Delta, United Airlines, and Singapore Airlines all use special soothing scents such as fir trees, lavender, chamomile, and citrus in their towels, soaps, lounges, and jet bridges to keep customers happy and calm. It also works as a method of branding, since each airline has their own special scent. The goal is to have people think of their airline when they smell, say oranges, for example. This subtle touch is hoped to boost loyalty towards their airlines in addition to keeping people from becoming restless during flight delays. More details at the link.