A French researcher claims to have found hazardous materials in the emissions of some Apple Mac computers.

A report in French newspaper Liberation suggests that an anonymous researcher at France's CNRS research centre studied the emissions from a Mac Pro computer and found that the machine emitted benzene into the air, which can be hazardous to human health.

The report states that, while the amounts of benzene were not sufficient to cause immediate health risks, prolonged exposure may lead to an increased cancer risk. The vapours caused irritation in the eyes and throat of the researcher.

The report also cites several Apple support threads in which users claim that their Mac Pro machines are emitting strange smells, although no mention of benzene is specifically noted.

Apple said that, while it is investigating the claim, the company has no prior knowledge of any reported benzene emissions in the Mac Pro.

The use of toxic chemicals has become a contentious issue for Apple in recent years. The company contends that it has long been working to remove hazardous materials from all of its models.

Apple bristled in 2007 when Greenpeace singled the firm out for what it said were sub-standard efforts in removing hazardous materials and recycling old machines.

Nevertheless, Apple has made its green efforts a centrepiece of some of its most recent hardware releases and updates.