The EC decay of 40K explains the large abundance of argon (nearly 1%) in the Earth's atmosphere, as well as prevalence of 40Ar over other isotopes.

So, when a mineral forms – whether from molten rock, or from substances dissolved in water – it will be initially argon-free, even if there is some argon in the liquid.

Since the rate at which this conversion occurs is known, it is possible to determine the elapsed time since the mineral formed by measuring the ratio of 40K and 40Ar atoms contained in it.

A 70 kg human body contains about 140 g of potassium, hence about 140g × 0.0117% ≈ 16.4 mg of 40K;[7] whose decay produces about 3850[8] to 4300 disintegrations per second (becquerel) continuously throughout the life of an adult person (and proportionally less in young children).

At the estimated 0.1 μSv, one banana equivalent dose is around 1% of the average American's daily exposure to radiation.