Another very good reason for connecting negative cable / clamp last either when connecting jumper leads or fitting a new battery is that if you connect negative first then if you touch any part of bodywork or engine with a tool while connecting the positive you have a short circuit and a big bang, same if you drop positive jumper lead onto body or engine if the negative already connected, big bang. So always connect positive first and the chances of getting a short are very low. If I was jump starting a car I would always connect straight to battery posts.
With modern maintenance free sealed valve regulated batteries used on cars and motorbikes they will only gas when being aggressively charged, and then a minute amount of gas compared to older types on non-sealed ( vented ) batteries.
Here is an extract from an article .
Practically every UPS (uninterruptible power supply) I know of has one [or more] SLA inside, so it's generally safe for indoor use. Here's a snippet from an APC white paper on the issue:
Valve regulated lead acid (VRLA) batteries [...] do not require special battery rooms and are suitable for use in an office environment. Air changes designed for human occupancy normally exceed the requirements for VRLA [...] ventilation. Vented (flooded) batteries, which release hydrogen gas continuously, require a dedicated battery room with ventilation separate from the rest of the building.
And bit later in the paper the difference in gas output is quantified as 60 times less for VRLA:
VRLA batteries are considered to be “sealed” because they normally do not allow for the addition or loss of liquid. A vented battery can give off sixty times more gas than a VRLA battery in normal use.
And the reason for this is that in a "sealed"/VRLA battery:
hydrogen recombines under pressure with oxygen into water inside the battery. Gas can only escape when internal pressure exceeds the rating of the pressure valve.
