Expedia.com is giving travellers who want to be friendly to the environment a way to go green as they buy their airline tickets.

The online travel agency announced Tuesday it will begin selling carbon offsets, a type of voucher which sponsors clean energy research and projects.

Customers can purchase the vouchers along with regular tickets for air travel.

A typical flight from New York to Los Angeles generates more than 900 kilograms per passenger of carbon dioxide. For $5.99, Expedia says consumers can mitigate the environmental damage of approximately 450 kilograms of carbon dioxide. The travel agency is offering the voucher to customers at cost.

Some critics of carbon offset programs say the programs are reactive and appeal to a limited, wealthy audience.

But advocates of the programs say they allow consumers to reduce their ecological footprint and that these programs permit organizations to create new forms of energy while giving the consumer options in offsetting greenhouse gas pollution.

The online travel agency has partnered with TerraPass, a provider of clean energy projects.

The offer will not be available to customers on the Canadian site, Expedia.ca.

TerraPass is part of a larger market that allows consumers to determine the quantity of greenhouse gases they are generating and neutralize the damage through funding energy projects such as wind farms, tree planting and methane capture plants.