Back in the days of no-holds-barred advertising by Madison Avenue's finest, anything went. Drinking alcohol made you sexy, smoking cigarettes was good for your lungs and every washing powder contained a magic ingredient that made your whites super-white.

And guess what? Those days are back, at least for green advertising. As more and more customers demand environmental responsibility from companies, few large corporations with any sort of public profile now dare to enter the marketplace without a blizzard of sustainability audits and low-carbon-emissions targets. But that being so, the risk of being conned by slick corporate "greenwash" has never been greater, as the volume of complaints to the Advertising Standards Authority (ASA) testifies.

The green claims coming from corporations can be absurdly general. Nearly everything we buy these days seems to be "sustainably sourced" or "environmentally friendly". Sometimes, though, they are crazily specific. Virgin Trains declares: "Our Pendolino trains emit 76% less CO2 than cars or domestic flights." But which cars, which flights, and how full are the trains?

Or plain bonkers. One brand of bottled water says its product contains "300% more oxygen".

In August, the ad industry's watchdog, the ASA, rapped oil company Shell's knuckles for trying to claim, in an advertisement in the Financial Times, that its $10bn investment in sucking tar sands out of the Canadian midwest was a contribution to a sustainable energy future. Tar sands contain bitumen. It takes a great deal of energy to turn them into something you can put in a fuel tank. Overall, the emissions from mining, refining and burning tar sands are between three and 10 times greater than for conventional oil. Shell's sleight of hand was to use the much-abused word "sustainability" to imply a green agenda when what it was really on about was keeping a sustainable flow of fuel out of its forecourt pumps. The ASA cried foul.

Earlier in the year, the agency also upheld complaints against Renault for branding its Twingo an "eco" car and picturing it with leaves blowing out of its tailpipe, even though its emissions are among the worst for a car of its size.

Advertising, says the ASA, "should always avoid the vague use of terms such as sustainable, green, non-polluting and so on".

In the real world, we have to admit, things can be nuanced. Lots of corporate claims - about carbon neutrality, for instance - hang on exactly what activities are being audited. Take Manchester airport, which was outed in a recent report from the sustainable development organisation, Forum for the Future. Last year the airport's owners pledged to make the airport carbon-neutral, with one small caveat: the target does not include the 200,000-plus flights into and out of the airport each year. As Forum for the Future observed, "this jars somewhat".

And what are we to make of Fiji Water's claims to be cutting the carbon footprint of its water by 25% and offsetting the rest? "Every drop is green," it says. But isn't the whole idea of bottling water on a remote South Pacific island and shipping it to your dinner table just a tiny bit barmy?

Equally questionable are the claims of financial institutions. Back in the days (oh, only a few weeks ago) when the City of London was concerned with something other than day-to-day corporate survival, the City of London Corporation launched a City Climate Pledge, under which finance houses would pledge to "measure and monitor" their carbon footprint. Good for them. Perhaps a few will follow the lead of HSBC, which has developed close links with the environment group WWF and has claimed since 2005 to be the world's first carbon-neutral bank.

But the pledge looks suspiciously like greenwash. Companies simply have to fill out a form detailing their CO2 plans and they can use the pledge logo. "Companies using the logo will be recognised as exemplar sustainable businesses [able to] attract consumers who are becoming more discerning about the credentials of businesses they deal with," says the flyer. Not bad for just filling out a form. Especially as there doesn't seem to be any follow-up or auditing process involved.

But as with the efforts of Manchester airport and Fiji Water, there remains a rather large elephant in the living room. The carbon footprint of finance houses is not about whether they offset executive flights or put double glazing in the boardroom; it is about their investment decisions. The press release put out by the City of London said the pledge would "encourage City organisations to use their global influence to affect the behaviour of companies around the UK and the world". Could the City be about to impose a freeze on cash for tar sands or coal-fired power stations? Not so fast. The pledge itself makes no mention of this.

