Walking in the forest when ferns are releasing spores
I think you're talking about bracken? A lot of sources confirm that the spores are indeed carcinogenic, but I also came across this page that “Regarding bracken mentioned in your newsletter, if you are worried about breathing spores, we understand now that bracken doesn't spore in this country [the UK] and its expansion is only by vegetative growth."
Unfortunately, I couldn't find any other corroborating evidence to support that, but check out the article on UK's Cancer Research site, which does suggest wearing masks on dry, hot days, but that it's "unlikely [to be] a major cause of cancer".
Biting the hand that feeds IT
