source outside the body can do little harm. The alpha particles
do not even penetrate
the outer layer of dead skin cells on the body. But once inside
the body (for example inside the lungs) they are very dangerous
particle causes a series of ionisations as it rips through matter.
As they are so highly
ionising they cause a lot of localised damage.
E.g. A tiny
speck of uranium dust contains millions of atoms (one millionth
of a gram contains 2,500,000,000,000,000 atoms!). Over a long
period of time these atoms decay, their alpha particles doing
considerable damage in a region very close to the source. Alpha
rays can only penetrate about 0.1mm into tissue, therefore a lot
of localised damage occurs and the chance of a tumour developing
or serious damage due to cell death is much higher than if the
damage was more widely (and sparsely) spread.
the body alpha sources are very difficult to detect because the
rays do not get out to the detector! You could swallow an alpha
source and it would not register on a Geiger
sources are very hazardous and are never used for medical applications.
They are particularly dangerous in powder form - the danger of
inhaling or ingesting them is then higher.
to look at an animated gif showing the spread of damage through the
lung from an inhaled grain of Plutonium.