MOST of them are sealed and there is no real danger.
If it is a wet cell battery, it is not sealed. Even if it filler caps or not, it has a vent to release gas, and still produces gas.
The only lead acid batteries that do not actively vent into the atmosphere are AGM style, and gell cell batteries. These can still gas, but only if overcharged.
If you've got a wet cell battery, you can charge it inside your house without issue if you do a few things to insure safety.
· Ventilate the room. You'll smell the sulfur with the hydrogen released if there is too much gas in the room. It's not life threatening until you can't tolerate breathing it anymore as it displaces oxygen.
· Put a drip pan of some kind under the battery, should it be too full it will over spill when charging as the acid expands when hot.
· Buy a battery hydrometer, it's the only way to tell of the cells of the battery are charged and it keeps you from over charging it.
· Only charge it when you're home, and awake.
If the battery is physically warm when your done charging it, it's starting to over charge. >120F is bad. The battery always releases gas, the act of charging and discharging causes it, and when the battery sits idle, it's discharging. When you over charge is when you get a lot of gas from the battery boiling.
It requires a lot of gas before it becomes a real health hazard, you can smell it when it does, worse of all, you can taste it when it's thick enough to cause you immediate issues. I've also blown up pleanty of batteries, its not really a fire issue you need to worry about but more that the acid gets blown everywhere. As long as you keep an eye on the battery when charging it's perfectly safe.
Also in an ideal situation you want to charge the battery without the caps on it, to avoid trapping the gas, but in your situation leave them on to avoid acid splatter.