The bridge's 40th birthday was marked in 2006

Dry air is being injected into the tiny space between the 8,322 wires which makes up each cable to soak up any moisture around them.

It is part of �20m work on the bridge, which opened in 1966, after inspections revealed they were corroding.

As part of work over the next two years, each cable is being also being wrapped in waterproof material.

The toll bridge, which carries the M48 between south Wales and South Gloucestershire, celebrated its 40th anniversary last year.

Chris Pope, who is project managing the work on the cables said all of the bridge's cables should be treated by the end of 2008.

"In 2006, we did an inspection of the cables and found significant levels of corrosion on the suspension cables in the middle of the bridge," he said.

"We want to protect the cables and so we are wrapping them in a waterproof material and then we will be injecting dry air into the cables.

Engineers are trying to halt the corrosion of the cables

"The air will pass through the wires and will dry out the moist air inside," he explained.

He said that the first phase of the work would be on the worst affected cables but that all of the cables would eventually be treated.

The levels of corrosion on the bridge cables was discovered following a Highways Agency investigation costing �3m.

Engineers said at the time they had opened up a "can of worms".

They began by examining six cables, then eight and finally checked 10. All are reported to be in a worse condition than expected.