The purity of gas is an indication of the amount of other gases it contains. A high purity refers to a low amount of other gases. Gases of higher purity are considered to be of better quality and are usually more expensive.
The purity of gas can be expressed as a percentage value or as a decimal fraction. The decimal fraction is an abbreviation of the percentage value, where the first digit represents the number of nines in the percentage value and the last digit represents the last digit of the percentage value. For example, a purity of 99.97% can be abbreviated as purity 3.7 and a purity of 99.99990% is the same as purity 6.0.