scope:

Introduction

In literature describing the physical and chemical properties and effects of fluorine compounds, the terms "fluorine" and "fluoride" are used synonymously to describe fluorine compounds. This guideline uses the term "fluorine" in those cases where a precise distinction between ionic and molecular forms or between gaseous and particulate compounds is difficult or unnecessary. The term includes all compounds of the element regardless of their chemical form.

In nature, fluorine occurs only in the form of compounds, e.g. in fluorite (CaF₂), cryolite (Na₃A1F₆) and fluorapatite (Ca₁₀F₂(P0₄)⁶) as the most important fluorine-containing minerals in the earth's crust. The main natural sources of gaseous and particulate fluorine compounds in the atmosphere are volcanoes, with hydrogen fluoride (HF) constituting the most significant fluorine-containing emission. Further fluorine compounds, e.g. silicon tetrafluoride (SiF₄), ammonium fluoride (NH₄F), sodium fluosilicate (Na₂SiF₆) and potassium fluosilicate (K₂SiF₆), are also important components of volcanic emissions.

Anthropogenic emissions of fluorine compounds from numerous industrial processes, however, pose the greatest danger to agricultural and forestry crops. The main source of fluorine-containing air pollutants are coal combustion and the electrolytic processes of the aluminium industry. Fluorine compounds are also released by melting and combustion processes during treatment of loams, clays, and other rocks and minerals, e.g. from brickworks, and also in the production of superphosphate fertilisers from primary phosphates. In addition hydrofluoric acid plants, enamelling plants, glassworks, glass etching plants and uranium processing plants may also emit fluorine compounds.

Water-soluble, gaseous compounds have the greatest phytotoxicity of all gaseous and particulate fluorinecontaining air pollutants. Hydrogen fluoride (HF) is of the greatest practical concern because it is so widespread, partly due to conversion of other equally phytotoxic compounds such as silicon tetrafluoride and hydrofluorosilicic acid (H₂SiF₆).

Hydrogen fluoride was known to be a phytotoxic air pollutant as early as the 19th century [1].The first detailed reports of vegetation damage caused by exposure to fluorine compounds are derived from areas surrounding fertiliser factories [1 to 5], brickworks [6 to 8] and glass and enamel works [5; 9].

It was not until after the First World War that fluorine emissions started to attract greater interest, as stronger effects began to emerge in gardens, plantations and forests. These were traced back to emissions from new electrolytic aluminium production facilities, the transition to larger plants for the manufacture of glass, bricks and ceramics, the production and processing of hydrofluoric acid and open-hearth furnaces in steel plants [10 to 12]. However, the harmful effects generally remained locally confined since most of the factories were in isolated locations and their exhaust gases were discharged via relatively low stacks. With growing industrialisation and the increased use of coal for power generation, fluorine compounds, distributed by higher stacks over larger areas, have become major components of air pollution in industrial and densely populated areas.

It must be pointed out, however, that fluoride emissions from large facilities in particular have been in decline in recent years. In the EU around 14000 t hydrogen fluoride were discharged to atmosphere in 2004 [13].

This summary shows that industrial development is largely responsible for vegetation damage caused by exposure to fluorine compounds. Two types of vegetation damage are recognised:

• damage occurring in areas surrounding specific, individual emission sources and

• damage occurring within larger industrial and densely populated areas exposed to a variety of pollutants including fluorine compounds.

The ecological and economic significance of the first type of damage frequently arises from the rapid loss of forest areas. In the vicinity of brickworks and ceramic works, for example, the total loss of spruce plantations covering an area of up to 200 ha at a distance of 2 km from the emission source was recorded [14]. In an area surrounding aluminium works several hundred to several thousand hectares of woodland were severely damaged or lastingly destroyed, making reforestation and land use impossible for a long time [15 to 18]. In Norwegian fjords pine forests were destroyed by the effects of hydrogen fluoride up to distances of 13 km from aluminium smelters and visibly injured up to distances of 32 km [19].

Present results of investigations into the harmful effects in the field concur that fluorine compounds are capable of inflicting severe damage on vegetation close to emission sources; visible harmful effects such as leaf discolouration, leaf necrosis or plant death diminish relatively quickly with increasing distance from the source.