Strontium, Cadmium, Stibine, Arsine.
None of them are good for you, but all them (and more) can be released by smelting lead-acid batteries.
Strontium and Cadmium are like lead, and fool your body into using the molecules in place of calcium in your body. ...Not good.
Stibine and Arsine can be fatal with a single exposure.
Stibine/Arsine toxicity excerpt from wikipedia:
Stibine binds to the haemoglobin of red blood cells, causing them to be destroyed by the body. Most cases of stibine poisoning have been accompanied by arsine poisoning, although animal studies indicate that their toxicities are equivalent. The first signs of exposure, which can take several hours to become apparent, are headaches, vertigo and nausea, followed by the symptoms of hemolytic anemia (high levels of unconjugated bilirubin), hemoglobinuria (high levels of hemoglobin in the urine) and nephropathy (kidney damage).
Note in this commercial operation, that all workers are wearing respirators, and access to automated equipment performing the initial smelt is tightly controlled (only the camera man):