Phosgene
Phosgene is the chemical compound with the formula COCl2.  This gas has no color. The gas was used as a weapon during World War I. It is responsible for most of the deaths related to poison gas during the war. Today, phosgene is used as a tool. It is used for organic synthesis.  When there is a small amount in the air, the smell is similar to that of freshly cut hay or grass.  Some soldiers during the First World War have stated that it smelled a little like May Blossom. When some compounds of chlorine and carbon burn or break down, small amounts of phosgene are given off.