Oh, it's perfectly safe. You can tell by the smell... a healthy sourdough culture smells healthy, a rotten culture smells rotten. The yeast/bacteria system is extremely tough and resistant to other microbes and molds.

Whenever you feed your sourdough starter, or add locally bought flour, or anything along those lines you are introducing locally available microbes.

A sourdough starter from SF won't be a SF sourdough for long just because of the microbes in the air/flour as you continue to feed it.

It becomes it's own living and breathing organism that you have to take care of or it dies.

I did it for years and my family's still here :-)