Pure helium is used in the blending of mixed breathing gases that contain an appropriate amount of oxygen suitable for diving applications. Helium is obtained from the ground during the production of natural gas. Some natural gas fields are rich in helium from millions of years of natural radioactive alpha decay of uranium and thorium in the surrounding granite rock. Companies that extract natural gas in those areas remove the helium as a byproduct. Helium refined from natural gas using modern methods is normally transported cryogenically as a liquid, and used to produce what is generically referred to as five nines, N5.0 or Grade-5 helium gas with 99.999% purity. There are variety of grades of compressed helium gas widely sold in returnable supply cylinders with the CGA-580 connector inert gas valve.
Grades of helium differ in the nature of their impurities, which in turn affects their suitability for specific applications. Different gas suppliers describe highly refined helium using different product names, but some common examples are Ultra High Purity (UHP), Semiconductor, Research and Laser grade. A few gas suppliers offer grades of helium that are specifically described as suitable for breathing gas mixture blending applications using names such as Grade-A or Diving Grade. Because there are no standards for product grades, you should examine the supplier specifications for the maximum contaminant levels among their different helium products. Based on the IANTD specification for oxygen compatible air, the helium (He) product must contain no more than 2 parts-per-million (ppm) for carbon monoxide (CO) and no more than 25 ppm total hydrocarbons (methane CH4 and other alkanes.) Trace contamination with other inert gases, oxygen, carbon dioxide, or water vapor will be of no concern; but there should be zero hydrogen sulfide (H2S) gas.
For diving gas blending applications we recommend avoiding helium grades where the contaminants are not specified; sometimes described as Balloon, Industrial, Medical, USP, or Zero (99.0%) grade. Diving physics tells us that any contaminants in mixed gases will have their partial pressures amplified by the absolute pressure when breathed at depth. This means as depth increases thus any unknown contaminants also become an increasing concern. From the early days of technical diving there are anecdotal stories of using contaminated helium to blend trimix that resulted in the onset of bizarre temporary symptoms at depth such as loss of visual acuity and intense skin sensations. The good news is because of modern production and transport logistics for liquid helium, the highly refined grades of helium gas are typically competitively priced relative to other grades.
See also How do the grades of compressed oxygen differ?
Is there a shortage of helium? Like natural gas itself, commercial helium gas is generally considered a non-renewable resource. although large untapped reserves of helium are believed to exist. Availability of refined helium is an indirect function of energy demand for natural gas plus demand from medical and semiconductor applications, so helium supply normally fluctuates with both demand for natural gas and demand for helium itself. There have also been recent supply chain disruptions due to geopolitical conflict. The supply and demand fluctuations occasionally cause transitory shortages. High asking prices are often blamed on the "helium shortage" but the price is usually very negotiable so it can be worthwhile to shop around between local compressed gas vendors. In our experience, technical divers can get all the helium needed when willing to pay the asking price.