You can pick up minute levels of arsenic from touching treated wood or from touching soil that is contaminated with arsenic. But it doesn't readily absorb through the skin. People also can inhale arsenic sawdust when working with the wood. Smoke from treated-wood fires is toxic, and the ash is hazardous. Some people have won legal settlements with the wood-treatment industry after being poisoned by the wood.
Arsenic poisoning can cause neurological problems, numbness and paralysis.
That's a tricky question, as any homeowner will soon find out if they call around for an answer. Sometimes, researchers are being paid by the very industry that produces a toxin. Sometimes, government regulators don't have the resources to do their own risk analysis. And, almost always, everyone involved argues over how, exactly, the risk is calculated. Many times, authorities contradict one another, with one state going with one number, another state going with a different number, and the federal government going with another.