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:
 Damage depends on absorbed dose
 Threshold exists
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 Xray), 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 (Svyear)  
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 
(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:
 Severity is independent of absorbed dose
 Threshold does not exist
 Probability of occurrence depends on absorbed dose
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.
The above information is provided by RHD of DH

