This article compares the harm to human body caused by three different types of ionizing radiations, i.e., alpha (α) particle, beta (β) particle and gamma (γ) ray. To learn more about the basics of ionizing radiation, please visit the Observatory website: https://www.hko.gov.hk/en/radiation/monitoring/ionizing.html

Alpha particle has greater mass, more electric charges and less penetrating power in matter. Even though the range of penetration of the most energetic alpha particle can be several centimeters in air, it cannot go through the cuticle of human skin. As such, the vulnerability to human body caused by external irradiation1 by alpha particles is comparatively less severe.   

However, once alpha particle enters the human body, its characteristic of depositing its energy over a short range becomes crucial. Under this condition, the alpha particle is surrounded by the living tissue of human body and the harm arising from such internal radiation exposure2 is mostly confined to the small area of tissue surrounding the alpha particle source. If the alpha particles accumulate in a certain organ, nearly all the energy released by the particles will be imparted to that organ rather than distributed to a larger area around it.  Hence the damages to the cells of that organ by alpha particles are substantially larger.

In contrast, the range of penetration of beta particles in air is larger than that of alpha particles. Beta particles with higher energy are capable of piercing the epidermis and penetrating several millimeters deep into the skin tissue. Hence, exposure to beta particles will cause more external irradiation hazard than alpha particles.  However, as the external irradiation brought by beta particles is mostly confined to the epidermis and outer skin tissue, such external irradiation hazard is not too severe.  

Naturally, beta particles also cause internal radiation exposure hazard but the hazard is relatively small when compared with that brought by alpha particles. This is due to the fact that the penetrating power of beta particles is greater than that of alpha particles. Thus, the energy released by beta particles is absorbed by a larger volume of tissue than that caused by alpha particles, bringing comparatively smaller damages to that organ.

Gamma ray is highly penetrating and has a relatively long range of penetration in air and matter. Even if a gamma source is located far away, it will still cause external irradiation hazard to the human body. For a human body exposed to gamma ray, all of his/her organs and tissues will probably be irradiated. Hence, the external irradiation hazard brought by gamma ray is more severe than that brought by either alpha or beta particles. 

With high penetrating power, gamma rays may even penetrate through the human body. As far as internal radiation exposure hazard is concerned, the high penetrating power means that the energy released by gamma rays and taken up by a small volume of tissue is comparatively smaller. Hence the harm to the organ is also smaller. Therefore, the internal radiation exposure hazard caused by gamma rays is not as severe as that induced by alpha or beta particles.

Since the radioactive source causing external irradiation hazard is outside human body, staying away from the source is the best way to minimize the hazard.  In addition, shortening the time of exposure and shielding are also effective protective measures to reduce the hazard caused by such external exposure.

Internal radiation exposure hazard is caused by radioactive source inside the body.  Radioactive material can get inside the body through inhalation, ingestion or by passing through the wounds of your skin.  Once radioactive substances enter the human body, it will produce radiation exposure the entire time they are inside the body until the material is no longer radioactive (i.e. it decays) or until it is removed naturally by the body through excretion.  For those radioactive substances with a long half-life or for those materials of which only a small amount could be get rid of by the body through excretion, the radioactive material will stay in the body for a longer time.  Furthermore, some radioactive nuclides have affinity for certain human tissues or organs and will accumulate in the organ, causing more damages. For example, iodine (beta particle and gamma ray emitter) and strontium (beta emitter) tend to accumulate in the thyroid gland and the bone respectively whereas plutonium (alpha emitter) mainly accumulates in the bone and the liver. 

Remarks:
[1] External irradiation occurs when all or part of the body is exposed to radiation from an external source.
[2] Internal radiation exposure results from radioactive material that gets inside the human body, e.g. through inhalation or ingestion.