I would have expected City institutions to be falling over themselves to sign up to the pledge. But 11 months after companies were first asked to sign the pledge, and three months after its public launch, the pledge's website is still promising that a list of signatories "will be available shortly", and further inquiries revealed that just one company - Deutsche Bank - had so far completed registration. Right now the pledge looks like a one-day PR wonder to green the City's image, with no substance at all. Or maybe they are all just a little busy right now.

To try to keep up with the welter of environmental claims, test the green spin and spot the green frauds, the Guardian is launching today a regular online column, Greenwash, and calls on readers to submit their examples of the fraudulent, mendacious, confusing, ignorant or just daft claims jostling for our attention.

Along the way, we may get to the heart of a dilemma that faces us all. Can we shop our way to sustainability? Are some products so green it is better to buy two of them rather than one? Or are our own consumer lifestyles, suffused in greenwash, the problem? Is there really no alternative to putting away our credit cards, pulling on our thickest jumper and heading for the hills?

We won't be limiting our investigations to corporations - we'll have politicians in our sights, too. Scraping away at the green patina on the new-look, Zac Goldsmith-inspired Conservative environmental policies, puncturing Brown's grumpy greenery and unpicking the carbon contortions of the coal-loving Celts. And now that both Barack Obama and John McCain claim environmental credentials, we'll be looking for greenwash at the White House too.

And we won't forget that, even in the corporate world, greenwash is not just a defensive mantra to help maintain business as usual. Some people are out there pushing the environmental agenda with sinister intent. Take, for example, the green rebranding of Steptoe and Son.

Last year the EU introduced the charmingly named WEEE directive, which stands for Waste Electrical and Electronic Equipment. The aim is to prevent millions of tonnes of toxic TVs, personal computers, toasters and other electronic goods being dumped into landfill each year. Instead, they have to be recycled. You may not have noticed this because there are no laws stopping you putting that laptop into your dustbin. But corporations and retailers are charged with making sure most of our electronic waste gets recycled. The question is how.

Some of this stuff is making its way to a handful of hi-tech metals recycling plants in Europe. But most is going to the developing world. Often it has paperwork claiming it will be refurbished and re-used, but nobody has the resources to police the system, so in practice much of it ends up in primitive workshops in India and west Africa and China, where it is stripped out, boiled up, dunked in acid or smashed to smithereens by unskilled, low-paid and frequently child labour. I have seen this "recycling" industry at work in Delhi, where barefooted children as young as eight dunk circuit boards in barrels of acid to remove traces of copper.

Last month, a charity called Computer Aid tried to blow the whistle on this. Computer Aid is one of the few organisations that is genuinely and safely refurbishing and reusing old computers, many of them going to schools in Kenya. It fears this "good" and socially responsible recycling will be undermined by the bad guys. Why is this happening? Partly because factories in India badly want the metals in your old computer. And partly because too many European companies have a no-questions-asked policy towards every broker and cowboy willing to take troublesome waste off their hands. As one industrial supplier told me last year, "A lot of these guys don't even have addresses, just mobile phone numbers."

Recycling may be a new term, but the trade is not new. Until the 60s, recyclers plied the streets of Britain with a horse and cart collecting old stuff that could be sold on. The characters in the television show Steptoe and Son were the archetypes. They have been succeeded by a generation of car-crushers and cable-burners. Nobody called it green then: now they do. But the same wide boys are in charge, so if anybody has their mobile numbers, do please get in touch.

We are too ready to suspend our critical faculties with anybody claiming to be green. But a great deal of recycling is not quite what it seems. What happens to recycled glass bottles, for instance? As we post them in the recycling bin, we presume they go to make new bottles and cut out the energy cost of making new glass from sand. My local supermarket bin in south London proudly proclaims that recycling one glass bottle "saves enough energy to power a TV for 20 minutes".

