Hydrogen fluroide :: all ecosystems

Key Concerns:

Hydrogen fluoride (HF) is the most phytotoxic of all air pollutants. It accumulates in the margins and tips of leaves, and in sensitive taxa it causes:

• Necrosis (death of parts of the margins and/or tip)
• Chlorosis (yellowing, usually between the veins, especially towards the margins)
• Red colouration, usually due to the pigment anthocyanin (again, near the margins)
• Distortion of the shape of the leaf, particularly the tip.

The effect of this type of visible injury on the performance and longevity of the species is very difficult to specify. There are relatively few examples known where visible injury has been shown to have a lasting effect on the affected plants, especially at the concentrations that currently occur in the UK. Limited (unpublished) measurements in plants in which the HF caused distortion of the leaves, but no other symptoms, showed that there was no effect on photosynthesis. In the absence of quantitative data linking symptoms to ecological effects, it is recommended that the precautionary principle should be applied: regard all visible symptoms as having some potential ecological effect.

There is some evidence that HF may reduce fertilisation of ovules by interfering with the growth of pollen tubes. This is most likely to happen in species that have exposed stigmatic surfaces that remain active for longer periods of time. The research was done on cherry and strawberry. The effect on the latter was to cause distortion of the fruit shape. Unfortunately there is insufficient data to prescribe critical levels for this effect.

The relationship between the concentration of HF needed to cause injury and the length of exposure time is a negative, log-log, relationship (Weinstein, 1977) i.e. high concentrations will cause injury if they occur over short periods of time, and it takes weeks or even months to cause injury at the lowest concentrations. The extensive database collated by McCune (1969a,b), is used here as the basis for the critical levels for 1 day, 1 week, 1 month and 3 months. However, lower concentrations are applied because of work published after McCune's assessment. There has been a great deal of reliable research done at higher concentrations (eg >1 ug m^-3), but less at concentrations around 0.5-1.0 ug m^-3. There are very few studies that have used <0.5 ug m^-3, and only one in the 0.2-0.3 ug m^-3 range. The latter was work done on grape using open top chambers in Australia. As it is now recognised that open top chambers effectively over-expose plants because of the turbulence, and because the work has not been confirmed, it is dificult to specify the lowest concentration that will cause injury. Therefore, the critical levels for shorter time periods (1 day, 1 week) are reliable, but they are much less so for 1 month, and certainly for 3 months. Due to the uncertainties the 1 and 3 month critical levels are given as 0.2-0.3 ug m^-3.

There are other air quality guidelines and NOELs (No Observed Effect Level) in existence, notably in the USA. Some are far too high to give protection to the most sensitive receptors. The most recent is WHO (2002) which specified only a single air quality guideline to protect plants, a No Observable Effect Concentration of "approximately 0.2 ug m^-3" over two growing seasons. Because of the reservations outlined above, 0.2 ug m^-3 gives a margin for error, but the logic of specifying two growing seaons is not obvious and is difficult to justify.

Additional Comments:

It is difficult to monitor HF at concentrations as low as 0.2-0.3 ug m^-3, especially if large areas have to be covered. In view of this, it is recommended that areas at risk should be surveyed using biomonitors and plant fluoride analysis (see Critical Loads for Fluoride deposition and Biomonitoring methods).

Ecosystem specific information:

Arable habitats - Plant species differ enormously in their sensitivity to HF. Some of the most sensitive taxa are: Gladiolus, Allium, Crocus, Tulipa, Lilium, Iris, Convallaria, Freesia, Hypericum and Polygonatum. The commoner arable crops are much more resistant (e.g. wheat and barley). Some maize and dwarf bean (Phaseolus) varieties fall into the moderately sensitive group, but are unlikely to be injured at UK concentrations. Root crops such as potato and carrot, and the brassicas, are not sensitive to the concentrations that are likely to occur in the UK.

Leaves are most sensitive during the period of expansion, and after that time they can usually tolerate much higher concentrations. Several vernal species (eg Crocus) are very sensitive so the critical level should be applied for the period of their leaf expansion in late winter / early spring. Most Allium species are as sensitive but they appear later. Gladiolus remains sensitive for the whole of the life of the leaf so it is one of the most sensitive species. However, there are very great differences in sensitivity between varieties.

