What is a toxin? It's any substance that can make you sick. You can breathe or ingest a toxin, or absorb it through your skin. The effects can range from mildly irritating to lethal, depending on:
Sometimes, the body won't react right away, and there's no sign of injury. Don't be lulled into a false sense of security just because the effects aren't immediate. Mercury, for example, accumulates in fat and keeps doing damage. In particular, it causes neurons to shrink (bringing on dementia).
How can you protect yourself? Learn about the toxins you might encounter, follow the recommended procedures, and wear the recommended PPE. If you suspect exposure, seek medical evaluation immediately.