Keeping the standard kilogram - a 4 cm high and 4 cm wide platinum iridium cylinder in Paris - clean is a formidable task. The cylinder has to be kept free from dust and contaminants like hydrocarbons (from air pollution) which can change its weight (Science, Vol 268, No 5212).

Now, Martin Seah and Peter Cumpson at the UK's National Physical Laboratory near London propose that exposing the standard to ozone and ultraviolet light will oxidise the hydrocarbons and free them so that they can diffuse out.