Based on expert judegement there is a rough correspondence between the 3 month critical level and the critical load; where one is exceeded then usually the other is also exceeded.

Bogs, wetland and heath - Plant species differ enormously in their sensitivity to HF. Some of the most sensitive taxa relevant to bogs, wetland and heath are: Allium, Lilium, Iris, Convallaria, Hypericum and Polygonatum. In the cases that have been properly investigated, there are also major differences between varieties and individuals of some species. If the most sensitive taxa are protected then all other, less sensitive taxa will be given a high degree of protection.

Leaves are most sensitive during the period of expansion, and after that time they can usually tolerate much higher concentrations. Several vernal species are very sensitive so the critical level should be applied for the period of their leaf expansion in late winter / early spring. Most Allium species are as sensitive, but they appear later.

There appears to be a rough correspondence between the 3 month critical level and the critical load; where one is exceeded then usually the other is also exceeded.

Coastal and rocky habitats - Plant species differ enormously in their sensitivity to HF. Some of the most sensitive taxa likely to be found around coastal and rocky habitats is Hypericum (St. John's Wort) and Allium. In the cases that have been properly investigated, there are also major differences between varieties and individuals of some species. If the most sensitive taxa are protected then all other, less sensitive taxa will be given a high degree of protection. Leaves are most sensitive during the period of expansion, and after that time they can usually tolerate much higher concentrations.

Expert judgement suggests a rough correspondence between the 3 month critical level and the critical load; where one is exceeded then usually the other is also exceeded.

Freshwater - Plant species differ enormously in their sensitivity to HF. One of the most sensitive freshwater taxa is: Iris. In the cases that have been properly investigated, there are also major differences between varieties and individuals of some species. If the most sensitive taxa are protected then all other, less sensitive taxa will be given a high degree of protection.

There appears to be a rough correspondence between the 3 month critical level and the critical load; where one is exceeded then usually the other is also exceeded.

Note -  critical levels apply ONLY to emergent vegetation - sedges, Iris, rushes, grasses etc, NOT to submerged or floating leaved species.

Grassland - Plant species differ enormously in their sensitivity to HF. Some of the most sensitive taxa found in glassland ecosystems are: Allium and Hypericum. In the cases that have been properly investigated, there are also major differences between varieties and individuals of some species. If the most sensitive taxa are protected then all other, less sensitive taxa will be given a high degree of protection.

Leaves are most sensitive during the period of expansion, and after that time they can usually tolerate much higher concentrations. Several vernal species are very sensitive so the critical level should be applied for the period of their leaf expansion in late winter / early spring. Most Allium species are as sensitive, but they appear later.

There appears to be a rough correspondence between the 3 month critical level and the critical load; where one is exceeded then usually the other is also exceeded.

Woodland - Plant species differ enormously in their sensitivity to HF. Some of the most sensitive taxa relevant to woodlands and hedgerow are: Allium, Lilium, Iris, Convallaria, Hypericum and Polygonatum. Some pine species, such as P. sylvestrisand P.mugho are almost as sensitive. Narcissus and bluebell, Endymion, are less sensitive again. Most common native trees such as Quercus, Alnus and Fraxinus are at least an order of magnitude less sensitive. In the cases that have been properly investigated, there are also major differences between varieties and individuals of some species. If the most sensitive taxa are protected then all other, less sensitive taxa will be given a high degree of protection.

Leaves are most sensitive during the period of expansion, and after that time they can usually tolerate much higher concentrations. Several vernal species (e.g. Crocus) are very sensitive so the critical level should be applied for the period of their leaf expansion in late winter / early spring. Most Allium species are as sensitive but they appear later. Several pines (e.g. Scots pine) are almost as sensitive, but the period of surveillance for them should be from the start of needle expansion until it ends in July/August. After that time, pines are much less sensitive. Second and later year needles are relatively resistant.

There appears to be a rough correspondence between the 3 month critical level and the critical load; where one is exceeded then usually the other is also exceeded.