The environmental effects of chemical films that reduce evaporation of water from reservoirs should be properly evaluated before the films are applied on a large scale (Nature 519, 18–19; 2015).

For example, when these films are driven by waves towards the shoreline (F. R. Crow and A. L. Mitchell Water Resources Res. 11, 493–495; 1975), they could accumulate in soils and sediments and then enter biological concentration processes, disrupting ecosystems (C. Franke et al. Chemosphere 29, 1501–1514; 1994). The film coating reduces the water's surface tension, interfering with floating plants, pollen transport and insect egg-laying. It has a heating effect that could alter flux between water layers, disturbing solutes and habitats.

Hydrophobic airborne pollutants, including particles of black carbon, will be attracted to the films, potentially changing the surface's reflectivity (albedo) and contributing to climate change.

We should learn from previous ill-judged large-scale chemical applications and ensure that the necessary safety tests and regulatory measures are in place first.