The estimated lethal dose of Polonium-210 by ingestion is around 0.1 micrograms, so swap it for the cadmium and that typical salad could kill 100 people.
With doses of ionizing radiation, there are like two to three orders of magnitude of various things we measure where the consensus is that they are likely OK for you (things large enough to move you within that range include[1] eating lots of bananas, having chest X-rays, flying in airliners, living in the highlands or in a place with a naturally high background, and having mammograms).
Then there are[2] multiple orders’ of magnitude worth of chasm that are considered[3] varying degrees of OK if you’re a particle physics experimentalist or radiochemist, nuclear reactor technician, or—worst of all—astronaut. At the high end of that, it starts to matter if you’ve received the dose all at once and in which place of your body and which kind of radiation it was. (I mean the units are supposed to take the last two points into account always, but here those correction factors can start to matter.)
Finally, there are a couple of orders of magnitude where you inevitably and gruesomely die at varying speeds, and after that nobody lived long enough to report.
The chasm is where you get single-percentage-point increases in multi-decade incidence of cancer and such, which is what you probably care about. (Don’t get me wrong, that can amount to a lot of dead people in the wrong circumstances, not to mention infertility.) Fortunately for humanity but unfortunately for your particular question, AFAIK we don’t have enough data to tell with any degree of certainty just how bad any particular point of that chasm is, and there’s no straightforward way to acquire that data.
As far as dramatic death, though, tens of nanograms of polonium inside your body (which is an especially nasty thing to have there) will absolutely kill you dead. That's on the order of 0.1 quadrillion atoms. Of course, those atoms are exceptionally easy to detect, comparatively speaking. As another point of reference, lethal doses of nerve agents are on the order of a milligram and up.
