They built a thin layer, or "nanostructure, " using minute particles of titanium dioxide, a substance that reacts with sunlight to break down dirt and other organic material and can be coated on cotton to keep the fabric clean.

CNN: Washing no longer dirty work

Matt Loxham, a medical biologist at Southampton University, collected minute dust particles of different sizes from at an underground train system in an unnamed European city.

BBC: Railway tracks

Another issue is that a second type of pollutant, aerosols such as the minute sulphate particles that form when sulphurous fuel is burned, promote the formation of clouds.

ECONOMIST: Changing science