Why is alpha and beta radiation measured in Sievert?

[cockpitvisit]

What's the point of measuring alpha and beta radiation in Sievert?

As I understand it, one Sievert is an equivalent of 1 Joule absorbed by 1kg of body mass.

But since alpha and beta radiation barely penetrate the human skin, most of the body mass isn't irradiated at all. So the dose in Sievert says nothing about the energy absorbed by the body, either absolute or per kg of body mass.

It seems it would be more sensible to measure absorption by a square meter of human skin. Why isn't it done this way?

 

[Astronuc]

What's the point of measuring alpha and beta radiation in Sievert?

As I understand it, one Sievert is an equivalent of 1 Joule absorbed by 1kg of body mass.

But since alpha and beta radiation barely penetrate the human skin, most of the body mass isn't irradiated at all. So the dose in Sievert says nothing about the energy absorbed by the body, either absolute or per kg of body mass.

It seems it would be more sensible to measure absorption by a square meter of human skin. Why isn't it done this way?

I'll let someone with Health Physics provide details, but alpha and beta emitting isotopes can be ingested. Alphas and betas would be a problem in the lungs, where alphas can do a lot of damage to the membrane.

Alpha emitters like some isotopes of radium (226), polonium (210), and other can be absorbed by tissue and bone.
http://en.wikipedia.org/wiki/Alpha_particle#Biological_effects

Similarly, beta-emitters can be absorbed by certain organs depending on their chemistry and thus strongly irradiated surrounding cells.

Some discussion about radiation here - http://hps.org/publicinformation/ate/faqs/radiation.html