What is the Safe Limit of Radioactivity for Humans?

http://en.wikipedia.org/wiki/Ionizing_radiation

I doubt you will ever get a definitive answer to that. It depends on your point of view. If you are the emmitter, your definition of "safe" is very different from what the definition would be for the receiver because ABSOLUTELY safe for the receiver would be zero. Anyhting more than that is going to fall into the category of "acceptable risk" and that is usually defined by the emmitters using a cost/benefit analysis.

It's not the level of radiation per-se, but the exposure limits. Each country and regulatory agency sets their own limits.

There is also the consideration for types of radiation i.e., alpha, beta, gamma, neutron...

Smoke detectors utilize less than or equal to 1 microcurie of (typically) Americium 141. Exposure times for those devices, well, how long have you had one in your house? <1% of the emissions is typically neutron, the rest is alpha, blocked by the foil envelope which surrounds the chamber.

I can only speak of the US DoD standards:

NAVMED P-5055 states: "The Administrative Control Level of 0.5 rem per calendar year is hereby established for the purpose of reducing the total man-rem from RASP sources of ionizing radiation."

you're decaying as we speak...

there is no absolutely "safe" level. ANY level can cause damage, but it is unavoidable. walking out into the sunlight or eating your food or breathing the air or drinking your water exposes you to radiation. Most things have a certain amount of radioactivity associated with them. Different standards groups have attempted to define a level at which the risks associated with that level of exposure are "acceptable" but the definition of "acceptable" is somewhat dependent on who you ask and what you base it on.

What's acceptable to an astronaut? What's acceptable to a cancer patient?

Good points. I was totally surprised when during my initial course of study to find out how many environmental challenges there were to exposure. Every time I eat a banana or change the litter box I absorb a little more.

Lord only knows about the Radon levels in each country.