Visitors to Chicago’s Field Museum can pay an extra dollar for admission to offset the carbon emissions they generated in getting there, according to Medill Reports. In the program’s first three hours, 100 out of 549 visitors offset their emissions. Out of that 100, nine people paid more than one dollar.

Some, who didn’t pay, said they didn’t understand the program or were unsure about the efficacy of offset credits.

Employees who greet visitors at the museum had five training sessions so they could answer any visitor questions.

The museum purchases carbon offsets through the Chicago Climate Exchange, which also calculated the $1 fee.