RADIATION!
Unless you eat the radioactive material, the
only emission from a radioactive nucleus that can reach you
across space is the γ-ray photons! Solid matter is almost
completely transparent to them.
A millisievert (mSv) is defined as "the
average accumulated natural background radiation dose to an
individual for 1 year, exclusive of radon, in the United
States." 1 mSv is the dose produced by exposure to 1 milligray
(mGy) of radiation. Americans actually get about 3.2 mSv
per year from all sources.
The whole-body exposure threshold for acute
hematopoietic syndrome or "radiation sickness" is 500 mGy. A
dose of ~3,000 mGy produces an acute gastrointestinal syndrome
that can be fatal without major medical intervention, and a dose
of ~ 5,000 mGy is considered the human LD 50 / 30, that is, the
lethal dose for 50% of the population in 30 days, even with
treatment. These are acute thresholds: the same dose
fractionated over a series of exposures or over a longer time
may produce less injury, as the body has a chance to repair
damage between exposures.
Nuclear weapons were detonated in the air
over the cities of Hiroshima and Nagasaki in Japan in 1945. The
health of the survivors, and of their offspring, has been closely
monitored. It is perhaps surprising, but no adverse
health effects have been detected. Death rates due to
various diseases such as cancer were normal over the years, and
no mutations or excess birth defects were ever observed, over 70
years and ongoing! Similarly, there are areas around Kerala,
India, where people have lived, for as long as humans have lived
in the area, on what is a hugely concentrated natural deposit of
Uranium and Thorium, getting doses anywhere from around 5 to 70
mGy. Again, no adverse
health effects of any kind have been detected in careful,
large-scale studies.
Radioactivity has proven to be enormously
useful in medical diagnostics and testing, and in
archeological, paleontological, geological and astrophysical
dating. Much of what we know about the time-scale of life on
earth, the origins of earth and solar system, etc., depends on
radioactive decay... a process which in turn depends on a few
fundamental physical constants, and is not only completely
dependable, but allows large numbers of cross-checks.
By the way, a 50% lethal dose of radiation
delivers about the same total kinetic energy to the body as a
rifle bullet, it's just that the rifle bullet does damage at the
organic level, while a whole-body radiation dose does damage at
the molecular
level.
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