You can breathe in a harmful substance, swallow it, or get it on your skin or in your eyes. Sometimes, there's no apparent effect and you may continue working in an unsafe manner thinking nothing's wrong.
If you're fueling a generator during a shutdown and gasoline splashes into your eyes, you experience what's called an "acute" effect. It's immediate and it hurts. But what if you get gasoline on your skin? It's a powerful hand cleaner, and you don't feel any effects; so what's the harm? Gasoline contains several very potent carcinogens and other toxins, and when absorbed through the skin or breathed in they do their damage over time. This is "chronic" poisoning.
Don't rely on pain or other immediate feedback to tell you if something is safe. Read the MSDS and container label before using it. Make sure you understand the necessary precautions. Know the emergency treatment and ensure you have the required emergency supplies on hand — before, not after, something goes wrong.
