Helium is one of the lightest and smallest gas molecules of the noble gases. Only hydrogen has a smaller or lighter molecule. Hydrogen is though both reactive and flammable at a high concentration. Helium's low molecular weight allows it to flow through the tiniest of leak paths. Helium is a inert gas, which means that it will not react with other substances. These properties of Helium make it the ideal tracer gas and are widely used in leak detection in various industries.
Other alternative tracer gases which we use depending on the application are Hydrogen, SF6, CO2.