Hydrogen is a naturally occurring gas which is totally non-toxic and has no adverse effects on the environment. Hydrogen can be found naturally or produced with very simple methods. In fact we all have some hydrogen being produced in our stomachs!
The normal background level of Hydrogen in air is as low as 0.5 ppm.

Other tracer gases either comes from finite sources, are expensive to produce or are potentially toxic. Therefore Hydrogen is the most environmentally friendly choice to make when considering Leak Testing with a tracer gas. A fact important to remember if your company holds an ISO14001 certificate or similar.