The internet is fraught with misinformation concerning the toxicity of houseplants! It's also full of sites that will list plants as "toxic" or "non-toxic" and leave it at that or throw a bunch of scary medical science terms your way, leaving many feeling ill-informed and worried about the consequences of a slip-up.
It's a legitimate concern - many of us are pet parents as well as plant parents, and we all want to make sure that we and those under our care are as healthy and happy as possible. However, while it's important to know which plants are toxic or non-toxic, it's also important to understand why and to what degree a plant is toxic.
90% of the time, the culprit here is a substance called calcium oxalate, known in plants as raphides. This substance also occurs naturally as a mineral called whewellite (which, interestingly, is actually the mineral that composes kidney stones).
Not all raphide crystals are the same shape. The shape of this tiny crystal has an impact on the symptoms that it causes in humans and pets, but there's no shape of it that's perfectly harmless.
In general, ingesting these crystals will cause symptoms that are usually along the lines of "vomiting, irritation, numbness" etc. This is because the crystals are abrasive and will irritate what they come into contact with.
Sounds scary, right? The key point to keep in mind, as mentioned before, is to what degree this substance will affect you or your pets on ingestion and how much of it you or they would have to eat before it could become a serious issue.
Raphides are a defense mechanism evolved by plants to discourage herbivores and omnivores from eating them, but it's not a mechanism designed to kill the offending party. By and large, you or your pet would have to eat quite a lot more of the plant than either of you could stand before you were in danger, particularly because calcium oxalate has a very distinct flavor that both you and your pets have evolved to very much dislike the taste of (not to mention the fact that your mouth is feels like its burning after only a small amount, which makes it hard to continue eating). And when we say "quite a lot", we mean quite a lot - we're talking "percent of body weight" amounts for most houseplants.
When a website lists a plant as "toxic" or "non-toxic", the presence of this substance is specifically what they're referring to. And they're not wrong - a plant that makes you throw up is obviously toxic, but again, there's a big difference between "this plant is toxic" and "this plant will kill you".
Now, that doesn't mean you won't experience symptoms - it's just that those symptoms will probably be restricted to the realm of vomiting and discomfort and not usually continue on to become anything truly serious. In the case of pets, you'll notice vomiting and, if they somehow ate more than a few mouthfuls, you might notice their gums bleeding (which can be scary looking). Don't panic! This bleeding is caused by the abrasions of those calcium oxalate crystals, which is only slightly more severe than brushing your gums way too hard with a toothbrush.
In a few specific examples, such as dieffenbachia (known colloquially as dumb cane), there are a number of symptoms particular to that plant. In the case of dieffenbachia, the crystals are a specific shape that results in symptoms that numb the mouth and cause excessive drooling and an inability to speak! This can seem quite scary, but again, it's very unlikely that you or your pet ate enough for it to become a problem (especially considering your mouth becomes slobbery numb mess and it becomes hard to chew).
In the case of plants like rubber trees, the culprit is latex and not calcium oxalate (although plants that produce latex are likely to also produce calcium oxalate). Symptoms can be more aggressive if you or your pet has a latex allergy, but the same rule of thumb holds true: it's very unlikely you or your pet could eat enough of the substance for it to become a problem. Latex in any form tastes truly, thoroughly, unimaginably disgusting.
If you or a child eats a plant that should not have been eaten, you or they will quickly feel terrible and the body will get rid of the plant material... in all of the usual ways.
Keep an eye on the persistence and intensity of these symptoms and use that to gauge the severity of the problem, but rest assured that these symptoms will clear as the body expunges the irritant. Unless the plant is specifically a plant that deadly poisons are made from (which, by and large, you can't easily buy), don't panic.
If you notice your pets consistently chewing on greenery (especially dogs) and then vomiting and you're having to fight them off of eating your plants, this is actually a well-known phenomenon that's sometimes hypothesized to have evolved in order to purposefully induce vomiting via these calcium oxalate crystals. No one is quite certain why they do this, but it's nothing to worry about and is very normal behavior. Symptoms to watch out for besides vomiting are lethargy, swelling, or bleeding (though, again, these symptoms will abate and don't necessarily indicate anything beyond ingestion of calcium oxalate). If you notice symptoms persist, seem disproportionate to the amount of plant eaten, or involve anything beyond what we've described here, that's when you take them to the vet.
Because calcium oxalate causes abrasions, if there are toxins present in the plant material they will be more efficiently absorbed through the abrasions in the throat - however, toxins are still not usually present in doses that would be easy to consume, and much of the plant material would still probably have to be consumed before it was a dire issue. And, you know, as long as you're not actively putting toxins into the plant, your houseplant probably doesn't have many of them. They are houseplants, after all, and it wouldn't be much of an industry if a significant number of houseplants were a health issue.
If you're wondering how herbivores can eat all these plants if they have raphides in them, it's an interesting discussion that has to do with the anatomy of the animal (many herbivores have more than one stomach, one of which is dedicated specifically to breaking down plant material) and the composition of their gut bacteria (which is more geared to digesting plants than the gut bacteria of omnivores like us). Some animals are even known to lick clay or other minerals before eating plants with a lot of needly raphides, which protects their already-tough mouth tissue from the burning sensation. If you're interested in learning about that, we highly recommend researching it - there's lots to learn from nature!
No rule is set in stone and there are obviously many instances of plant toxicity that results in serious medical issues, but knowing the facts and the rarity of this is important to take the edge off of that particular concern. If you're truly worried, you can consult your doctor or veterinarian about the house plants that you have or want to buy! You can also feel free to get into contact with us and we'll be happy to give you all the facts you could ever want about your plant.