Hmm, whatever chemical solder flux is made out of. Then there's some fun chemical solvents like PVC primer. None of these are dangerous but all of them fuck with your respiratory system, the primer with your brain too. Though I guess the mercury in an old thermometer is more dangerous than all of these.
For honest danger, HCL drain acid which won't eat through metal but has no problems with skin or meat. Oh, and thermite. Amazing that it's both legal and easy to make, equal volumes aluminum powder and iron oxide powder(rust) mix well, light with magnesium striker, run away and enjoy the fireworks.