See also lead. Its similarly in all of us.

1,000,000 ppm

Tree fiddy

Preferably none, but I guess the max would be when it began affecting brain function.

All of it, if it helps with neuroplasticity

0

Plastic is very durable, will it make my brain last longer?
Also plasticity of the brain is a crucial function, if it benefits the plasticity that would be great too.

/s

“Will allow” is absolutely irrelevant, if it’s in the environment there is no way to control it individually.