The lethal dose of radiation (LD) is an indication of the lethal amount radiation. In radiation protection, the median lethal dose, LDXY, is usually used. For example, the dose of radiation expected to cause death to 50 % of the irradiated persons within 30 days is LD50/30. LD1 is the dose expected to cause death to 1% of the irradiated persons, consequently, LD99 is lethal for all (99%) persons irradiated. It is also very important, whether a person receives some medical treatment or not. The greater an acute radiation dose is, the greater is the possibility of it killing the individual. For a healthy adult, the LD50 is estimated to be somewhere between 3 and 5 Gy.
- 2.5 Sv – Dose that kills a human with a 1% risk (LD1), if the dose is received over a very short duration.
- 5 Sv – Dose that kills a human with a 50% risk within 30 days (LD50/30), if the dose is received over a very short duration. Cause of death will be loss of bone marrow function.
- 8 Sv – Dose that kills a human with a 99% risk (LD99), if the dose is received over a very short duration. At around 10 Gy, acute inflammation of the lungs can occur and lead to death.
The lethal dose data given above apply to acute gamma doses delivered in a very short time, e.g., a few minutes. More dose is required to produce the effects listed above, if the dose is received over a period of hours or longer.
Deterministic Effects and Dose Limits
In radiation protection, dose limits are set to limit stochastic effects to an acceptable level, and to prevent deterministic effects completely. Note that, stochastic effects are those arising from chance: the greater the dose, the more likely the effect. Deterministic effects are those which normally have a threshold: above this, the severity of the effect increases with the dose. Dose limits are a fundamental component of radiation protection, and breaching these limits is against radiation regulation in most countries. Note that, the dose limits described in this article apply to routine operations. They do not apply to an emergency situation when human life is endangered. They do not apply in emergency exposure situations where an individual is attempting to prevent a catastrophic situation.
The limits are split into two groups, the public, and occupationally exposed workers. According to ICRP, occupational exposure refers to all exposure incurred by workers in the course of their work, with the exception of
- excluded exposures and exposures from exempt activities involving radiation or exempt sources
- any medical exposure
- the normal local natural background radiation.
The following table summarizes dose limits for occupationally exposed workers and for the public:
According to the recommendation of the ICRP in its statement on tissue reactions of 21. April 2011, the equivalent dose limit for the lens of the eye for occupational exposure in planned exposure situations was reduced from 150 mSv/year to 20 mSv/year, averaged over defined periods of 5 years, with no annual dose in a single year exceeding 50 mSv.