In general, radiation is energy generated in the change of an atom's excitation state or in nuclear reactions, and is in the form of electromagnetic or particle radiation. The creation and interaction mechanisms of different forms of radiation differ from each other even though they are all commonly referred to as "radiation".

Radiation can be categorised into two groups, based on the effect the radiation has on the subjected target, such as living tissue.

Non-ionising radiation is e.g. visible light and infrared and ultraviolet radiation.

Ionising radiation can be either electromagnetic radiation, such as shortwave gamma and X-radiation, or particle radiation. The interaction of ionising radiation and matter cause chemical alterations in tissue, which come up as biological injurious effects.

Radiation dose

The radiation dose describes the health hazard caused by radiation. Its unit is sievert (Sv). The dose is often given in thousandths of sieverts, i.e. millisieverts (mSv) or in millionths, i.e. microsieverts (µSv).

The dose rate indicates the amount of radioactive dose received by a person within a certain period of time. The unit of the dose rate is sieverts per hour (Sv/h).

Some examples of radiation doses​ ​
​Dose rate ​What the rate causes
​0,01 mSv ​The radiation dose received by a patient having his/her teeth X-rayed
​0,1 mSv The radiation dose received by a patient having his/her lungs X-rayed
2 mSv ​The annual dose of cosmic radiation received by a person working in an aeroplane
​3.2 mSv ​The average annual radiation dose for Finns caused by indoor radon, X-ray examinations, etc
​20 mSv ​The highest permitted dose for a radiation worker over a period of a years
1 000 mSv The dose which may cause symptoms of a radiation sickness (e.g. tiredness and nausea) if received within 24 hours
​6 000 mSv The dose that when received within under 24 hours causes radiation sickness and can be fatal


Some examples of external dose rates
​Dose rate Example
​0,04-0,30 µSv/h Natural background radiation in Finland
​​0,2 - 0,4 µSv/h The dose rate which, if it is exceeded, causes an alarm in an automatic radiation monitoring station in the Finnish automatic external dose rate monitoring network. Each station in Finland has its own station-specific alarm level, which is determined separately for each station. Alarm levels in Finland are between 0.2 and 0.4 μSv/h. The differences are mainly caused by the level of natural radioactivity in the soil surrounding each sensor.
​5 µSv/h ​The dose rate in an aeroplane flying at an altitude of 12 kilometres
​5 µSv/h ​The highest dose rate measured in Finland during the Chernobyl accident
​10 µSv/h Some protective measures are needed, e.g. avoiding being outdoors unnecessarily
​30 µSv/h ​​The dose rate measured at a distance of one metre of a patient that has undergone isotope treatment. When the dose rate is less than 30 µSv/h, the patient can be discharged.
​100 µSv/h It is necessary to take protective measures, e.g. to shelter indoors