Within the last week there was reason for me to do a bit of web searching, checking a couple encylcopedia and dictionaries, just to be sure of the difference between baking soda and baking powder. From what I' gathered from multiple sources, baking powder is generally made of baking soda with something else added. Often times, the somthing else is aluminum, and this apparently helps to provide baked goods with a second "rise" effect. Of course there are alternatives that are more natural ways to get this same rise effect without using aluminum. However, these require a little more time and effort...perhaps requiring 5 minutes of extra effort to pour & mix in some yogurt or other ferment, instead of a quick shake of pre-mixed baking powder..... in other words, the aluminum in baking powder is another one of those "a convenience to save you time to do all those other things to do in your busy day" things.
Still, I do recall the warning from Hulda that common brands of Baking Soda have been tainted over the years, not with aluminum but with some other additive/preservative.