Hydrogen cylinders contain pure hydrogen unless they are specifically manufactured for and marked as a mixture. The purity grade is usually between 99.5% and 99.9999%. The balance is typically inert gases (such as nitrogen) with just ppm levels of other contaminants, but this can vary depending upon the production source. When emptied, the residual is still the same purity of hydrogen, just at lower pressure. If emptied to atmospheric pressure, there would still be one atmosphere of hydrogen within the container unless evacuated or purged to another gas. (See Properties section for related FAQs). Note that when a cylinder is reduced close to or at atmospheric pressure, it is very susceptible to air migration into the cylinder, especially if ambient temperature changes can create a slight negative pressure in the cylinder. Air should never be allowed into a hydrogen cylinder until it’s been properly purged with an inert gas such as nitrogen, helium or CO2.