When discussing what types of food items can be used for the eiruv, the Gemara emphasizes that foods that are dangerous to eat should not be used.
Rav Hamnuna said: One may not establish an eiruv with raw beets, as Rav Ḥisda said: Raw beet kills a healthy person. The Gemara asks: Don’t we see people eating it and they do not die? The Gemara answers: There, it is referring to a beet that was only partially cooked, which is dangerous.
The Gemara follows this discussion with another quote from Rav Ḥisda that cooked beets are good for the heart, good for the eyes, and certainly good for the intestines.
The beets commonly referred to in the Talmud are Beta vulgaris cicla, a garden vegetable. Its leaves can be cooked and eaten, and have a flavor similar to spinach.
Another vegetable that may not be valid for the eiruv because of the potential danger involved in eating it is the onion. The Gemara relates that while the onion itself can be used for the eiruv, its leaves are potentially dangerous and cannot be used. A baraita is brought that teaches that onions should not be eaten because of “the snake that is in it.” The baraita continues with a story that Rabbi Ḥanina ate half an onion with half of the “snake” that was in it, and became ill to the extent that he was close to death. His colleagues then prayed on his behalf and he recovered, since the generation needed his teaching and leadership.
The “snake” that the Gemara understands to be the danger lurking in the onion is subject to much speculation. The Ritva suggests that it is a worm that is found in the leaves of the onion that is potentially lethal. According to most traditions, however, it refers to a sprouting onion, which looks very much like a snake. It is difficult to come to a clear conclusion regarding the Gemara’s contention that eating onions generally, or their leaves specifically, presents a danger, since experience shows that onions are eaten with no ill effects. Nevertheless, onions contain the chemical n-propyl disulfide (CḤ12S2). Ingestion of even relatively small amounts of raw onions can, theoretically, cause toxicity from this chemical, which denatures hemoglobin leading to the destruction of red blood cells. In our case, which discusses making up two full meals solely from onions, there is certainly the possibility of poisoning. People with specific sensitivity may even be in danger of death.