It can be, but the smell drives people away quickly at concentrations well below danger, doc.................
http://www.goodway.com/hvac-blog/2009/08/ammonia-as-a-refrigerant-pros-and-cons/
Sorry, but there are countless documented cases of death caused by ammonia leaks back when it was used as a refrigerant... Go back to ammonia you will have people dying once again when systems leak.