Easyjet customers alone bought 1% of UN accredited offsets in the last year.
When booking a plane ticket to next month's United Nations Climate Change Conference in Copenhagen, a little button that offers the chance to "offset" the carbon dioxide emissions appears.
According to Easyjet, it will cost £1 each way to make the necessary reduction in emissions elsewhere.
But where does that money go?
Many are unsure, and some businesses are hoping world leaders will take steps to reform the industry.
The offsetting concept emerged from the 1997 Kyoto protocol.
It was used by firms who had to make emissions reductions and were allowed to pay for a portion to be made elsewhere.
They bought credits certified by the UN as guaranteeing an emissions reduction somewhere else.
It is possible to pay to offset emissions from cars.
But credits are also bought by firms who are not forced to make reductions, or by consumers.
These can sometimes be certified by other standards boards, not linked to the UN.
This is called the "voluntary market" in carbon credits.
Besides flights, you can now offset car journeys, home energy usage and food consumption.
You can even buy offsets as a Christening present, according to JP Morgan's Climate Care website.
Easyjet and Virgin both estimate that some 3% of their passengers offset the carbon from their flight, though numbers have come down during the recession.
Even so, Easyjet passengers alone would purchase close to 1% of the UN accredited offsets issued in the last 12 months, a significant amount for just one airline.
So far, 345 million tonnes of carbon dioxide (C02) or its equivalent in other gasses have been reduced through the UN scheme.
Within the voluntary market, New Energy Finance estimates the number is 111 million tonnes of CO2.
Confusingly, though, only a small fraction of that is used for offsets bought by consumers and blue chip firms.
Where your money goes
Where does my money go?
Paying for a project which reduces emissions
Possible profit for a firm which invested in the project
Proving the emissions have been reduced
Buying and selling the credit
The admin and staff costs of the firms involved
With such a complex market the price varies.
For a trip to Copenhagen it costs between £10 and £30 to offset each tone of carbon dioxide emitted.
Some offsets are traded as commodities, so some of the money goes to those who buy and sell them - the brokers.
Brokers can charge between 15% and 25% of the total price to track down and find the right offset for the right company and run their offices.
Virgin's credits are provided by Myclimate, which takes a 15% administration fee.
Easyjet buys the credits itself - and claims it charges no administration fee.
The offsets they buy may already have been traded several times by banks, with the price going up and down with the market.
Much of the money goes to the people who invest in the project that reduces emissions; companies such as Eco-Securities and First Carbon or their financial backers.
They invest in a project, for instance a new hydro electric dam, and then sell the emissions reductions to brokers or even directly to firms such as Easyjet.
The money customers pay goes back to the investors and their local partners.
Their profit, or loss, depends on how many tonnes of carbon dioxide they can save and what they can charge.
No firm contacted by the BBC could specify exactly how much money from an individual credit went to implementing an individual project.
Critics say much of the investment would have taken place anyway.
Within the UN certified scheme there are more than 200 types of projects that might be approved.
The Easyjet website mentions a small hydro electric plant in Equador.
JP Morgan's Climate Care pioneered energy efficient cooking stoves in Africa to reduce emissions.
Yet most credits come from relatively few types of projects.
The Equador hydro electric project would generate only a small fraction of Easyjet passengers' offsetting needs.
So far, 76% of all the UN certified emissions reductions have come from projects that stopped the release of industrial gasses such as HFC's.
These gasses are relatively cheap to prevent and cause far more damage than carbon dioxide - so they earn more credits.
They are released in the manufacture of fridges in China for example.
In that case the fridge manufacturer - as well as the investor - would probably earn money from the credit.
The final part of the cost lies in the certification.
My Climate estimates that "transaction costs" such as this account for 10% of what people pay.
The UK government recommends firms buy only UN certified credits.
This process requires that investors successfully register their projects with an independent board and then verify their reductions after inspection by independent auditors.
Ecosecurities claim the UN verification process alone can cost upwards of £20,000.
"Sometimes it feels like a bit of a criminal investigation," says Belinda Kinkhead, head of the companies project implementation.
"They think you are guilty unless proven otherwise," she says of the UN process.
Ms Kinkhead says the UN verification process is so expensive that some smaller projects simply cannot be implemented within it.
UN credits are therefore sometimes seen as overly expensive and perhaps lack an exciting "story".
Nobody wants to talk about refrigerators on their website.
These issues led to the emergence of cheaper and more flexible forms of verification.
They offer different levels of proof that emissions reductions have taken place.
The lack of government certification led to confusion.
"I believe this 'Wild West approach' has undermined people's confidence and that's been detrimental to the market as a whole", says Paul Dickinson, managing director of the Carbon Disclosure Project, which gathers data on businesses environmental policies.
In 2003 a group of non governmental organisations set up the "gold standard" .
Lisa Hodes, the group's director of US markets, says not all credits do what they say on the tin.
"A tonne of carbon dioxide [reduced] is not necessarily a tonne of carbon dioxide".
Their process places a greater emphasis on sustainable development than the UN mechanism.
They say some of the money they invest in projects goes to the communities involved.
Other "premium" standards include the Voluntary Carbon Standard and the Carbon Action network.
Some sellers have formed the International Offset Reduction Alliance, which sticks to these standards.
These tend to be more expensive, between £5 and £15 pounds a tonne.
So far only 8 million tonnes of credits have qualified under the scheme, according to New Energy Finance.
On the other end of the spectrum some credits currently cost pennies.
Few of these are sold to consumers or blue chip firms.
"If you go for prices that are in the gutter, that is a sign of what you are buying," says Lenny Hochschild, managing director of US firm Evolution Markets who buy offsets on behalf of big US companies.
Even UN certified projects have been criticized for failing to prove they provided emissions reductions which would not have happened anyway.
Friends of the Earth and the World Wildlife Fund (WWF) have both questioned how additional some projects are.
The groups have also warned that offsetting may create perverse incentives; encouraging firms to pollute in order to earn money by implementing a pollution reduction programme.
But the UN and WWF also claim many projects do provide genuine emissions reductions.
The quality of the credit and its cost ultimately depends on where it comes from.
The confusion has led one small travel firm - responsible travel - to stop offering offsets entirely.
The market for offsets - both voluntary and compulsory - is still very small, with most demand still coming from companies that have to buy them.
Ecosecurities say business is held back by lack of certainty over the future of the market.
Jonathan Grant looks at the market for Price Waterhouse Coopers.
He argues that the UN offsetting system "probably won't get the political attention it really needs to be effective in any post 2012 regime" at Copenhagen.
In the meantime journalists looking to travel to Copenhagen can always take the ferry.