Spam emails use up an estimated 33 terawatt hours of power each year, according to a report on the state of the spam industry by security vendor McAfee.

The Carbon Footprint of Email Spam Report (PDF) states that 62 trillion spam emails were sent in 2008, and that the energy used to send and delete them could power 2.4 million American homes. Each spam email generates 0.3 grams of carbon, the report said.

McAfee estimates that, while spam filters can cut the carbon footprint of spam by 75 per cent, it is far better to shut down spam at the source. The closure of the McColo spam operation last year caused a dramatic drop in spam levels, which had an unexpected environmental benefit.

"The most obvious benefit of the McColo shutdown for practically anyone with an email address was an immediate reduction in unsolicited junk messages," the report said.

"At the same time, the planet experienced a less obvious environmental benefit. For every spam email not sent, an associated reduction in electricity use, and therefore carbon emissions, took place."

Overall, the McColo shutdown saved the equivalent energy of taking 2.1 million cars off the road.

Businesses are also taking a financial hit from spam, the report said. A typical medium sized firm uses 50,000 kilowatt hours to run its email system, but over a fifth of that power is used to deal with spam.

The average business email user is responsible for 131Kg of CO2 per year in email-related emissions, and 22 per cent of that is spam related. Users viewing and deleting spam is the largest energy drain associated with spam, at almost 18 billion kilowatt hours or 52 per cent of total spam energy.