Walkers Crisps is the first firm to put carbon footprint figures on its products, with nine more companies set to follow. How are these figures calculated?

On taking a food item off a supermarket shelf, consumers can instantly read in detail the impact it will have on the body. But what about the effect on the planet?

In April, Walkers Crisps began labelling its cheese and onion bags with a carbon footprint - how many grams of greenhouse gases were emitted in its production - and that has been rolled out to other flavours.

The calculations are done by the Carbon Trust, a private company set up by the government to reduce the UK's carbon footprint.

It spent several months working out that 75g of greenhouse gases are given off in the production of a 33.5g bag of Walkers crisps, taking into account the energy used in:

1. FARMING: Planting the seeds for sunflower oil and potatoes, the manufacture of fertilisers and pesticides, ongoing management of the growing process, the diesel used by the tractors to pick the potatoes, and storage of the potatoes in sheds and farms.

2. MANUFACTURE: Potatoes taken from fields to a factory in Leicester, where they are cleaned, chopped up, cooked and bagged.

3. PACKAGING: Sourcing the aluminium and plastic that goes into the packaging, then making and printing the packets.

4. DISTRIBUTION: Taking bags of crisps in lorries to retail stores.

5. DISPOSAL: From kerbside litter bin, into the back of a dustbin lorry and off to landfill.

Nine more companies, among them Coca-Cola and Cadbury, are committed to following Walkers when the methodology used by the Carbon Trust is approved next year.

Boots already reveals footprint figures on certain products at the point of sale, and Innocent Smoothies has the information on its website.

"Ultimately the aspiration is that everything you can buy will have a carbon measure with it - 75g is the first number out there and actually there's not much context for it," says Euan Murray of the Carbon Trust. "But when we can start making comparisons across different products, then we can make choices as consumers."

And businesses can identify "hotspots" in the production process in order to reduce greenhouse gas emissions, energy use and costs, he says.

Tread lightly

The label comes with a two-year commitment to reduce the size of the carbon footprint. Although firms will be able to do their own calculations, their sums will be checked by the Carbon Trust. Will consumers care enough to change shopping habits?

Yes, says Mr Murray, many are already starting to take an interest. Research last year suggested two-thirds of shoppers want to buy products with a low carbon footprint.

A Walkers spokeswoman says the company's own survey shows nearly 80% of consumers are aware of the labels with 20% dismissing it as "purely a gesture". And the crisps manufacturer has promised to reduce water use per kilo by 5% year on year, and energy use by 3%.

But Chris Goodall, author of How to Live a Low-Carbon Lifestyle, thinks labelling should be broader, indicating whether the product is from a high-carbon industry like dairy or beef, or has clocked up the food miles from being transported long distances by air.

"These are the signals people need on the packets, not a number that is, frankly, pretty meaningless."