This issue is actually not isolated to just barbecues and barbecue brushes although the fact that it gets into your food does make it worse.

A few weeks ago my friend invited me to their high class HOA pool that had a sand beach. Wading around in about a foot of water I felt something pierce my foot when I took a step. When I pulled my foot out of the water to see what it was I could see it was a thin piece of metal and it had gone in all the way to my bone.

I concluded it probably came off a metal brush they use around the pool. Maybe something like this: https://www.amazon.com/Performance-Tool-W1148-Stainless-3-Pi...

That's when I realized that these types of metal brushes are a huge environmental hazard in general. They should probably be illegal in certain settings -- sand beaches being one of them.