When Does Radium-226 Become Safe?

[Moonflower]

When does 226Ra become "safe"?

Hi, I was working on an assignment which deals with radioactive decay.
Half life of 226Ra is 1600 years (it varies from source to source, but this is the one I chose), and the equation I got for half-life is y=Ce^(-0.000433216988t), C being the initial value and y being amount left in grams, and t being the amount of time passed in years.

When does 226Ra become "safe", where the radiation from it is no longer considered to the public? I know 226Ra is a toxic element, so I'm expecting pretty low...
also, if possible, give the units in grams, like x micrograms per gram, not using other units like pCi/L.

Thanks!

 

[mgb_phys]

Safe depends on the amount of radiation.
When there is either a small enough total amount of the material, or it has decayed to a point where there is a small enough amount of the radioactive isotope left to meet the safety limits for your country.
Generally with Radon the halflife isn't really a factor because new Radon is being produced from the Uranium in the native rock so there is a constant amount of radioactive Radon present.

In mining there is a monthly safe limit - this assumes 160hours exposure (40hours/week * 4weeks) and is equal to roughly 3.5 mJhours/m^3 or a concentration of 250Bq/m^3

Household exposure where you are exposed to the gas for >12hours a day and there is no monitoring would be higher, and so the safe limit in a home must be lower.