While responding to a L BLEED DUCT EICAS warning a CRJ-900 flight crew inadvertently deployed the passenger O2 masks when they actuated the EMER DEPRESS switch prior to descending through 15;000 as directed by the checklist.

Narrative

Climbing through approximately FL180 we got a L BLEED DUCT Warning. This plane had the same warning on its inbound flight. I was pilot flying; so at this time the radios and airplane were given to me so the Captain could run the checklist. I could hear him running it while changing to a few different Center frequencies and flying the airplane. The Captain said we needed to descend to 10;000 FT now. I relayed our needs to Center and down we came. The checklist started with 10;000 FT max altitude and he said 'we are on our way' As we were descending (16;500-17;000 FT) he read the EMER DEPRESS portion of the checklist; had me confirm he was on the correct switch; which I did; and he pressed it. I did not think of the 15;000 FT limitation for the pressing the emergency depress switch. We descended to 10;000 FT and returned safely to the airport. I could hear from the conversation between the Flight Attendant and the Captain that the passenger oxygen masks had deployed. We both took the checklist's 10;000 FT item to mean max altitude for sustained flight and forgot about the 15;000 FT limitation. Our interpretation of the 10;000 FT max altitude item of the checklist was that it was for sustained flight and not a point in the checklist that needed to be accomplished before we continued said checklist. Also; neither of us remembered the 15;000 FT limitation for the EMER DEPRESS switch. Perhaps clarifying this checklist with a 'yes or no' option would help.