The problem with the word "cleanest" is that its definition entirely relative.

If you were to look at the criminal records of Reggie Kray, Ronnie Biggs and Nick Leeson I'd suggest that Leeson's is the cleanest - but I still wouldn't want him to manage my pension.

So for ExxonMobil to claim in its now banned TV ad that liquified natural gas is "one of the world's cleanest fuels" was always going to be a major risk.

Regardless of the fact that vetting body Clearcast approved the ad and the fact that anyone with even a passing knowledge of energy and carbon emissions would assume that Exxon meant that the fuel was being compared to other fossil fuels, the term should still have sent alarm bells ringing in the oil giants' marketing department.

The Advertising Standards Authority (ASA) has made it patently clear that any green claim that can be misinterpreted in any way will be treated harshly and the vagueness inherent in the phrase "one of the world's cleanest" was always going to attract their attention. As soon as the ASA decided that the viewer could easily believe that LNG was being compared with the solar, wind and nuclear energy sources mentioned in the advert it was bound to rule the advert misleading and slap a ban on it.

It appears that ExxonMobil has made an innocent mistake and it is easy to feel sorry for the company, which as an environmentalist is not an emotion that comes naturally. In this light, it is also understandable that it is considering an appeal; but that does not it make it right to do so.

The ASA does not lose many appeals and there is simply no way for the company to construct a watertight case in defence of the advert's claims. The term cleanest is relative and it is not clear from the advert what it is relative to - that's Exxon's mistake and it needs to suck it up.

The lesson for other firms is that when making green claims any term that it is equivocal in any way has to be avoided. Words like sustainable and environmentally frinedly have already been flagged as potentially misleading by the ASA, but a company's due diligence has to go much further than that.

Clarity is everything and if Exxon had simply said that LNG is one of the world's cleanest fossil fuels or had even included some verifiable data on its carbon footprint then its advert would stil be being broadcast.

Adverts have to be accessible for their intended audience, but they also have to be as precise and carefully worded as any legal contract. Those firms who forget this will see the publicity benefits that should be associated with green initiatives evaporate faster than Nick Leeson's millions.