[kidd]

There is lots of radioactivity around the home.

Each and every smoke detector has Americium-241.

Salt substitutes contain potassium...of which 0.01% contains naturally radioactive potassium-40.

Lantern mantles contain radium, as do some watch hands.

The human body has over 100 grams of potassium-40 and emits over 4000 beta particles per second.

[InMemoriam]

Wait, it is 0.0001 of naturally occurring potassium and the body contains > 0.1kg of it?!?

0.1 kg / 0.0001 = more than 1000 kg of total potassium in a human body.

No.

[TChris]

Lantern mantles contain radium, as do some watch hands.

Lantern mantles contain thorium, not radium.