Some coal is high in fluorine and also has iron pyrite. The combination is deadly – when burned it make hydrofluoric acid. (H/T Chris Phoenix
Excess fluorine consumption can often cause dental fluorinosis, a condition in which excess fluorine is deposited in the teeth, discoloring them. In more severe cases, fluorine deposition in the bones can lead to osteofluorosis, which can cause disfigurement, deformity, and chronic pain.
One area in which osteofluorosis is distressingly common is Guizhou, China. Over the past decade, this disease here was linked to the combustion of high Fluorine coal. Studies showed that the clay that was intermixed with the coal was high in F, and a steady stream of recommendations has come along describing how this must be getting aerosolized in smoke ,and adhering to food, particularly corn and chilies hung up in houses to dry.
Flourine appeared inside uncut chilies, and sometimes was associated with silica- particulate matter which should not be able to penetrate food and is biologically inactive.
An adult suffering osteofluorosis in Majiazhuang village in Zhijin County.
Child suffering dental fluorinosis
Almost half of the total rural area of Guizhou Province and many regions within the 11 adjacent provinces in southwestern China have a long history (at least 70 years) of endemic fluorosis, including dental fluorosis and osteofluorosis along with its associated deformities and disabilities. Over decades of research, this specific type of endemic fluorosis has been defined as coal-burning fluorosis, which is distinct from drinking-water fluorosis. It is generally acknowledged that indoor burning and combustion of high-fluorine coal leads to food contamination, and fluorine then enters the human body. However, the exact chemical form of fluorine during its release and transfer to the body is still unknown. In the present study, 21 domestic coal samples from outcrop and semi-outcrop coal collected in five villages with fluorosis were analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The total mass fraction of sulfur in the samples ranged from 0.24%–5.58% and total fluorine content ranged 90.2–149.2 mg/kg.