A couple of posts ago, I used a Geiger counter to demonstrate the radioactivity of Tech99, with a half life of 6 hours. The Geiger counter was beeping like crazy a few hours after the procedure, but two days later, only 0.3 percent of the Tech99 remained.
Half life is an important physical property of radioactive materials. The shorter the half life, the more intensely radioactive the element. The longer the half life, the less radioactive the element. For example, Tech99 has a half life of 6 hours and is very radioactive, but thorium has a half life of 14.5 billion years, so it is not very radioactive and there is still a lot of it around since it was created at the beginning of the universe. Tech99 does not exist naturally.
My daughter is still a little radioactive, just like me and you and everybody else. We have Carbon14 and Portassium40 in us, from the things we eat. Our cells can’t tell the difference between “natural” radiation and “man-made” radiation. The effects are the same, but depend on dose and dose rate.