X-rays penetrate living tissue and cause ionisation.

X-rays deposist energy in matter.

Ionising radiation can initiate a cancer and can cause hereditary defects.

The risk of harm from x-rays is proportional to the dose received.

The dose received in most forms of diagnostic x-rays are very low.

X-rays cause harm by causing changes to cells in the body.

X-ray machine operators must observe strict safety guidelines.

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.


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.