Answer:
When you are in the laboratory and take a direct sniff the chemicals you are using, you run the risk of damaging your mucous membranes or your lungs. When its necessary to smell chemicals in the lab, the proper technique is to cup your hand above the container and waft the air towards your face.
Gas is a naturally odourless substance, but the completely harmless artificial smell is added to make it more detectable. The substance is called mercaptan and gives off a strong sulphur like smell.