[Contribution of the incidence of urban sewage spreading to dietary chromium].

The sewage sludges contain fertilizing matters that justify their use in agriculture. Simultaneously, they hold minerals that are directly proportional to the nature and intensity of industrial activity. The more abundant are chromium, lead, zinc; cadmium and mercury are also found. The concentration in chromium varies from 0.2 to 20.0 g/kg of dry matter. Its assimilation by the rootlet and, later, its intestinal absorption, are essentially dependent on its chemical state; the hexavalent forms have a high biodisponibility and are more toxic than the trivalent compounds. In vegetals, the assimilated chromium is stopped in the root, associated to iron hydrates; a small fraction reaches the leaves and a weaker part comes up to the grain. (Table I). If the concentration of chromium--mainly hexavalent salts--is very important in soil, the mechanism of regulation in the rootlet becomes inefficacious and chromium diffuses in the whole plant (Table II). The development of spraying of sludges and composts will increase the quantity and efficiency of chromium in vegetals, because of various factors: the wastes of many industries: chromium plating plants, tanneries, painting and dyeing industries throw out hexavalent chromium; if the sewage sludges are purified by an irradiation treatment, it will tend to oxidize the whole chromium in hexavalent forms; at last, the presence of sewage sludges in the arable soil favours the assimilation of chromium by inhibiting that of iron (Figure 1). Then, in absence of iron hydrates that regularize the diffusion of chromium in the plant, this latter tends to overrun more easily the aerial tissues and organs of vegetals. On the whole, the spraying of sewage sludges supplies the soil with chromium that will easily be assimilated by the plant, considering that it will be mainly at an hexavalent state. Compared with other trace elements held in the sludges, chromium is assimilated by the plant in a clearly more important proportion (Table III). That is why the consequences of spraying concern more particularly chromium than other elements, even if they are toxic. The intensification of spraying will contribute to increase the chromium intake that varies from 100 to 200 micrograms/day and whose 60% are provided by foods of vegetal origin. The consequences of a more important intake will be dependent on chemical state of chromium. If the hexavalent salts are predominant, an increase of intake will provoke a high intestinal absorption, a raising of chromium rate in liver, kidney, spleen, bone, etc. This situation will eventually be responsible for a chronic intoxication.(ABSTRACT TRUNCATED AT 400 WORDS)