Tetra stands for the Terrestrial Trunked Radio system - a secure
digital radio system being connected across Britain for the emergency
services, the military and companies.
are masts being erected?
O2 Airwave, the company creating the police's Tetra, is installing
3,350 base stations on masts and buildings in Britain.
does Tetra work?
It uses radio signals broadcast at 400MHz. Base stations relay
signals to and from handsets. Four users can broadcast on the same
channel simultaneously by confining transmissions to
"bursts" at 17.6 times per second (Hz).
There have been reports that early users suffered headaches,
sickness, migraines, disturbed sleep and skin rashes. People living
near masts say they have suffered similar stress-like symptoms. Some
campaigners have expressed concern at the 17.6Hz frequency used by
There are two possible risks to health from a radio frequency
radiation - a heating effect on tissue and a non-thermal effect on
cells. The first is well established, well researched and governed by
international regulations. The second is poorly understood.
is the evidence for a heating effect?
Prof Colin Blakemore, the head of the Medical Research Council and a
leading brain researcher, says the heating effects are negligible
more than three feet from the antenna. Because the beam is pointed
away from the ground, exposure below the mast is also negligible.
About 100 yards away, the point where the beam usually reaches ground
level, the radiation is several hundred times weaker than the
radiation from a mobile phone next to the head, he says.
handsets typically emit radio-frequency radiation at a peak of 0.25
watt - within safety guidelines. But if four signals were sent
simultaneously from a handset, the signal would be more powerful.
is the evidence for non-thermal effects?
Studies on dead chick brains in the mid-1970s appeared to show that low-power
16Hz radio frequencies - similar to those used in the handsets -
triggered movement of calcium ions in cell. Calcium plays a crucial
role in nerve tissue.
studies of 16Hz frequencies, including some at the Defence
Evaluation Research Agency at Porton Down, have been contradictory.
Blakemore says that the early studies were not well designed and
have been hard to reproduce. The significance to health is therefore unclear.
industry says that the masts emit frequencies around 400MHz and do
not pulse. But campaigners say they do.
is the official line?
The chick research was highlighted in a report on mobile phone safety
in May 2000, under Sir William Stewart's chairmanship. He said 16Hz
should be avoided in mobile phone systems.
major report into Tetra from the National Radiological Protection
Board in 2001 concluded that the system was unlikely to pose a health
hazard, but said it had "too many limitations" to give
assurance of no hazard.
Dr Gerald Hyland, a retired theoretical physicist at Warwick
University, says some radio frequencies interfere with the natural
"resonance" of cells and could be harmful. Most mainstream
biologists and physicists disagree with him.
do protesters cite the Salzburg resolution?
In 2000, 19 academics gathered at an international conference
proposed new rules for mobile phone and radio base stations. They
called for exposure to radio frequency radiation to be kept at levels
9,000 times lower than guidelines from the International Commission
on Non-Ionizing Radiation Protection.