Any environmental concerns about hydrogen venting from experiments?

The key concern with any hydrogen release is the risk of creating a flammable mixture. There should be no environmental issues if you properly vent hydrogen to a safe area where it is diluted in air below the flammability limit before contacting an ignition source. Very small quantities of hydrogen are frequently releasing into a fume hood. Releases have to be small enough so that the vent air is sufficient to dilute to below the lower flammability limit. The fume hood face velocity should be in excess of 100 ft/min (30 m/min). 

Larger quantities of hydrogen should be released through a properly designed and constructed hydrogen vent stack. Standards and codes such as CGA G 5.5 and NFPA 2 provide guidance for vent stack design and installation.