The scientists from two government-run research institutes, Eawag and Empa, claim that built-up areas are responsible for a considerable proportion of the toxins designed to harm plants and animals.

Many exterior paints and materials for rendering buildings, for example, contain biocides to protect them from algae, fungi, weeds and vermin.

These substance can leach out or the original material when exposed to sunlight and rain – and are then washed away by rainwater which carries them into the watercourses.

In Switzerland alone, the authorities estimate that up to 600 tonnes of biocides are added to paint and façade renders every year.

Lab experiments designed to simulate exposure to the elements found that the leaching is at its worse the first time it rains following the application of the paint or render.

A linked study tracking the levels of pollutants in rivers or streams found that some of the polluting compounds could not be attributed to farms as they did not have any agricultural applications or concentrations did not match seasonal patterns of use.

According to the study, there is no doubt that the biocide concentrations measured in the facade runoff and additionally estimated using a computer model have toxic effects on algae and aquatic plant and animal life.

Contaminated facade runoff can enter watercourses directly via drains or stormwater sewers.

Substances that inhibit algal growth on facades exert the same effects in waterbodies – even when heavily diluted.

Sam Bond

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