The creatures on this world tend to favour arsenic as a replacement for phosphorous in their biology. There's an overabundance of it in the ground soil, and very little phosphorus.
The equivalent of mushrooms on this world pack their spores with arsenic as an essential chemical nutrient for their spawns survival.
Is it possible that the resultant arsenic spore cloud would be dense enough to cause arsenic poisoning to anyone who breaths it in, over an appreciable amount of time, say a couple of hours? Assume that unlike the above gif, the spores become well mixed in the air (this may or may not impact how much arsenic they can contain), and the cloud rises to at least a few metres. More like the mist seen here
Mushroom spores are, from some googling, about 40 micrograms in weight, and the LD50 of 13 mg/kg. Some mushroom spores contain phosphorous at levels of 10-20 g/kg, so the arsenic in a single spore was a 1-to-1 replacement, each spore would contain 0.8 micrograms.
At those levels you'd need 16,250 spores per KG of person to get an LD50. A human of 80kg would need 1.3 million spores, and "a single basidiomycete mushroom is capable of releasing over 1 billion spores per day". That means in an hour something like 41 million spores per hour if it was released evenly throughout the day. It feels like this is more than enough spores.
However I don't know if my substitution of arsenic for phosphorous makes much sense, nor how dense the cloud of spores (spores per volume of air) would be?
Is this arsenic spore cloud something that could reasonable be lethal?
Please note: this question is tagged reality-check
This tag should clue in any potential answerers based on it's tag wiki: "Asks if a given concept is realistic in a given context. Answers should say yes or no, with supporting info. " (emphasis mine). As such I am not soliciting discussion or suggestions on ways to improve this concept. The context above should be taken as an inviolable fact, and not something to be iterated on as part of this question.