Well, it would if they turned the glass back into new bottles. But it turns out that often they don't. Much of London's recycled glass is actually crushed and sold to construction firms as a substitute for sand, or an ingredient in a substitute for asphalt such as Glasphalt - "specially treated so it won't puncture tyres," as one recycling website puts it. That's a relief, but how many assiduous recyclers trying to do their bit for the environment realise they are actually helping build new roads?

How many more green scams, cons and generous slices of wishful thinking are out there? We want to name and shame them before the whole green movement gets a bad reputation. "Green" has another meaning after all - naive. And we cannot afford that.

Greenwash: Fred Pearce on the con of green electricity

How to spot a fake

It's greenwash if it's ...

• Ludicrously general

All claims for products being environmentally friendly or pollution-free should have evidence that clearly supports them.

• Overspecific

Be on guard against numbers or other "facts" that can only be true in specific circumstances, such as Virgin Trains' claims to have 76% fewer emissions than planes or cars (see main feature). Often these come with a discreet asterisk referencing an obscure study - look the study up, or ask the Guardian's new Greenwash column to do so on your behalf.

• Reliant on nature pictures

Most pernicious, perhaps, are attempts to green products by association, such as cars driving through verdant meadows.

• Backed up by a tame boffin

Companies like to spotlight their researchers working on renewable energy, even if it makes up less than 1% of their business. Anybody can keep a tame boffin: to mean something, such research must be a significiant part of the company's business. (Incidentally, has anyone met a researcher? We would like to hear.)

• Simply absurd

If a claim sounds absurd, it probably is. Inside knowledge can help you blow the whistle, but a nose for the absurd is just as good.

How to spot the real thing

It might just be true if ...

• What the company says matches what they do

Rob Harrison of Ethical Consumer thinks the most important question is: "Is their position consistent right across the group?" Some car manufacturers advertise their green cars but continue to lobby for lower carbon-reduction targets. Look for companies who match what they say and do, such as Peugeot, one of the only car manufacturers likely to come in under the European emissions targets in 2010.

• The company is in partnership with an independent ethical organisation

Forming a partnership with an independent organisation is voluntary and doesn't have to mean much: groups such as the Ethical Trade Initiative or the Carbon Disclosure Project have little power. All the same, it's an indicator of willingness. Some NGOs are more picky: their validation is, therefore, worth more.

• They take that extra, obsessive step

Evangelistically green companies such as Lush, Co-operative Food or Sawdays can never resist the chance to talk about the environment, and they've all taken radical steps within their own organisations. Lush uses minimal amounts of packaging and gives money to anti-road groups, Co-operative Food has clad its entire HQ in solar panels, and Sawdays publishers operates from zero-carbon offices.

• They are green innovators

DIY Kyoto came up with the Wattson, a device for measuring how much electricity your house is using. BSkyB, headed by James Murdoch, has brought in the auto-standby device, which senses if the box hasn't been used in the two hours after 11pm, and automatically goes to standby.

• They set themselves targets

Quantifiable targets set for some specific time in the future - "we will reduce our water use by such and such by 2010" - are an indicator of seriousness: they are handing you and the press a stick to beat them with if they miss the target. In January, Tesco, for example, pledged to reduce its carbon footprint by 50% by 2020.

• They take action even if it may harm business

Marks & Spencer's decision to charge for plastic bags, British Gas's encouragement of reduced use: these are not immediately obvious comfort zones for money-making enterprises. You can be cynical, or you can clap.

• They have been audited

Some companies get a third party in to check they're doing it right: Eurostar called in Environmental Resources Management (ERM) to go over their plans with a toothcomb before they kicked off their big Tread Lightly initiative. Some companies - the Guardian, for example - are so obsessive they will even audit their social report. This could be seen as just showing off.

• They make it easy for you to find all this out

If you can't easily find the information, be suspicious and ring up the company. Other possible sources of info: Ethical Consumer, FTSE4Good Index, the Dow Jones Sustainability Index, and the ASA (which may be considering cases against the company you're looking into).

Bibi van der Zee

• Please send your examples of greenwash to greenwash@theguardian.com