How many rad is dangerous for a human?

Answer 1

This question is like asking how much smoking is harmful. There is no exact answer. Damage begins immediately. More exposure equals more damage. And this is exactly the problem with radon. It doesn't kill directly like, say, poison from a venomous reptile. The radioactive inert gas has a short half-life, and, when breathed in at measureable concentrations even below the recommended limit, some radon undergoes decay in the lungs. This irradiates surrounding tissue. The radioactive daughters of radon will also cause damage when they decay inside the body. The recommended limit of radon contamination in air is 4pCi/l (4 picocuries/litre of air). Even at this level and a bit lower, a wise homeowner would immediately get a consultation and seriously consider an option to deploy radon mitigation technology to reduce the level even more.

Answer 2

Exposure to 1,000 rad can be fatal to a human, while lower doses can still cause various health issues such as nausea, vomiting, and an increased risk of cancer. It is important to limit exposure to radiation to as low as reasonably achievable to minimize health risks.

Answer 3

According to the Wikipedia article, Rad,

"A dose of under 100 rad will typically produce no immediate symptoms other than blood changes. 100 to 200 rad delivered in less than a day will cause acute radiation syndrome, (ARS,) but is usually not fatal. Doses of 200 to 1,000 rad delivered in a few hours will cause serious illness with poor outlook at the upper end of the range. Doses of more than 1,000 rad are almost invariably fatal[2]. The same dose given over a longer period of time is less likely to cause ARS. Dose thresholds are about 50% higher for dose rates of 20 rad/h, and even higher for lower dose rates."

Refer to the related link for the entire article.