Deterministic effects and stochastic effects

From the biological effects of radiation on human body, radiation effects are generally divided into two categories: "Deterministic effects" and "Stochastic effects".

Deterministic effects

Based on a large number of experiments involving animals and other researches, further supplemented by theoretical studies, it was discovered that severity of certain effects on human beings will increase with increasing doses. There exists a certain level, the "threshold", below which the effect will be absent. This kind of effects is called "deterministic effects".

Characteristics of deterministic effects:

Example: cataract, erythema, infertility etc.

Deterministic effects and dose relationship

Severity of deterministic effects depends on dose. However, thresholds exist, only above which the effects will occur. The International Commission on Radiological Protection (ICRP) considers that if the annual radiation doses to the lens of the eyes of radiation workers are restricted to 150 mSv (equivalent to 150 mGy for X-ray), cataract is unlikely to occur during his/her life assuming a working period of 50 years. For other major organs, the annual dose limits for preventing deterministic effects are as follows.

Threshold for deterministic effects (Sv)
	Effects	One single absorption (Sv)	Prolong absorption (Sv-year)
testis	permanent infertility	3.5 - 6.0	2
ovary	permanent infertility	2.5 - 6.0	> 0.2
Lens of eyes	milky of lens cataract	0.5 - 2.0 5.0	> 0.1 > 0.15
Bone marrow	Blood forming deficiency	0.5	> 0.4

(Source : 1990 Recommendations of the International Commission on Radiological Protection
(ICRP Publication No. 60))

 

Stochastic effects

The severity of stochastic effects is independent of the absorbed dose. Under certain exposure conditions, the effects may or may not occur. There is no threshold and the probability of having the effects is proportional to the dose absorbed.

Characteristics of stochastic effects:

Example: radiation induced cancer, genetic effect

As stochastic effects of radiation have no thresholds and can cause cancers or genetic modifications, of which the curing rates are rather low to date, they become a major subject of research in radiation protection.