The International Air Transport Association says fewer people are sending goods by air.
It said total volumes are down 2 percent compared with the end of 2014.
The association blamed lower growth in Asia, and a re-balancing of the Chinese economy toward domestic consumption.
It said most of the fall happened earlier this year and the drop stabilised in August.
Air freight is sometimes seen as an economic indicator since it carries high-end items, which people can choose not to buy, while necessities produced in bulk are sent by ship.