mylar, a brand of biaxially-oriented polyethylene terephthalate (BoPET) developed by DuPont in the 1950s, was specifically engineered to have much lower gas permeability to keep balloons afloat longer.In high-tech industries like aerospace and nuclear physics, preventing helium leakage is a critical engineering challenge. Engineers use specialized metal alloys and hermetic seals to contain liquid helium, which cools superconducting magnets in MRI machines and the Large Hadron Collider. Even with these precautions, a small percentage of the gas is lost over time due to helium's relentless ability to migrate through solid barriers.