4.1 Biological Effects of Radiation

Mechanisms of Cellular Damage

Radiation damages living cells primarily through two main mechanisms:

• Direct Action: The radiation particle or photon directly strikes and damages a critical molecule within the cell, most notably DNA. This is a common mechanism for high-LET (Linear Energy Transfer) radiation like alpha particles.
• Indirect Action: The radiation interacts with water molecules within the cell (which make up about 80% of a cell's mass), creating highly reactive free radicals. These free radicals then attack and damage DNA. This is the predominant mechanism for low-LET radiation such as gamma rays and X-rays.

Types of Health Effects

The health effects of radiation exposure are generally classified into two main categories:

• Stochastic Effects: These effects are random and have no threshold dose. The probability of their occurrence increases with dose, but their severity is independent of the dose. The main stochastic effects are cancer and hereditary (genetic) effects.
• Deterministic Effects: These effects occur only after a certain threshold dose is exceeded. Once the threshold is reached, the severity of the effect increases with the dose. Examples include radiation sickness, skin burns, and cataracts.

The Dose-Response Relationship

The relationship between radiation dose and biological effect is often represented by a dose-response curve. For deterministic effects, this curve has a clear threshold. For stochastic effects, it is generally assumed that the relationship is linear without a threshold, meaning even small doses carry some risk, although the risk is very low. This is the basis for the ALARA principle ("As Low As Reasonably Achievable") in radiation protection.