Because x-rays can penetrate living tissue and cause ionization as they travel, they can be a health hazard.
The symbol shown opposite, is commonly seen in
hospitals. It warns people of the presence
of ionising radiation (like x-rays)
Cell damage
When ionizing radiation, like x-rays, interacts with living tissue, it
transfers energy to molecules of cellular matter. Normal matter has no
net electrical charge. Ionisation is the removal of electrons from atoms
resulting in ion pairs forming. If a significant level of ionisation
occurs in human tissue, it may initiate a cancer or the possibility of a
hereditary defect in future generations. The severity of the injury
depends on
· the frequency of the radiation,
· the absorbed dose,
· the rate at which the dose was absorbed
· the radiosensitivity of the tissues involved.
Energy is required
to cause ionisation and so when ionising radiation interacts with
matter, energy is deposited in the matter. The amount of energy
deposited per unit mass of matter is called the radiation dose.
Dose
For biological
tissue, dose is measured in units called sieverts
(Sv). The average annual dose received by each of us from natural
background radiation is approximately 2·6 mSv. The International
Commission on Radiological Protection recommends that members of the
public should not receive more than 1 mSv additional to background. The
dose received in most forms of diagnostic x-rays are very low, e.g. the
dose to the patient from a chest X-ray may be as low as 0·02 mSv.
The biological effects of a large dose of radiation delivered rapidly
differ greatly from those of the same dose delivered slowly. The effects
of rapid delivery are due to cell death, and they become apparent within
a short time. Protracted exposure is better tolerated because some of
the damage is repaired while the exposure continues, even if the total
dose is relatively high.
Damage to most body cells will affect only the individual exposed to the
radiation. Any damage to reproduction cells however can also be passed
onto future generations. The risk that exposure to ionising radiation
will cause a problem is proportional to the dose received.
An x-ray tube
operating at about 40,000 V and 25 mA, is converting 1 kW of power, and
the x-radiation coming from that tube is extremely dangerous. If you put
your hand within 1 or 2 cm of that tube, and allow the full blast of
radiation from the window to strike your hand, even for a short time you
will get 3rd degree burns and these burns will not heal.