Radiation can prove hazardous through external exposure (handling or standing too close to radioactive matter) or ingestion (consuming radioactive food or water). Furthermore, some radioactive substances have additional chemical effects.
The health risks of a high dose of radiation are straightforward: it causes cell death and damage, and above a certain point will kill a human being. These are called deterministic effects. Except for the case of Alexander Litvinenko, all instances of acute radiation poisoning have been accidental; so there isn’t really a body of research involving humans. However, these effects are routinely used in (for instance) radiation therapy for cancer, and this plus the informal data from various exposures that have occurred have given a good picture of the danger to humans. (However, even though sieverts and other units specifically measure biological damage, it’s not always straightforward to correlate risk with dosage.)
25 microsieverts (millionths of a sievert) is the world-wide average cumulative yearly dosage from environmental background radiation. It is also the maximum allowable one-time dose from a single airport screening.
One millisievert (thousandths of a sievert) per hour is the US Nuclear Regulatory Commission’s threshold definition for a high-radiation area.
A cumulative dose of 0.1 sieverts (Sv) at a rate of 0.1 Sv/hour is the threshold at which deterministic health effects start to occur. These effects include so-called radiation burns, death of white blood cells and bone marrow, and general incapacitation.
Above the 0.1 Sv dose, effects increase with more radiation and mortality starts occurring at 1.5 Sv. About half of exposed people die (LC50) at 3 Sv of exposure. Above 5 Sv, almost everyone dies.
Below doses of 0.1 mSv, deterministic effects do not exist; however, there are other effects that can only be stated as statistical possibilities: for instance, there is a general scientific consensus that each sievert of cumulative low-level dose increases a person’s chances of getting cancer by 5.5% over their lifetime. Beyond that, there is little agreement. (While it is beyond our scope to engage the scientific controversy over the effects of low-level radiation, we do note elevated cancer rates in areas that have been exposed to chronic non-acute radiation.)
Strontium-90 is of extra concern because it has similar chemical properties to calcium: it can accumulate in bones and stay there for the rest of a person’s life, irradiating their bone marrow. Furthermore, it decays to an even more virulently radioactive isotope: yttrium-90, with a half-life of 64 hours. Thus, in calculating the radioactivity of strontium-90, one usually includes the yttrium-90 radiation along with it.
Caesium-137 has a different set of special chemical properties: it very quickly forms compounds that dissolve readily in water, thus making it a major contaminant of the water supply – for animals and crops as well as for humans. In Fukushima, for example, where most of the radiation leakage is through contaminated water, caesium is the major contaminant being found in meat and vegetables produced nearby.
Because caesium-137 and strontium-90 are so common and have these properties that make them especially dangerous to humans, the danger to people from major nuclear accidents is often expressed in terms of the quantity of caesium-137 and strontium-90 that was released.