scope:

Introduction

Inorganic fluoride compounds are considered as classic air pollutants, emitted from specific - mostly industrial - sources [7]. Typical emission sources for gaseous and particulate inorganic fluorides are residential heating using coal, coal-fired (power) plants and in particular specific industrial processes as: production and use of hydrofluoric acid (HF), production of aluminium, glass, phosphate fertilisers, ceramics including bricks and potteries, electronic microchips); VDI 2310 Part 3, [30; 55; 60]. Although emissions of inorganic fluorides from these sources have been efficiently reduced during the past 30 years, increased emission of fluorides along with their injurious effects have been repeatedly observed near specific sources even in recent years [10; 31; 58].

Typical concentrations of fluorides and hydrogen fluoride are clearly under 0,1 μg/m³ in remote and rural regions, and only slightly higher in urban areas without specific industrial emission sources. In the vicinity of specific emission sources mean concentrations frequently range from 1 μg/m3 to 3 μg/m³ with peak concentrations - mostly during accidental malfunctions - reaching up to 23 μg/m³; VDI 2310 Part 3, [14; 30; 60].

In highly sensitive species of higher plants (e.g., St. John's Wort, Crocus, Gladiolus, Spruce, Apricot) visible leaf injury due to inorganic fluoride air pollution occurs at concentrations as low as 0,5 μg/m3 to 1 μg/m3 within a few days. The formation of visible injury is preceded by an accumulation phase during which the fluorides, predominantly taken up by the leaves' stomata and easily soluble in water, are transported by the transpirational flow towards the margins and tips of the leaves. The characteristic injury symptoms of atmospheric fluorides are beige to light brown necroses of the leaf tips and margins which are sharply separated from the intact leaf tissue [59]. The uptake of fluorides via the roots from the soil, where fluoride compounds are frequently present in high concentrations (partially > 1000 μg/g), but with very low availability, is generally low [13; 19; 60].

In less sensitive plant species, e.g., most grasses along with clover, fluoride compounds from gaseous and particulate sources can be accumulated to levels several times higher than the normal values (which range from 2 μg/g to 20 μg/g dry matter) without symptoms of visible injury [23; 24]. In cattle the uptake of fluoride-contaminated grass or fodder containing fluoride > 30 μg/g dry matter can result in dental and skeletal damage (fluorosis), whereas fluoride contents > 250 μg/g dry matter may cause acute poisoning and death of the animals (VDI 2310 Part 26). Accumulative bioindicators (e.g., Standardised Grass Exposure, VDI 3957 Part 2), featuring low sensitivity and high accumulation potential towards fluoride air pollution, can be efficiently used to assess the exposure and potential risks by atmospheric fluorides e.g., for livestock. For mammals fluoride uptake by inhalation is generally insignificant as compared to ingestion through food and water [30; 60].