Is there a big risk of fire when a person uses metallic tools inside a fume?

Using tools inside a fume hood that may have a flammable gas mixture should be prohibited. A properly operating hood of the right capacity should keep the mixture of hydrogen in air inside the hood below the Lower Flammability Limit (LFL) of hydrogen further reducing any risk. 

If the use of tools is necessary, the source of hydrogen should be isolated before the work begins even if the concentration of hydrogen is expected to be below the LFL. It is best practice to leak test equipment before introducing hydrogen to minimize the probability of leaks. If spark resistant tools are suitable for a specific task for working with hydrogen systems, use of such tools will lower the probability of producing a spark.