Currently you only get damage from fire, but since the fire in the smelter is basically out of reach, this isn't implemented for furnaces yet... but it's true that the exhaust gases would be extremely hot in reality, so it makes sense that the player gets damage if he stands right on top of a furnace. We will change that with the next update
I can already see the complaints of people getting burned while refilling the coal into the smelter ![]()