The Which? Report showed that:

• it confirmed claims it first published in April by the same magazine that using hands-free earpieces could more than triple the brain's exposure to radiation compared to a conventional mobile phone call.
  
  
  
• Although handsfree kits can reduce radiation by 10% to 90% in some positions, in others they could increase them by a factor of 1.5% to 3.5% depending on where you position the phone and kit. The increased emissions were however still within British and European safety levels
  
  
  
• The original Which? report was dismissed in August by the UK government, which commissioned research that showed the hands-free kits cut exposure levels.
  
  
  
• But Which? said the methodology used in the government research was flawed as the government study used only one position that did not allow the wire to hang straight down, Which? said.
  
  
  
• Further research using a more realistic model confirmed hands-free kits could act as an aerial that channeled radiation to the ear.
  
  
  
• The study found that the level of emissions depends on the distance between the tip of the phone's aerial and the earpiece, which varies according to how the phone is held. The government tests did not allow for this, Which? said.
  
  
  
• Unfortunately, there is no way that consumers can work out the best position to reduce radiation.
  
  
  
• Levels of SAR inside the head can be increased by mobile phone hands-free kits by up to 3.5x. 
  
  
  
• Those handsfree kits using ferrites fitted to the wire just below the ear seem to reduce radiation levels. Ferrites are metallic/ceramic compounds, designed to absorb electro-magnetic fields.
  
  
  
• By changing the position of the wire, emission levels are reduced.
  
  
  
• Technical report of Which? handsfree tests (PDF format - 112K)
  
  
  
• Only the Plantronics Headset was shown to reduce SAR levels

Here is how the radiation level was tested in the Which! test:

[Graphic courtesy Which! Magazine, UK]

Which? found that with the kit wire hanging straight down, the critical factor in determining radiation emissions is the length between the earpiece and the mobile phone antenna.

Which? was also able to uncover a potential solution to the higher levels of emission. Having carried out some preliminary testing, we found that fitting ferrites on the hands-free kit wire below the ear seems to reduce the levels of radiation into the ear. 

Hoping to identify why different tests (SAR tests) indicate that hands-free kits reduce radiation, Which? also carried out this type of test at the same laboratory used by the DTI. We discovered problems in the way some SAR test rigs are erected for tests on hands-free kits, including that the wire cannot hang straight down - as in normal use - in positions where we found maximum emissions.

Another important difference between the Which? tests and SAR tests is that the maximum SAR reading from a mobile phone appears at the cheek and jaw, but the maximum SAR reading from a hands-free kit appears in a different position - at the ear. The Which? tests take both readings inside the head in line with the ear. Because radiation may affect the tissue in the brain differently from the tissue in the cheek and jaw, Which? wants to see further research carried out.

"We were able to reveal some good news for mobile phone users - ferrites fitted to the wire just below the ear seem to reduce radiation. So we're looking to the hands-free kit manufacturers to carry out further research and - if they are proven to work - then incorporate ferrites into hands-free kits."

Notes:
¹ Which? published results of tests on two hands-free kits that found the kits tripled radiation into the ear (Ring of Truth, Which? April 2000).
² The DTI published a report on 8 August which stated that hands free kits reduced exposure to electromagnetic fields.
³ Which? tested the following phones and hands-free kits:

⁴ Ferrites are metallic/ceramic compounds, designed to absorb electro-magnetic fields.
⁵ SAR (Specific Absorption Rate) tests measure the rate of energy that is absorbed, or dissipated in a mass of dialectic materials, such as the biological tissues.
⁶ Because some SAR test rigs do not allow the wire to hang straight down as it would in normal use, the crucial vertical difference of the wire between the earpiece and the antenna cannot be taken into account. Even the SAR tests carried out by the Australian Consumers' Association, which taped the wire to the test body, did not explore the variation of the distance of the wire.

• Technical report of Which? tests (PDF format - 112K)
  
• The test protocol used by Which? is available so that other scientists can replicate the tests. It can be found as part of a technical report here (PDF format - 724K