Internet users have long known that spam emails - offering everything from cheap medicines and sex aids to get-rich-quick schemes - are an unwanted annoyance, but new research suggests that they are also hugely damaging to the environment.

More than 80% of the world's email traffic is now spam and the transmission and receipt of unwanted email gobbles up 33bn kilowatt-hours of electricity a year, according to anti-virus software specialist McAfee. That is the equivalent of the electricity used by 2.1m US homes.

The report follows research published yesterday by rival online security firm Symantec showing that cyber criminals are now trading stolen credit card details for as little as 4p, while a person's full identity can change hands between gangs for 50p.

Symantec's Internet Security Threat Report recorded a 192% increase in spam detected across the internet, from 119.6bn messages in 2007 to 349.6bn in 2008, with 90% of those messages generated by "bot networks".

Bot networks are collections of computers that have been hijacked - often without their owners realising - by malicious software found on websites or within spam. They can then be controlled by gangs to relay spam messages. Many internet security analysts fear that the Conficker internet worm, which has infected millions of PCs worldwide, could ultimately be intended to create a massive bot network.

McAfee estimates 62tn spam messages were sent globally last year. The greenhouse gases involved in providing enough electricity to generate, send and then delete this unwanted traffic was the same as the emissions from 3.1m cars, according to McAfee's Carbon Footprint of Spam report, published today.

"As the world faces the growing problem of climate change, this study highlights that spam has an immense financial, personal and environmental impact," said Jeff Green, a senior vice-president at McAfee.