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Manganese and the Environment

Manganese is an essential trace element in the metabolism of all living organisms. Nevertheless, excessive doses are toxic, and the resultant disease may arise in the pulmonary system or the central nervous system (CNS).

Manganese exposure is usually via inhalation (the risk varying with the manganese species involved and with particle size).

The manganese industry has adopted specific measures to protect its workers. These include : reducing exposure levels and time of exposure, the use of exhaust ventilation, having workers in isolated control rooms or in air-conditioned or filtered air cabins, making available better protective equipment.

Many countries have issued restrictions concerning the permissible amount of airborne manganese in dust and fumes (threshold limit value) and these seem likely to become tighter in the future.

Manganese and the Human Body

There is general agreement amongst health professionals and dieticians that manganese is essential to ensure the health and well being of humans and animals. The human body contains from 12 to 20 milligrams of manganese. Estimations of the human requirements for manganese vary considerably, but are based on studies of the balance between intake and excretion necessary to maintain this level.

Data from several studies suggest that manganese intake of from 0.035 to 0.070 milligrams per kilo of body weight provides the needed balance. However, a 1988 study conducted by researchers at the University of Texas at Austin found that a minimum of 3.5 milligrams per day seems necessary.

Human consumption depends on the amount of certain foods consumed. The typical English winter diet (with substantial tea intake) provides up to 8.8 mg of manganese per day, while studies of women in Japan, Canada, New Zealand and the USA suggest average daily intakes from 2.5 to 4 mg per day.

Manganese deficiency has been demonstrated in animals and has been noted in humans in association with vitamin K deficiency. Its main manifestations in all species studied are impaired growth, skeletal abnormalities, disturbed or depressed reproductive functions, lack of muscular coordination among newborns and defects in lipid and carbohydrate metabolisms.

The following fresh food groups (in descending order) are most important in manganese content : nuts, whole cereals, dried fruits, roots, tubers and stalks, fruits, non-leafy vegetables, meat, poultry products, fish and seafoods. Leafy vegetables also rank high on the list when expressed in dry-weight terms. Tea has a very high manganese content, ten times that of cereals.

The wide range of manganese contained in cereal grains and products depends on the species and on the milling process used. In a US study, wholewheat containing 31 ppm manganese yielded 160 ppm in the germ, 119 ppm in the bran and only 5 ppm in the white flour.

Half a cup of oatmeal, 30 grams of shredded wheat or raisin bran cereal, a quarter of a cup of pecans or a third of a cup of peanuts, and half a cup of cooked spinach, contain each over a milligram of manganese.

Sweet potatoes, red lima or navy beans and pineapple juice are other sources. There is very little or no manganese present in dairy products and highly refined sugar-containing foods.

Manganese and Animal health

Grazing cattle do not seem to suffer from manganese shortage, probably because their manganese requirements are met by the herbage they consume. Manganese deficiency is a more serious problem in domestic animals such as poultry, non-grazing cattle and pigs. The main reason for this is that protein supplements of animal origin (dried milk, fish meal, mean meal) are usually low in manganese.

To promote strong legs in poultry, and to keep up normal egg production, it is necessary to supplement their feed with about 40 mg of manganese per kilo of weight. For ruminants, the requirement for optimal skeletal development is only 20-25 mg/kilo. Soybean meal, important in poultry raising, contains 30 to 40 ppm manganese.

Manganese, Plants and Soil

All plants require manganese for growth and reproduction. Manganese is the element chosen by nature to catalyse oxygen evolution in photosynthesis. When there is a lack of manganese, the structure of the chloroplasts is markedly impaired. Manganese also plays a role in lignin synthesis and in thephenolic acid metabolism, helping to suppress both leaf and root diseases. Some plants are particularly susceptible to manganese deficiency.

Manganese content in the soil can vary widely. It is only 50 ppm in some localities, but can reach 10,000 ppm in unleached alkali soils. Only the divalent cation (Mn+2) which is soluble and mobile in the soil, is easily available for absorption by plant roots. The tetravalent cation (Mn+4) is virtually insoluble. Both leaf-manganese and soil-manganese analyses are used to determine manganese deficiencies or toxicity.

Manganese deficiency in most crops is indicated by an overall chlorosis of the leaf, which leaves only the main veins and midrib outlined in dark green healthy tissue. The degree to which plants can absorb manganese is more important than its simple chemical presence in the soil.

Manganese sulphate is considered the most efficient source of manganese for fertilizer production, although manganese monoxide is also used. These are added in regions deficient in manganese. Manganese sulphate and other compounds can also be used in solution to spray on foliage.

It has been demonstrated that a fungal infection of wheat can develop as a result of manganese deficiency. To correct it efficiently, manganese must be brought to the roots of the plant. The hypothesis is that manganese has a direct effect on the pathogen through toxicity, and strengthens the host plant by modifying its resistance and susceptibility.

It should also be noted that an excess of absorbable manganese which sometimes occurs in acid soil, can be toxic to some species. The problem is generally solved by adding lime.