Is it ok to charge battery inside car or home with no one in it?

I read that when charging a battery(charger to battery) or battery to battery(I imagine) a toxic chemical/gas is released from the battery. As such you should ventilate and not be in the home whilst battery charges.

I’m going to try the battery to battery method and want to leave it in the car. My concern is the chemicals that come out might get absorbed into the seats or react with something in the car and I will inhale it later etc or it might react with a cleaning chemical etc. Not sure what chemicals come from the battery but is it a concern or after charging for a while with windows closed(as I cant be around for too long)will the chemicals go quickly once u open the windows?