To offset carbon dioxide, a Japanese airline is asking its passengers to go to the toilet before boarding.

(CNN) -- To offset carbon dioxide, a Japanese airline is asking its passengers to go to the toilet before boarding.

All Nippon Airways is asking passengers to use the restroom before flying to reduce weight.

The unusual request by All Nippon Airways (ANA) is part of its "e-Flight" promotional program to reduce the amount of carbon expelled on 38 domestic routes and its twice daily international flights to Singapore.

"Asking passengers to go to the toilet (before boarding) is just a small part of the program," said spokeswoman Megumi Tezuka, which includes using recycled paper cups and plastic bottles instead of glass.

"We are making these items lighter -- and making the passengers lighter, a little bit," Tezuka said.

The airline estimates that if 50 percent of passengers relieved themselves before boarding, it would reduce carbon dioxide by 4.2 tons a month.