Don't know what level is harmful to humans but some do develop an insensitivity to the odor. Likely not a concern with small amounts from a reef but nonetheless it's harmful and should be avoided.
It develops as a by product of sulfate reducing bacterial activity. This occurs when organics are present and the oxygen including the oxygen in nitrate are exhausted in a particular location. Then bacteria seek oxygen for energy from SO4, sulfate which is present at 2700ppm in salt water. As they reduce it H2S ,hydrogen sufide gas, is a by product. This article by Randy Farely has more detail:
http://reefkeeping.com/issues/2005-12/rhf/index.php
It develops as a by product of sulfate reducing bacterial activity. This occurs when organics are present and the oxygen including the oxygen in nitrate are exhausted in a particular location. Then bacteria seek oxygen for energy from SO4, sulfate which is present at 2700ppm in salt water. As they reduce it H2S ,hydrogen sufide gas, is a by product. This article by Randy Farely has more detail:
http://reefkeeping.com/issues/2005-12/rhf/index.php